Technical feasibility study on the chromium recovery from electroplating effluents

Table of Contents

Abstract

Preface

Nomenclature

1 Introduction and Purpose

2 Chromium
2.1 History of chromium
2.2 Production and properties of chromium
2.2.1 Chromium processing and application
2.2.1.1 Chromium in the refractory industry
2.2.1.2 Chromium compounds
2.2.1.3 Chromium pigments
2.2.1.4 Chromates as rust prevention
2.2.1.5 Chromium tanning
2.2.1.6 Electroplating
2.3 Chromium in the environment
2.3.1 Chromium in the air
2.3.2 Chromium in aquatic systems
2.3.3 Chromium in humans, animals & plants

3 Separation of Chromium
3.1. Chemical precipitation
3.2. Membrane separation
3.3. Adsorption
3.4. Ion exchange
3.5. Solvent extraction
3.5.1. Principles of solvent extraction (SX)
3.5.2. Extractants
3.5.2.1 Alamine 336
3.5.3 Solvents/diluents
3.5.4 Modifiers
3.6. Anionic liquid ion exchange (A-LIX)

4 Analysis of chromium:
4.1 Atomic flame Absorption Spectroscopy (AAS)
4.1.1 Interferences in atomic flame absorption spectroscopy
4.2 Colorimetric analysis

5 Materials and Methods
5.1 Identification of a suitable solvent mixture for extraction experiments
5.2 Establishing extraction isotherms for chromium recovery
5.2.1 Preparation of Cr(VI) stock solution
5.2.2 Preparation of solvent mixture
5.2.3 Determining optimum pH for chromium extraction
5.2.3.1 pH adjustment of aqueous phase (feed)
5.2.4 Determining the effect of competitive anion (SO42-) on Cr(VI) extraction
5.2.5 Effect on Cr(VI) concentration on extraction efficiency
5.3 Determining the loss of reagents during extraction
5.3.1 Solubility of solvents as function of SO42-
5.3.2 Solubility of solvents as a function of temperature
5.4 Total Suspended Solids (TSS) in real effluent
5.5 Determining sulfate concentration in real wastewater
5.5.1 Titration experiment
5.5.2 Sulfate determination by colorimetric analyses (La Motte)
5.6 Oxidation of chromium(III) species
5.7 Quantitative element analyses (AAS)
5.7.1 Sample preparation for the AAS
5.7.2 Installation and setup of the instrument (Perkin Elmer AAnalyst 200)
5.8. Colorimetric qualitative chromium analysis

6 Results and Discussion
6.1 Solubility of reagents
6.2 Effect of pH and solvent on Cr(VI) extraction
6.3 Effect of competitive anion (SO42-) on Cr(VI) extraction
6.4 Effect of Cr(VI) concentration on extraction efficiency
6.5 Effect of temperature and NaOH concentration on the solubility of Na2SO4
6.6. Determining the loss of reagents during extraction
6.6.1. Solubility of solvents as a function of SO42-
6.6.2. Solubility of solvents as a function of temperature
6.7. Determining sulfate concentration in real effluent
6.8. Total Suspended Solids (TSS) in real Effluent
6.9. Metal extraction experiments with single and multi-elemental solutions
6.10. Metal extraction experiment in real electroplating wastewater

7 Conclusion

8 References

9 List of tables

10 List of figures

11 Appendices i

Abstract

Chromium is an element that is widely used for the manufacturing of stainless steel, in leather tanning and electroplating. Hexavalent chromium is toxic and cause of considerable damage if released into the environment. Coupled with depleting reserves and stricter environmental laws it becomes necessary to reduce, reuse and recycle this important commodity.

The objectives of the study are to test the suitability of Alamine 336^[1] in the anionic liquid ion exchange (A-LIX) process for the recovery of hexavalent chromium from electroplating effluents by comparing the performance of conventionally used kerosene and palm oil as a renewable and biodegradable solvent.

This study showed that refined palm oil is a potential candidate to replace more hazardous solvents such as kerosene. 1:1 stoichiometrical extraction can be carried out over a pH range of 1.0 – 4.0 without significant loss in extraction efficiency. Alamine 336 has a high affinity for chromium species CrO3SO42- and HCrO4-, while sulfate anions have no effect on chromium uptake. Visual Minteq has proven to be a useful tool and was successfully applied to solve relatively complex metal speciation and solubility problems encountered in solvent extraction.

The findings of this study provide a significant step towards a greener recycling technology and zero emission plants in the electroplating industry.

Preface

This report is submitted as a documentation of the Diploma Thesis in Environmental Engineering accomplished during the 8th and 9th semester at the Malaysian Institute of Chemical and Bioengineering, Universiti Kuala Lumpur, Malaysia in cooperation with the Hochschule Wismar – University of Applied Sciences, Technology, Business and Design, Wismar, Germany.

I would like to thank my research supervisors, Prof. Dr. M. Wilichowski and Prof. Dr. J. Dietrichs, for their support and guidance throughout my research work.

Special thanks go to Dr. R.T. Bachmann for his excellent engagement and his constructive advice during all the time of my project work. His patience and nature to explain scientific coherencies motivated me a lot and the cooperation was exemplary.

I also like to appreciate A.B. Tengkiat from the University of Santo Tomas for his professional support.

The valuable experiences which I have gained in this country will accompany me on my further way. I like to say thanks to all the students at the department of environmental engineering for their nice and friendly bearings and their help I was allowed to adept all the time.

Wismar, 20 November 2015

Daniel Wiemken

Nomenclature

Abbildung in dieser Leseprobe nicht enthalten

1 Introduction and Purpose

Rapid industrialisation and growth in population over the past two hundred years exert an increasing pressure on natural resources and the environment. Billions of tons of controlled and scheduled waste are generated every year (Research and Markets, 2008) by the industrial sector worldwide which is often either pre-treated on-site or at a licensed contractor prior to disposal in landfills. This practice if continued is leading to resource depletion and creates a potentially harmful legacy for future generations. In order to move towards a more sustainable development as outlined in the Bruntland Report (1987), waste reduction, reuse and recycling coupled with pollution prevention measures play an important part to slow down if not reverse this practice.

Heavy metals such as cadmium, mercury, lead and chromium are not degradable or renewable like biomass hence if they are to be used in future processes reuse and recycling are the only options. At present, heavy metals are used in the chemical industry sector for applications ranging from batteries to catalysts and surface coatings, and can be found at various concentrations in gaseous, liquid or solid waste.

Chromium is of particular interest owing to its legislative status and unique chemistry. Chromium exists in nature primarily in one of two oxidation states (Bartlett, 1998; Rai et al., 1989). There are other chemical oxidation states of chromium, which include 0, II, IV, and V, but they are considered transitory compared to more stable Cr(III) and Cr(VI) species. Hexavalent chromium is a strong oxidizer which can react with DNA causing mutation, while the trivalent, organically complex form is a dietary supplement to help with proper glucose metabolism, weight loss and muscle tone (Bartlett, 1998). Unlike many other metals, Cr(VI) can combine with oxygen to form water-soluble, negatively charged anions known as yellow chromate (CrO42-) or orange dichromate (Cr2O72-), which adsorb to positively charged sites in contrast to cationic metal species (Bartlett, 1998). Therefore, hexavalent chromium species are not strongly bonded in many soils under alkaline to slightly acidic conditions, for example. Thus, they can be very mobile in subsurface environment while other metals precipitated out and exert toxic effects on biological systems.

Various well-established methods may be used to treat industrial effluents and contaminated water such as reduction and precipitation (Kongsricharoern & Polprasert, 1995), reverse osmosis (Frenzel et al., 2006), evaporation (Frenzel et al., 2006), ion exchange (Cavaco et al., 2007) and adsorption (Huang & Wu, 1977; Malkoc & Nuhogly, 2006). While these processes are able to remove the pollutants from the waste stream to meet stringent discharge limits, they may produce concentrated waste and be less suitable for efficient and selective material recovery. Solvent extraction, on the other hand, is one of the most effective conventional methods extensively used in separation science (Cerna, 1995), with potential to selectively recover metals from industrial effluents that can also be operated in a closed loop. Various reviews dealing with solvent extraction of chromium and other heavy metals are available (Ritcey G., 2006); (Rao & Sastri, 1980); (Cerna, 1995). Several ion-association forming systems such as triisooctylamine (Bojanowska, 2002), tetrabutylammonium bromide (Aliquat 100) (Akama & Sali, 2002; Venkateswaran & Palanevelu, 2004), trioctylmethylammonium chloride (Aliquat 336) (Pribil & Vesley, 1970; Alonso & Pentelides, 1996; Sze & Lam, 1997; Park et al., 2002; Galan et al., 2005), trioctylamine (Alamine 336) (Adam & Pribil, 1971), triphenylsulphonium (Halal et al., 1966) and triphenylphosphonium (Halal et al., 1966) and (Burns et al., 1987) have been studied to extract chromium in the anionic form.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1 Chemical structure and formula of various extractants used in chromium recovery processes

In this study Alamine 336, a tertiary amine, will be used for hexavalent chromium extraction because of the lack of publicly accessible information. By inspecting eq. 1 and eq. 2 Alamine 336 should be able to extract anionic species of hexavalent chromium. According to supplier information (Anon, 2007), the Alamine series of reagents exhibit the following general extraction chemistry:

Abbildung in dieser Leseprobe nicht enthalten (eq. 1)

Abbildung in dieser Leseprobe nicht enthalten (eq. 2)

The first equation illustrates amine salt formation with R representing a variety of hydrocarbon chains, and the second equation represents true ion exchange. The extent to which B- will exchange for A- is a function of the relative affinity of the two anions for the organic cation and the relative stability of the anions in the aqueous medium. Typically, extraction favours larger lowly charged anions over smaller more highly charged anions.

A number of extractants have been used for the recovery of Cr (VI) such as aqueous solution of (NH4)2SO4 (Akama & Sali, 2002), benzene (Agrawal et al., 2008), chloroform (Pribil & Vesley, 1970; Bojanowska, 2002; Agrawal et al., 2008; Dhakal et al., 2005), dichloroethane (Burns et al., 1987), dichloromethane (Venkateswaran & Palanevelu, 2004), hexane (Dhakal et al., 2005; Agrawal et al., 2008), kerosene (Lo et al., 2008; Dhakal et al., 2005; Galan et al., 2005; Agrawal et al., 2008), methyl isobutyl (Maeck et al., 1962; Venkateswaran & Palanevelu, 2004),toluene (Dhakal et al., 2005), tri-n-butylphosphate (Tuck & Walters, 1963), and xylene (Agrawal et al., 2008).

Most of the solvents reported are acceptable for use in lab-scale extraction processes but are not suitable for industrial scale extraction processes due to economics, health, safety and environmental concerns. Of all the solvents cited above kerosene appears to be the preferred solvent of choice.

Kerosene, a mixture of various hydrocarbons obtained by distillation of petroleum at boiling point 175 to 330°C, is also used as an aircraft gas turbine and jet fuel by both commercial airlines and the military service, as heating oil, and as a spray oil to combat insects on citrus plants (Bland & Davidson, 1967). With regards to its use as solvent in commercial extraction operations the lighter aromatic compounds in kerosene such as benzenes, toluene and xylenes are still a cause of concern due to their toxicity, volatility and water solubility.

In order to render the solvent extraction process safer and more environmentally friendly a renewable and harmless solvent for metal recovery would therefore be highly desirable. In this study the extraction performance of refined palm oil and kerosene as solvent is investigated and compared using Alamine 336 on Cr(VI) extraction from acidic synthetic wastewater solutions. In addition, experiments are extended to best matching synthetic multi element solutions and finally extractions were carried out in diluted effluent. Software based speciation modelling in dependence of altered pH, SO42- and Cr(VI) concentration also was part of this survey.

2 Chromium

2.1 History of chromium

The history of chromium^[2] began over 200 years ago. Four Siberian gold mines have been working for the elements gold, copper, silver and lead since 1752. In 1761 Johann Gottlob Lehmann obtained samples of an orange-red mineral that he termed “Siberian red lead” while visiting the Beresof mines located in the eastern slopes of the Ural Mountains. But the remoteness of these sources delimited the use of it. In 1770 Peter Simon Pallas visited the same site as Lehmann and found this mineral that had useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. A bright yellow made from crocoite (PbCrO4) also became fashionable. In 1797 Louis-Nicolas Vauquelin received samples of crocoite ore. At first he only extracted chromium oxide (CrO3) by mixing crocoite with hydrochloric acid (HCl). In 1798 Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gemstones, such as ruby, or emerald (Guertin, Jacobs, & Avakian, 2005).

During the 1800s, people used chromium compounds mainly for coloring paints and clothes dyes. A particularly popular colour was chromium yellow or lead chromate. Soon other chromium-containing chemicals were being used for processes such as tanning leather. At first, crocoite from Russia was the main source but in 1811 a larger chromite (FeCr2O4) deposit was discovered by Isaac Tyson near Baltimore in the United States. This recovery enabled the economical use of the ore and the United States became the largest producer of chromium products until 1848 when large deposits of chromite where found near Bursa Turkey. Chromium was used for electroplating as early as 1848 but this use only became widespread with the development of an improved process in 1924 (Lepora, 2006).

Today, chromium is a strategic metal of the twentieth century but it is also used in a dozens of industrial processes (Table 1), creating thousands of consumer products.

Table 1 Typical application fields of chromium nowadays (Guertin et al., 2005)

Abbildung in dieser Leseprobe nicht enthalten

2.2 Production and properties of chromium

On earth chromium is a widespread element occurring in many minerals. It is also indispensable to life for many organisms. Chromium takes place 21, with a concentration of 100 mg/kg, of the frequency distribution on elements in the lithosphere; and is the 26th most common element in seawater (Kogel et al., 2006). Though, in the earth core it takes position seven with a concentration of 5.000 mg/kg (Salem & Katz, 1994). The 50 times multiple concentration of chromium in the core is a result of the origination of our earth. When the earth was created and asteroids smashed together, the early planet was a ball of hot liquid. Heavy substances, like most metals including chromium, sank to the centre of the growing planet (Lepora, 2006).

Chromium exists in all oxidation states between –II up to +VI, however, only the stable ternary and hexavalent compound, as well as metallic chromium are of practical relevance. It is a silvery, ductile, extensible and malleable metal (Gauglhofer, 1984). Further metal specifications are given in Table 2.

Table 2 Thermal, electrical and mechanical properties of chromium (Wakaki et al., 2007)

Abbildung in dieser Leseprobe nicht enthalten

Chromium is conductive and has an atomic radius of 129pm. The trivalent oxidation state is the most stable in acidic and aqueous solutions; especially hexavalent chromium is stable in alkaline environments. +V compounds already disproportionate in aqueous solutions to +III and +VI bondings. The specific stability of trivalent chromium refers to the fact that Cr(III) compounds have a pure alkaline character and that they are strong reducing agents. Cr(VI) compounds comprise a pure acidic character, thus they are strong oxidizers. On the other hand, the Cr(III) compounds are amphoteric in redox- as well in acid-base behaviour.

Chromium almost never occurs as a pure metal in nature. Instead, it is found combined with other elements such as iron, lead, and oxygen in natural compounds called minerals and ores. Most chromium ore is buried deep underground; scientists believe that there is a plentiful supply for the future but extraction up to the markets is subject to high energetic efforts and high costs. The main ore of chromium is the mineral chromite. This chemical is a compound of iron, chromium, and oxygen (FeCr2O4). Chromite and other metals are still most common in the liquid centre of the earth. Sometimes, the hot liquids from deep inside the earth ooze up through cracks in the seafloor. This creates a layer of chromium minerals along with other rocks. Over millions of years, movements of the crust of the Earth push these rocks up from the seafloor to make dry land. These rock layers are the chromium ores that are mined today (Lepora, 2006).

Recovery of chromite originating from chromite-ore includes surface and underground mining methods and ranges from hand sorting to gravimetric and electromagnetic separation processes. Chromite for refractory, chemical and metallurgical markets involves crushing and grinding, and size sorting by pneumatic and hydraulic methods (Rao et al., 2004).

Beneficiation of the recovered chromite becomes necessary when it is mixed with other minerals due of geological conditions or when mechanized mining methods are nonselective. Beneficiation depends on the end use and may include increasing chromic oxide content, the chromium / iron ratio, or alumina content. Reducing silica content or eliminating waste rock associated with chromite-ore also may desirable. A variety of techniques that are used to accomplish these tasks further depend on the physical properties of the minerals present. Beneficiation cannot change the chemical characteristics of the chromite mineral because chromite-ore is a mixture of minerals, however, altering the mineral mixture can change its characteristics for its respective market. A deposit producing lumpy ore in which chromite mineral is easily distinguished visually may require only hand sorting and screening; otherwise, heavy media separation can be used. A deposit that yields chromite-ore thoroughly mixed with gangue minerals may require milling and sizing followed by gravimetric or electromagnetic separation to produce marketable chromite products. (Kogel et al., 2006). The flow chart in Figure 2 illustrates the variety of processes that are required to produce marketable products from chromite-ore (FeCr2O4).

Abbildung in dieser Leseprobe nicht enthalten

Figure 2 Flow-chart of required steps in the production of metallic chromium and some of the most important commercial chromium compounds (Nriagu & Nieboer, 1988)

The high mechanical efforts that have to be made for the ore mining and further process treatments require a high demand on power which accompanies with high carbon dioxide emissions. Also the missing infrastructure and inaccessibility as well as political instability, especially in emerging markets, make chromium production an expensive and complex business.

To sum up, the extensive use of chromium in industrial processes is the dominant source of anthropogenic chromium emissions in liquid, solid and gaseous forms, which can have significant adverse biological and ecological effects (Hoet, 2005). The durable increasing world request for finite products needs a sharper look at industrial metabolism.

2.2.1 Chromium processing and application

Chromium is used in the ferrochromium (85 %), chemical (8 %) and refractory industry (7 %) (Papp, 1998). Chromium is primarily obtained from chromite ore. South Africa, Kazakhstan and Zimbabwe are the main exporters of this mineral commodity owning 6500 million tons or 97 % of the proven chromite reserves worldwide (C.M, 2007; Coakley, 2002; Ellmies, 2001; Papp, 1994). The annual worldwide production of chromium increased exponentially since recording started hundred years ago (Figure 3). (Papp, 1994) estimated that 26 % of the chromium produced worldwide in 1993 has dissipated in the environment. The following sections introduce the major uses respectively industries that deal with chromium.

Abbildung in dieser Leseprobe nicht enthalten

Figure 3 Annual worldwide production of chromium since 1900 (U.S.G.S., 2009)

2.2.1.1 Chromium in the refractory industry

The refractory industry needs ores with a maximized concentration of Cr(III)- and Al(III) oxide, a maximum of manganese and a minimum of iron. Chromium corundum stones, mix crystals from Al2O3 and 5-10% Cr2O3, were particular used in partitions of the furnace. Pure chromium oxide stones only find application in special realms (e.g. fibreglass production) due to its high price.

2.2.1.2 Chromium compounds

Resource for all chrome chemicals is the chromite. For the transformation of chromite to other chromium compounds, the following oxidising digestion that leads to alkaline chromates is applicable in industrial scale.

Abbildung in dieser Leseprobe nicht enthalten (eq.3)

Soda can be partly replaced with sodium hydroxide. Lime or iron oxide from the digestion residue is needed to aspirate by spongy soda and sodium chromate^[3] and hence allows air admission to the reaction mixture. The ore that is enriched by flotation is ground to small particles (dp < 0,1mm). Nowadays the digestion takes place in rotary furnaces.

The complete reacted mixture has a contingent of 30-45% sodium chromate, which is equal to a yield of 75-85%. The digested mineral is ground and leached with water. In this way chromate solutions with 500 g/L are obtained. Insoluble iron,- silicon,- and aluminium oxides or hydroxides remain in the solid residue. Today this residue only contains a few mils Cr(VI). The conversion in sodium dichromate (Na2Cr2O7) takes place by ignition with sulfuric acid:

Abbildung in dieser Leseprobe nicht enthalten (eq. 4)

After evaporating the resulting solution to 70 mass percent almost the whole sodium sulfate precipitates out. Further concentrating leads to the crystallisation of sodium dichromate dehydrate. The reaction of sodium chromate with carbon dioxide in aqueous solution under pressure also leads to sodium dichromate:

Abbildung in dieser Leseprobe nicht enthalten (eq. 5)

The precipitated sodium hydrogen carbonate is either calcined to soda by 200°C – 300°C or transformed to soda by use of caustic soda, which is in turn submitted to the alkaline-oxidizing digestion:

Abbildung in dieser Leseprobe nicht enthaltenO (eq. 6)

Abbildung in dieser Leseprobe nicht enthalten (eq. 7)

2.2.1.3 Chromium pigments

Lead chromate (PbCrO4) is used since the 19th century as a yellow pigment. By adding “Prussian Blue” (Fe4[Fe(CN)6]3) a chrome green pigment is obtained. Meanwhile there is a multitude of chromium pigments, ranging from yellow to red. They are adopted in colours, printer cartridges and floor coverings.

Chrome green consists in general of Cr2O3. It is produced by reduction of sodium dichromate with charcoal, sulfur or other organic materials.

Abbildung in dieser Leseprobe nicht enthalten (eq. 8)

Abbildung in dieser Leseprobe nicht enthalten (eq. 9)

It is durable obverse acids and bases and is used for coloration of rubber products, cement, ceramics and roof tiles. It is a licensed cosmetic also for coloration of contact lenses. It is also used in printing inks for bank notes and protective mimicry for military crafts. (Salem & Katz, 1994, Swaddle, 1997)

2.2.1.4 Chromates as rust prevention

Soluble chromates are used for rust prevention in coolant circles-, systems, and towers. They support the generation of a thin layer on the metals, which protect the surface from corrosion. The best protection is achieved in light alkaline ambience with chromate concentrations of a few mils.

Chromic acid stain also avoids atmospheric corrosion of aluminium and magnesium. For that purpose it colorizes the metal and acts as a primer for further coatings. Zink chromate found great application as a primer for construction steel and in shipbuilding. Zink chromate is also used as a primer for the construction of aeroplanes (Nriagu & Nieboer, 1988).

2.2.1.5 Chromium tanning

The tendency of Cr(III) to build complexes with alkaline carboxyl and amino groups in proteins makes it interesting for the leather tanning industry. Alkaline Cr(III) salts, especially the oligomeric Cr(III) hydroxyl sulfates with the configuration Cr4(SO4)5(OH)2 links the carboxyl groups from the collagen in the animal skin. This process increases the temperature resistance and decreases the extension of the material.

Cr(III) hydroxyl sulfates, for instance, are formed. by reduction of sodium dichromate with sulfate dioxide:

Abbildung in dieser Leseprobe nicht enthalten (eq. 10)

Around the end of the last century the commercial development of the chromium tannery started. The use of chromic tanning agents provides some advantages versus the use of herbal agents. Herbal agents require some days for successful tanning processes. With chromium, tannery processes can be accomplished in few hours. Furthermore chromic leathers are of sturdier heat resistance. Herbal tanning leather is used in shoe soles and leather belts; chromium tanning leather is soft and flexible (Salem & Katz, 1994).

2.2.1.6 Electroplating

Wastewater streams from these industries are heavily loaded with hazardous substances, particularly hexavalent chromium. For this purpose it should be verify whether it is possible to remove these species before the typically used “one-way” treatment is applied (reduction, precipitation and subsequent dumping of the sludge).

Electroplating is defined as coating a metal surface with another metal by applying electric current (electrolyses). Electroplating baths (or tanks) contain metal ions that are reduced to a metallic state on the metal being plated. The metal being plated serves as the cathode. The anode is usually the metal providing plating by dissolution in the bath solution. If an inert metal serves as the anode, the bath solution is replenished by the addition of a salt of the metal as it is consumed by plating. Electroplating baths contain metal salts, complexing agents, pH buffers, as well as organic and organometallic additives.

A typical bath composition of a chromium bath is as follows:

Chromic acid (225-300 g/L H2CrO4)

Sulfuric acid (2.25-3.9 g/L H2SO4)

Operating temperatures 45-60°C

Plating current 1.55-3.10 kilo amperes per m² DC^[4]

Anodes lead with up to 7% antimony

A great variety of metals or alloys are used for plating; those that are commonly used include copper, chromium, nickel, zinc, cadmium and other precious metals (Wang et al., 2004). Recovery of these metals, except cadmium, is also part of extraction experiments from synthetic and real wastewater and is analysed by atomic flame absorption spectroscopy (AAS) in this present study.

In surface engineering chromium layers are affixed to multitude of materials and are used in a wide variety of businesses. The relatively easy controllability of the processes has extensively forced the distribution of chrome-plating (Winkler, 2009).

In principle there are two classes of chromium plating: “decorative”, in which thin coatings serve as a non-tarnishing, durable surface finish; and industrial “hard” chromium, where heavy coatings are used to take advantage of the special properties of chromium, which include resistance to heat, wear, corrosion, erosion, low coefficient of friction and anti galling.

Decorative chromium is almost always plated on top of either nickel or copper and nickel-plated layer. These sub-layers of copper and nickel tend to seal off the substrate so that the micro-cracking chrome deposit does not present corrosion problems. Decorative chromium deposits are used on such items as automobile bumpers and trim, household appliances, furniture’s and many other articles that require a bright and aesthetic appearance. The normal thickness of decorative chromium is in the range of 0.0005mm – 0.0018mm. The total deposit including the copper and nickel under-layers is typically 0.013mm thick (Such & Dennis, 1993; Svenson, 2009).

Hard chrome is used as a wear resistant coating not only on steel but also on a wide variety of other metals. Hard chromium differs from decorative chromium not only because of its use but also because of the difference in deposit thickness. A typical hard chrome deposit is in the range of 0.013mm – 0.25mm. The electro-deposition of hard chrome is a recognized means of prolonging the life of all types of metal parts subjected to wear, friction, abrasion and corrosion. These parts can be protected when newly manufactured, or they can be salvaged when they are worn and would otherwise be scrapped. As an example, it is much less expensive to reclaim worn hydraulic components by rebuilding tolerances with a hard chrome deposit than it is to buy or fabricate the part from scratch. Hard chromium, because it has a low surface energy, is more often deposited on sliding or revolving parts and is therefore used on things like engines, pumps, compressors, hydraulic or pneumatic rods, etc. The hard chromium deposit is also highly resistant to corrosion, and a large number of applications where it is used to protect parts from corrosion are in popular demand.

Chromium cannot be deposited from a solution containing only chromic acid and water. There must be at least one or more acid radicals present to act as catalyst and aid in the cathodic deposition of chromium. Chlorides have also been tried as catalysts along with a few other halogen radicals but drawbacks of these are the more precise need to control their level and problems with several etching parts. Chlorides are now regarded as major contaminants and care must be exercised to keep chloride contamination out of the chrome bath. For successful operation, the ratio of chromic acid to total catalyst acid radicals must be maintained within defined limits, preferably about 100:1 in the case of sulfate and other ratios for fluoride and related catalyst ions. It is generally immaterial what other substances the catalyst is combined with when enters it the bath or from what sources it may be derived, but the material must be soluble (Such & Dennis, 1993, Svenson, 2009).

Some sulfate is ordinarily present in all chromium plating baths, since it is contained in even the best commercial grades of chromic acid. Sulfuric acid is the most common source of sulfate. The definition of a catalyst in any chemical equation is “an agent which allows the reaction to proceed toward its desired end but it’s not itself used up in the reaction”. The sulfate ion (SO42-) in the chrome-plating bath acts as a true catalyst in that it is regenerated continuously and the only way its concentration decreases is through drag-out. Other catalyst ions, like fluoride, are consumed in the electrolysis so in reality they are not true catalysts, but act like catalysts (Svenson, 2009).

Looking at the occurring reactions, chromium baths can be prepared by adding hexavalent chromium in the form of either sodium dichromate (Na2Cr2O7 • H2O) or chromium oxide (CrO3). When sodium dichromate is used it dissociates to produce the dichromate ion (Cr2O72-). When chromium oxide is used, it immediately dissolves in water to form chromic acid according to the following reaction:

Abbildung in dieser Leseprobe nicht enthalten (eq. 11)

Chromic acid is considered as a strong acid, although it never completely ionizes. Its ionization has been described as follows:

Abbildung in dieser Leseprobe nicht enthalten (eq. 12)

Abbildung in dieser Leseprobe nicht enthalten

Abbildung in dieser Leseprobe nicht enthalten (eq. 13)

Abbildung in dieser Leseprobe nicht enthalten

Moreover, the dichromate ion (Cr2O72-) will exist in equilibrium with the acid chromate ion as follows:

Abbildung in dieser Leseprobe nicht enthalten (eq. 14)

Abbildung in dieser Leseprobe nicht enthalten

Theoretically, HCrO4- is the predominant species between pH 1.5 and 4.0, HCrO4- and CrO42- exist in equal amounts at pH 6.5, and CrO42- predominates at higher pH values. Chromium plating wastewater is generally somewhat acid, and the acid chromate ion HCrO4- is predominant in this wastewater.

The waste streams generated by the plating process can be subdivided and classified into eight categories (Wang et al., 2010):

1. Concentrated acid wastes
2. Concentrated phosphate cleaner wastes
3. Acid rinse water
4. Alkaline rinse water
5. Chromium rinse water
6. Nickel rinse water
7. Concentrated nickel wastes
8. Concentrated chromium wastes

Apparently most hazardous waste from a metal finishing plant originates from wastewater which is generated by rinsing operations. These effluent streams in turn, require further treatments by cleaning, plating, and stripping operations. The savings associated with reducing rinse water use are primarily from reduced water, sewer, and sludge disposal fees. Improved rinse efficiency should reduce costs, chemical use and sludge generation. These are dependent on rinse water hardness and the sludge precipitation and the chemicals used in the wastewater treatment system.

Multiple rinse tanks can be used to significantly reduce the volume of used rinse water. A multistage counter-current rinse system uses up to 90% less rinse water than a conventional single stage rinse system. In a multistage counter-current rinse system, work piece flow moves in cross flow to the rinse water stream. Figure 4 illustrates the use of a three-stage counter-current rinse system.

Abbildung in dieser Leseprobe nicht enthalten

Figure 4 Three-Stage counter-current rinse system (DIANE, 1992)

Abbildung in dieser Leseprobe nicht enthalten (eq. 15)

Abbildung in dieser Leseprobe nicht enthalten Rinse water demand (fresh water) L*h-1

Abbildung in dieser Leseprobe nicht enthalten Diversion L*h-1

C0 Concentration of a substance in the process bath; or its diversion

Cn Concentration in the last rinsing stage

N Number of stages

Considering the whole process, rinse water treatment is needful and is a not underestimated process to achieve discharge limits for industrial wastewater streams, reduce the demand on freshwater, concentrate and recover valuable finite elements and finally to achieve cost advantages over competitors.

An introduction of these groups of chromium appliance (including electroplating) should bear a sentience for its wide field of usage and its necessity for a suitable treatment respectively recovery process. A sophisticated process management (e.g. implementation of rinse water cascades) will help to organise these industries to work more eco-friendly and finally to safe costs for treatment.

2.3 Chromium in the environment

With approximately 100 mgCr/kg, chromium compounds are components in the lithosphere and theirs alteration is the major reason for the natural chromium concentration in ground- and surface water, soils, dust and in the air. Volcanism e.g. is another natural source of emission. The annually global chromium emission by volcanism is estimated to 3.900 tons and emissions by dust drifts are expected to 50.000 tons annually (Pacyna & Nriagu, 1988).

However as expected, the chromium concentrations in the environment are mostly affected by anthropogenic sources. They were estimated to a global mass of 75.000 tons, additionally to 54.000 tons non-anthropogenic sources. In the 1970s, major atmospheric emissions of chromium were from the chromium alloy- and metal producing industries, and fewer amounts came from coal combustion, municipal incinerators, cement production, cooling towers, use of chromium containing phosphate fertilizers and landfill dumping of chromium contaminated sewage, sludge and consumer products (Eisler, 2000). Table 3 gives an overview of chromium concentrations for major chromium processing industries and clarifies the necessity for an ecological treatment of these waste streams, especially from electroplating industry.

Table 3 Typical chromium concentrations in industrial waste streams (Salem & Katz, 1994)

Abbildung in dieser Leseprobe nicht enthalten

2.3.1 Chromium in the air

Chromium accesses by natural as well as anthropogenic pathways in the atmosphere, its distribution is quite inhomogeneous. The lowest concentration (0,005 ng/m3) was measured above the Antarctica. The values over the Arctic are six to ten times higher than what is found in the higher industrialization and the bigger landmass of the northern hemisphere (Pacyna & Nriagu, 1988, Nriagu et al., 1988). Generally, chromium concentrations above the oceans are measured between 0.1 – 1 ng/m³, in rural regions the concentrations are diluted to an average of 4 ng/m³ and in the cities about 20 ng/m³. The highest annual average chromium concentration of 157 ng/m³ was measured in 1977 in Baltimore (USA) (Pacyna & Nriagu, 1988, Nriagu et al., 1988). Even in particular industrial areas of Hong Kong concentrations of 3300 ng/m³ are reported (Chuang et al., 1979). It is expected that one third of the atmospheric chromium exists in the hexavalent oxidation state (Salem & Katz, 1994). Vapours caused by volcanism can contain 250 ng / m³ (Mount Etna, Sicily) or 4500 ng/m³ (Tolbachik, Siberia) e.g. the chromium accesses the soils and waters by deposition.

Hard chrome plating industries emit high concentrations of Cr(VI) which can be treated by well designed and maintained venturi scrubbers. If a zero tolerance of emissions is claimed, it is necessary to explore innovative technology from other types of processes and installations (Wilson, 1997).

2.3.2 Chromium in aquatic systems

In aquatic environments, the major sources of chromium are the electroplating and metal-finishing industries and publicly owned treatment plants; relatively minor sources are iron and steel foundries, inorganic chemical plants, tanneries, textile manufacturing, and runoff from urban and residential areas. Chromium in phosphate, used as fertilizers is also an important source of anthropogenic emission in waters (Guertin et al., 2005). The toxicity of chromium to aquatic biota is significantly influenced by abiotic variables such as hardness, temperature, pH and salinity of water, biological factors such as species, life stage, and potential differences in sensitivities of local populations (Ecological Analysts, 1981).

In both freshwater and marine environments, hydrolyses and precipitation are the most important processes that determine the fate and effects of chromium, whereas adsorption and bioaccumulation are relatively minor.

Both, Cr(III) and Cr(VI) can exist in water with little organic matter; Cr(VI) is usually the major specie in seawater. The ratio between these two forms depends on various processes, which includes precipitation/dissolution reactions, and adsorption/desorption reactions (Guertin et al., 2005). Under oxygenated conditions, Cr(VI) is the dominant dissolved stable chromium species in aquatic systems. The hexavalent form exists as a component of a complex anion that varies with pH and Eh and may take the form of chromate (CrO4-2), Hydrogen chromate (HCrO4-), or dichromate (Cr2O7-2); with dichromate predominating in acidic pHs (EPA, 1984). These ionic Cr(VI) forms are highly soluble in water and thus mobile in the aquatic environment. All stable Cr(VI) anionic compounds strongly oxidize organic matter on contact and yield oxidized organic matter and Cr(III). Trivalent chromium tends to form stable complexes with negatively charged inorganic or organic compounds, and hence is unlikely to be found uncomplexed in aqueous solution if anionic or particulate compounds are present.

Many aqueous environments do contain oxidizing agents, such as MnO2 and Mn3+ in sufficiently high concentrations to produce measurable yields of Cr(VI). In oxygenated surface waters, not only pH and the oxygen concentration are important, but the nature and concentrations of reducing agents, oxidation mediators, and complexing agents play important roles. These factors seem to be responsible for the occurrence of significant Cr(III) quantities in many oxygenated surface waters (Guertin et al., 2005).

In acidic to light alkaline conditions, chromate can be adsorbed by hydroxide groups in surface waters with presence of minerals like ferrous oxide (Fe2O3) or aluminium oxide (Al2O3) (Zachara et al., 1987). By protonation of the hydroxide groups the adsorption is increasing on these solid particles, by decreasing pH. The adsorption depends on the mineral itself and is highest on ferrous oxides. Adsorption is reliant to the amount of available spots and consequently depending on the particle size of the adsorbing material and also of the concentration of competing anions like sulfate or carbonate (Zachara et al., 1989; Stollenwerk & Grove, 1985). Precipitated Cr(III) hydroxides remain in the sediments under aerobic conditions. Under low pH and anoxic conditions, however Cr(III) hydroxides may solubilise and remain as ionic Cr(III) unless oxidized to Cr(VI) through mixing and aeration (Eisler, 2000).

The behaviour of chromium species in aquatic sediments is also an important fact that needs to be considered for an evaluation of its risk potential. This is widely discussed in the literature like (Nriagu & Nieboer, 1988; Allen et al., 1998; Guertin et al., 2005) and is not part of this paper.

Parameter limits for effluents which are discharged into inland waters, published by Malaysian Federal Subsidiary Legislative are given in Table 4. Standard A corresponds to effluents which are discharged into any inland waters within the catchment areas and Standard B becomes effective for industrial discharges into any other inland waters.

Table 4 Parameter limits for industrial effluents and common metal species in electroplating wastewater streams (Federal Subsidiary Legislative, 1979)

Abbildung in dieser Leseprobe nicht enthalten

The (World Health Organization, 2004) is also regulating a limit of 0.05 mg/L^[5] Cr(VI) in drinking water. However, according to the US EPA total chromium (including Cr(VI), Cr(III) and its other forms) is regulated below 2 mg/L (Salvato & Dee, 1992). Actually (Sadaoui et al., 1997) define the wastewater norm for Cr(VI) to be restricted strongly to about 0.1 mg/L that will become more severe in the future. Due to its high mobilization and toxicities in the environment, hexavalent chromium is the strictest regulated metal specie.

2.3.3 Chromium in humans, animals & plants

Chromium is an important trace element for humans and mammals, but not for plants. The human body naturally contains 5 – 50 mg trivalent chromium (Gauglhofer, 1984). Complex bonded trivalent chromium is a component of the glucose metabolism. Its absence leads to diabetes. Chromium deficiency favours arteriosclerosis. For adults the appropriate dose of chromium intake is between 50 and 200 µg/day (Nriagu & Nieboer, 1988). In contrast, hexavalent chromium is strongly toxic.

At the beginning of the last century it was discovered that Cr(VI) compounds are heavy skin irritating substances that can ulcerate. It was monitored, that workers who were exposed to chromic acid (H2CrO4) vapours had frequent perforations in the nasal septum. Since 1935 it was pointed out that there is a higher risk of lung cancer by occupational exposure of Cr(VI) compounds (Nriagu & Nieboer, 1988). Employees in chromate processing/producing industries suffered a lot by bronchial carcinomas. Allergic skin irritations are also found to be an effect of occupational handling with chromium compounds. Skin sensitizing can happen by antirust agents in air conditioners, etching solutions in the lithography and by wood preservatives (Salem & Katz, 1994). Reports from Europe, Scandinavia, Asia and North America all emphasize the high incidence of lung cancer and other respiratory diseases among workers involved in the manufacture of chromates (Eisler, 2000; Nriagu & Nieboer, 1988; Guertin et al., 2005).

Cr(VI) is more toxic because it is more mobile than trivalent form, is a strong oxidiser, and diffuses readily into tissues. It also participates in the non-specific binding and consequent inactivation/malfunctioning of important biological molecules. Cr(VI) was also found to inhibit respiration by inactivating several other enzymes which are involved in other physiological processes (Shahid et al., 1998).

Chromate (CrO42-) can gather better in the human bowel system than Cr(III) and permeates the cell wall. With its oxidation power it can directly damage the cell nucleus and can be complexly bond in its reduced form to the DNA. By changing the genotype, the result is uncontrolled cell growth and as a consequence cancer. Chromium storage in the organism is little. The trivalent chromium that is essential for the glucose metabolism is stored in small quantities in the liver. Hexavalent chromium is reduced by the organism. Higher chromium concentrations are only found in the hair and while chronic lung damages. Cr(VI) compounds are 10 to 100 times more toxic than Cr(III) compounds (Salem & Katz, 1994). Acute animal tests, such as the LC50^[6] and LD50^[7] tests in rats, have shown that Cr(VI) has extreme toxicity from inhalation (LC50=30-140 mg/m³) and oral (LD50 < 100 mg/kg BW^[8] ) exposure. Chromium(III) exhibits moderate toxicity from oral exposure, with LD50 values of 200-2000 mg/kg BW and much lower toxicity from acute dietary exposures (EPA, July 2000).

For humans, the OSHA^[9] workplace permissible exposure limit (PEL^[10], 8-hour-time-weighted average) for chromic acid and hexavalent compounds is 0,05 mg/m³ (carcinogen). For bivalent and trivalent chromium the PEL is stated to 0,5 mg/m³ (Olsen, 2004).

Crops contain on average 0,2 – 1 mg/kg chromium in dry mass. Cr(III) is not available in high amounts for plants in soils. On the other hand Cr(VI) can be captured significantly better but remains predominantly dissolved in the vacuoles of the radix. With an increasing chromium concentration the radix gets damaged and the concentration of the residual plant also increases. Oat plants for example die by a Cr(VI) concentration of 750 mg/kg in soils (Rengel, 1999).

3 Separation of Chromium

Separation processes are central to the petroleum, chemical, petrochemical, metallurgy, pulp, pharmaceutical, mineral and other industries. Major portions of capital and operating expenses of such industries are associated with one or more separation processes; consequently, the impact of separation process technology on corporate profitability is large in most of these industries. The growth of new industries, based on biotechnology or electronics, for example, requires the development of new separation techniques and application of historically successful technology in environments (Rousseau, 1987). There is a wide range of well developed separation techniques available. The choice of used technology is depending on costs of the system, permitted concentrations for waste discharge, and selectivity of the process.

The focus in this chapter is particularly on the chromium separation from wastewaters, especially from electroplating effluents. Improvements of the treatment become necessary due to the rapid development of those industries in Malaysia. In addition the government encourages the establishment of modern plating plants to complement the country's industrial growth (MIDA, 2010).

The applied separation technique in this study is the liquid-liquid or solvent extraction which was stated as one of the best and most suitable methods which can be used for extraction of Cr(VI) from wastewaters (Stas, J., 2007); some of the most competing methods are only introduced in a summarized form.

3.1. Chemical precipitation

Precipitation of metal loaded wastewaters involves adding chemicals in order to alter the physical state of the dissolved or suspended metals and to facilitate their removal through sedimentation. The incidental precipitates may then be further treated for metal recovery. Commonly used chemicals to effect precipitation are caustic soda, lime ferrous and sodium sulfide, soda ash, sodium borohydride, and sodium phosphate. Some wastewater constituents, e.g. hexavalent chromium, cannot be effectively precipitated without first reducing the metal chemically to a more favourable form (trivalent chromium) for precipitation. Typically used reducing agents are sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sulfate.

The following chemical reactions, using ferrous sulfate, illustrate this method of treatment (reduction, neutralization, precipitation):

Abbildung in dieser Leseprobe nicht enthalten (eq. 16)

Abbildung in dieser Leseprobe nicht enthalten (eq. 17)

Abbildung in dieser Leseprobe nicht enthalten (eq. 18)

In order to enhance settling times of the precipitated metal coagulation chemicals usually are added. Examples of coagulants currently used by industry include lime, alum, and synthetic polyelectrolytes. Solubility products of some metals which are typical for electroplating wastewater are listed in Table 5.

Commonly, chemical reduction followed by precipitation is used to treat metal bearing wastewaters from electroplating, pigment manufacture, the photographic industry, feather tanning, wood preservation, the electronics industry, battery manufacture, and nonferrous metal production. Approximately 75% of all electroplating facilities use precipitation in the treatment of their wastewaters (EPA, 1991). This process is more than a few decades old, and chemical feed reagents are being improved to yield better metal removals from the aqueous phase.

Specific waste characteristics that could affect the performance of chemical precipitation systems include the concentration and types of metals, the concentration of total dissolved solids, the concentration of complexing agents, and the concentration of oil and grease (Nemerow & Agardy, 1998; EPA, 1991).

While the precipitation process is effective, it simultaneously generates large amounts of sludge that needs further treatment whereas new chemicals and energy is needed. In relation to the fact that heavy metals are no renewable substances, the demand to more eco-friendly technologies like recover-reuse systems must be the future.

Table 5 Solubility products for common metal ions found in electroplating effluent (Wilichowski, 2008)

Abbildung in dieser Leseprobe nicht enthalten

3.2. Membrane separation

The commercially available membrane processes for the removal of metal species from industrial wastewaters include microfiltration^[11] (MF), ultrafiltration^[12] (UF), reverse osmosis (RO), and electrodialyses (ED). MF and UF are used in combination with chemical treatment for the physical separation of metal sludge’s. RO and ED are used to recover plating compounds from rinse water and to enable reuse of these waters.

MF and UF membranes cannot be applied directly to recover those metals that are present as dissolved solids in wastewaters. UF however, can be used as a pre-treatment method for RO units to avoid fouling of the RO membranes.

A major advantage of membrane separation processes is that the metal content of the effluent can be extremely low assumed an appropriate pre-treatment was done. Each application requires studies on the treatability to integrate the chemical pre-treatment and the MF/UF membrane system. Well-run precipitation / UF and MF systems can achieve metal removals greater than 99 %.

RO systems consist of several modules which are connected in series, parallel, or a combination of both. However, it is noticeable that the application of RO to the treatment of metal-containing wastes is limited by the pH range in which the membrane can operate. For example cellulose acetate membranes cannot be used on waste streams with a pH > 7. The amide or polysulfone membranes nevertheless have a pH range of 1 – 12. Influences like colloidal matter, low-solubility salts, and dissolved organics can seriously inhibit the effectiveness or RO. Pre-treatment steps such as pH adjustment, carbon adsorption, chemical precipitation, or filtration are therefore recommended to ensure extended service life of RO systems. Commercially these systems are commonly used to recover brass, hexavalent chromium, copper, nickel, and zinc from metal-finishing solutions (EPA, 1991).

In conclusion, RO offers effectively filtration of ions through a semi-permeable membrane at high pressures, which provides an alternative mean of concentrating metal impurities for subsequent removal. This approach can be capital intensive (high pressures) and furthermore any solids, together with organics, have to be removed prior to treatment. A technique that runs under high pressures has a high subsequent energy demand.

3.3. Adsorption

Adsorption is usually applied in industrial scale processes like gas scrubbing and the separation of gas/steam mixtures. Adsorption is a surface phenomenon whereas a solid phase bonds molecules out of a fluid or gaseous phase on its surface by van der Waals forces. A high specific surface of the adsorbent (e.g. biochar with a specific surface of 0.4 – 1.6 km²/kg) is necessary for a good adsorption. Figure 5 clarifies the process and terms of adsorption (Wilichowski, 2008).

Abbildung in dieser Leseprobe nicht enthalten

Figure 5 Schematic diagram of the adsorption process

When discussing the fundamentals of adsorption it is useful to distinguish between physical adsorption and chemisorption. In the case of physical adsorption relatively low van der Waals forces fix the adsorbate onto the solid surface. During the chemisorption chemical bonds arise among molecules of the adsorbate and adsorbent caused by electron exchange and splitting (Yang, 2003).

Normally the process of adsorption is reversible and is known as desorption. If the adsorbent gets in contact with a fluid or gas phase, which contains an adsorbate, its concentration in the fluid phase will be reduced. The concentration of the adsorbate increases in the surface area of the adsorbent and will be adsorbed. In the beginning the adsorption ratio is very high. But this phenomenon decreases with the run of time because the adsorpt increases at the solid surface. The desorption ratio is contrary to this. A complete solid surface allocation^[13] is therefore not possible. Resulting is an adsorption balance, which means that adsorption is equal to desorption. This is a dynamic balance because the amount of the adsorbed molecules is like the desorbed ones.

The process of adsorption has not been used extensively in treatment of wastewater so far, although demands for a better quality of treated wastewater effluent, including the reduction of toxicity have led to an intensive examination and use of this process on activated carbon (Metcalf & Eddy, 2003). Besides activated carbon the adsorption of hexavalent and trivalent chromium from wastewaters can also be interestingly accomplished by a lot of different adsorbents like saw-dust and fly ash from thermal power plants, agriculture by-products such as insoluble starch, xanthate, peanut skin, walnut expeller meals, modified cotton, modified bark, paddy husk, paddy straw, flour waste or onion skin (Pathe et al., 2005).

3.4. Ion exchange

Ion exchange is used to remove heavy metals and can also be used to remove any inorganic cation or anion from effluent streams. Normally ion exchangers use a bed of a resin or a gel bead material to hold ions, and exchange the ions in the wastewater for hydrogen or hydroxide ions. The exchange beds needs to be regenerated by replacing the exchanged ions with hydrogen or hydroxide ions by recharging with acid or caustic. The reactions are as follows, with R representing the ion exchange resin:

Abbildung in dieser Leseprobe nicht enthalten (eq. 19)

Abbildung in dieser Leseprobe nicht enthalten (eq. 20)

The formulas above assume that the cation bed is regenerated with sulfuric acid and the anion bed is regenerated with sodium hydroxide. Regeneration by the use of strong solutions of acid or caustics results in the reversal of the driving force and in a turnaround of the above equations. These equations represent strong acid and strong base resins. Weak acid and base resins are used in water treatment, nevertheless to achieve removal of heavy metals, strong acid resins are required.

The usual train is to exchange the cations first, which leads to the formation of mineral acids, including in most cases carbonic acid. Normally degasification and exchange of the anions follows. In the case where heavy metal removal is the only requirement, neutralization could follow the degasification step. Ion exchange resins are rated in grains exchange capacity per cubic foot of resin. This value could be obtained from the manufacturer and the calculation of required bed volume is subsequent made by simply dividing the grains of ion exchange required by the bed capacity rating (Stephenson et al., 1998).

The major advantages of ion exchange processes are the very low running costs. The employed regenerant chemicals are cheap and if well maintained, resin beds can last for many years before a replacement is needed. The process has also some disadvantages in that there are substances occurring in some water (such as organic matter or Fe3+ ions) which can foul the resin, but in general the advantages of the process outweigh the disadvantages. In addition, the process is very environmentally friendly because it deals only with substances already occurring in water (Alchin & Wansbrough, 2008).

3.5. Solvent extraction

The term solvent extraction^[14] refers to the distribution of a solute between two immiscible liquid phases in contact with each other, i.e., a two-phase distribution of a solute. It can be described as a technique, resting on a strong scientific foundation. Scientists and engineers are concerned with the extent and dynamics for the distribution of different solutes – organic or inorganic – and its use scientifically and industrially for separation of solute mixtures (Rydberg & Cox, 2004).

As applied to refining operations, particular uranium, solvent extraction commenced in the 1940ies (Ritcey G., 2006). These plants were small by present plant sizes. Shortly after the first solvent extraction plant was installed to treat hydrometallurgical solutions at the mine site for the recovery of uranium. The obtained success in this first generation of uranium operations eventually led to the application of solvent extraction to copper operations in 1969. Afterwards hundreds of SX plants have been installed for the recovery of many metals. With only a couple of exceptions, all plants were mixer settler in design. Considerable research on the chemistry and engineering of the solvent extraction process was achieved throughout the succeeding years. Based on process improvements many reagents have been developed as well as numerous contactor systems, constructed from upgraded materials and furthermore equipped with sensing and control devices became marketable in order to enforce the mass transfer (Ritcey G., 2006).

Generally SX is primarily used when the separation could not accomplished by distillation (e.g. cause of azeotropic formation or thermal sensitivity) or is not economical beneficial.

However the main applications of SX in treatment of wastes are in the recovery of recyclable fractions from waste streams to provide regenerated process water. This reduces the effect on the environment by minimizing the amount of effluent for downstream treatment besides the reduction in fresh water consumption. The most important liquid effluents where SX could be applied are from various metal finishing operations: plating, pickling, etching, and the rinse waters arising from the cleaning of work pieces (Rydberg & Cox, 2004). In the treatment of those wastes, the objectives must be:

- Purification of a metal from unwanted contaminants either by extracting the desired metal from the impurities or by extracting the impurities from the desired metal.
- Concentration of the metals in order to reduce downstream processing costs and to meet regulatory discharge limits.
- Conversion into a metal form which eventually promotes the recovery.
In any given solvent extraction process, one, two or all three objectives may be accomplished.

Selecting a proper reagent and operating under optimised process conditions should cover these objectives usually.

A variable number of mixer-settler stages, adjustable temperatures and flow rates as well as the ability to use wash stages allows the optimization of process conditions as well as design flexibility (Cognis, 2007).

3.5.1. Principles of solvent extraction (SX)

SX always consists of a minimum of two stages, even the intensive mixing of aqueous and organic phase and the respective, preferably completed, separation of both fluids phases.

During the mixing the mass transfer takes place, i.e. the desired extract moves from the first solvent (feed or aqueous phase) into the second solvent (extracting agent or organic phase). Assumed that the solvent is not (or in small parts) soluble in the feed solution (Rydberg & Cox, 2004). The principle of solvent extraction is illustrated in Figure 6. To achieve an excellent mass transfer, and there with a high extraction performance, a preferably high surface area between the liquids is necessary. To obtain a great surface area, one of the liquids is dispersed in small drops with an agitator. However, it must be avoided, that an emulsion occurs the separation of which is difficult or even not possible. During the separation (settling) the dispersed fluid has to coalescence to a homogeneous phase and must be able to separate by sufficient different densities from the other liquid. The output is significantly influenced by the speed of separation (Rydberg & Cox, 2004; Lewinsky, 2007).

Abbildung in dieser Leseprobe nicht enthalten

Figure 6 Mass-transfer in liquid-liquid extraction (Wilichowski, 2008)

The assumption for evaluating a separation by extraction and design of the extraction process is the knowledge of the distribution equilibrium, which define the distribution of the extract between the two phases.

The Nernst partition coefficient is valid for adequate diluted solutions and reciprocal insolubility of raffinate and acceptor (Rouessac, 1994):

Abbildung in dieser Leseprobe nicht enthalten (eq. 21)

CE concentration of the solved substance C in the A-charged acceptor phase (extract) [kmol/m³]

CR concentration of the solved substance C in the B-charged donator phase (raffinate) [kmol/m³]

K^[15] Nernst distribution coefficient

For higher extract concentrations, K becomes a function of the concentrationAbbildung in dieser Leseprobe nicht enthalten.

The conceivable basic design of a one stage extraction process is a mixer – settler system.

The process is characterized by an intensive mixing of the feed and solvent solution, followed by the separation of both phases as mentioned above. On the other hand, there is a wide variety of continuously working extraction processes in packing, - head, - or agitation columns. If the raffinate and extract are in distribution equilibrium, the extraction stage is called as a theoretic or ideal stage. The transfer between the extractant C and the acceptor respectively donator phase can be expressed as (Wilichowski, 2008):

Abbildung in dieser Leseprobe nicht enthalten (eq. 22)

Based on this important equation, we can conclude the following specifications for a successful extraction process:

- High flow speed and strong turbulences to increase the mass transfer coefficient k
- Great interface by high dispersion of one phase in small drops
- Using counterflow columns to yield a high equilibrium deviation ΔX over the whole apparatus
- Long contact times among both phases

If a chemical reaction is swapping the physical mass transfer it is called a reactive reaction. In this case the mass transfer is defined by a chemical reaction. Major applications of reactive reactions are the separation of heavy metals, organic acids or pharmaceuticals out of product streams and low concentrated effluents (up to a few ppms) as well as the enrichment and conversion of salts and the production of ultra pure precious heavy metals.

3.5.2. Extractants

The single most important variable to extract valuable components from feed solutions is the choice of a suitable extracting agent; commonly called the reagent. Reagents which offer higher selectivity’s simultaneously will require fewer extraction stages and lower solvent-to-feed ratios to effect a separation, thus a highly selective reagent is resulting in lower capital and operations costs for the process equipment (Marcus & Sengupta, 2002). As illustrated in Figure 1 (introduction) it is obvious that there is a variety of available reagents that have been studied and are well reported in the literature.

To achieve a satisfactory solvent extraction the engaged reagent must meet a number of criteria. The most important of these have been summarised as follows (Sudderth & Kordosky, 1986):

- A selective extraction of the desired metal from the feed solution.
- The extracted metal must be easy to strip from the loaded reagent phase.
- The reagent must be chemically and physically stable.
- The reagents must meet today’s stringent environmental and work place regulations.

The extraction and strip kinetics must be sufficiently fast to allow these processes to take place in an industrially acceptable time frame (Cognis, 2007; Kodorsky et al.). The reagent

- must be soluble in a preferable relatively inexpensive and eco-friendly diluent/solvent; kerosene respectively palm oil is used in this study.
- must be separated from the aqueous phase.
- must not release deleterious species back from the strip section to extraction.
- must be tolerant of crud and should not promote crud formation.
- must have reasonable costs to promote an economically attractive recovery.

The high demands to meet sufficient of these criteria to an acceptable level exhibits high requirements to the chemicals which have been found suitable for commercial use. Out of the wide variety of developed and in laboratory scale tested reagents only very few of these have found commercial acceptance. However the application of SX to metallurgical processes is not only limited to the properties of the reagent. In principle there are five classes of metal extractants as characterized by structure, extraction mechanism and the metal species extracted (Sudderth & Kordosky, 1986):

- Ion pair extractants
- Chelating extractants
- Neutral or solvating extractants
- Organic acid extractants
- Ligand substitution extractants

These different mechanisms are only introduced in the following while the focus in this paper is on the ion pair extractants like Alamine 336 which was used in this survey. An extensive overview including the extraction chemistry and formula structure is attached to the appendices.

Chelating extractants:

Chelating refers to “claw”, which is a graphic description of the way in which the organic extractant binds a metal ion. i.e., the extractant chemically bonds to the metal ion at two sites in a manner similar to holding an object between the ends of the thumb and the index finger. In many cases, upon bonding with the metal ion, the extractant releases a hydrogen ion into the aqueous solution from which the metal was extracted. One of the important parameters controlling the equilibrium position of this reaction is the acid content of the aqueous phase.

Neutral or solvating extractants:

Extractants of this class are basic in nature and will coordinate to certain neutral metal complexes by replacing waters of hydration, thereby causing the resulting organo-metal complex to become organic soluble and aqueous insoluble. Solvating extractants have an atom capable of donating electron density to a metal in the formation of an adduct, and are classified according to that ability:

R3PO > (RO)3PO > R2CO > ROH > R2O

Trialkylphosphine oxides > trialkylphosphates > ketones > alcohols > ethers

It takes little imagination to see that the above list is only a brief representation of the organic compounds that could function as solvating extractants. In general, extractions with solvating extractants are limited by:

- the metal’s ability to form neutral complexes with anions,
- the co - extraction of acid at high acid concentrations, and
- the solubility of the organo - metal complex in the organic carrier.

Other important extractants in this class are tributylphosphate (TBP), dibutyl butylphosphonate (DBBP), 2,2'-dibutoxy diethyl ether (di-butyl carbitol) and methyl isobutyl ketone (MIBK). Di-butyl carbitol and MIBK are used for the extraction of gold from acid chloride solutions for example.

Organic acid extractants:

The chemistry of this type of extractant has some characteristics which resemble chelating extractants and some which are similar to neutral or solvating extractants. Diethylhexyl- phosphoric acid (DEHPA) is representative of this class of extractant. Reagents which belong to this class are the organophosphoric, -phosphonic and -phosphinic acids, their respective mono- and di-thio derivatives, organosulfonic acids and carboxylic acids.

Ligand substitution extractants:

Ligand substitution extractants are similar to neutral or solvating extractants in that they donate an electron pair to a metal ion, but they are different in that these extractants form inner shell complexes with metals and, in doing so, will displace other ligands. Other reagents of this type include dialkyl sulfides (R2S) such as di-n-hexyl sulfide.

The obtained purity of the product after successful extraction is a key factor in determining the price. For that reason a selective extraction processes become highly desirable. One of the most affecting factors is the thermodynamic selectivity of the reagent for one metal over other competing metals. Other stated factors which have an influence on the selectivity of a solvent extraction as opposed to the selectivity of an extractant are listed below:

- The chemistry and structure of the reagent molecule which determines thermodynamic selectivity.
- The pH in extraction and stripping and the possibility of selective stripping.
- Differences in the extraction kinetics of competing metals.
- The use of scrubbing circuits for the loaded organic.
- The mechanical design of the equipment and the circuit layout employed.
- Entrainment and crud effects.

The recovery of Cr(VI) by solvent extraction is widely discussed in literature and a lot of experiments have been carried out by varying the used extractants and solvents from different environments (Table 6).

Table 6 Comparison of some of the reported Cr(VI) solvent extraction methods in literature (Kalidhasan et al., 2009)

Abbildung in dieser Leseprobe nicht enthalten

TPPB: tetraphenylphosphoniumbromide; TBAB: tetrabutylammoniumbromide; TBP: tributylphosphate; TBADBDTC: tetrabutylammoniumdibutyldiethyldithiocarbamate; TOA: trioctylamine; DPC: diphenylcarbazide; TBA: tribenzylamine.

Toxic chlorinated solvents were used in most of the reported methods. The percentage of recovery is in the range of 79-95% and the reusability of the solvent (org.-phase) is for about 5-6 cycles. Some of the methods suffer from draw backs such as the third phase formation during the extraction (Kalidhasan et al., 2009).

3.5.2.1 Alamine 336

Alamine 336, a mixture of tri- n -octylamine and tri- n -decylamine which is capable of forming oil soluble salts of anionic species at low pH. It is a tertiary amine that belongs to the ion-pair extractants. Tertiary amines are very stable extractants however under highly oxidising conditions and in the presence of nitrate ions they can degrade to form nitrosamines. Besides reducing the amine concentration nitrosamines are also carcinogenic.

In this study Alamine 336 was chosen as the extracting agent for the extraction of chromium and other metal anions. Alamine reagents contain basic nitrogen which is capable of forming amine salts with a wide variety of inorganic and organic acids. Tertiary amines of the general formula R3N, where R represents a variety of hydrocarbon chains, find the widest use in metal recovery processing by solvent extraction. The general reactions which are shown below in two steps, protonation and exchange, describe this behaviour (Cognis, 2009):

Abbildung in dieser Leseprobe nicht enthalten (eq. 23)

Abbildung in dieser Leseprobe nicht enthalten (eq. 24)

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^[1] Alamine 336 is a registered trademark of Cognis Cooperation Mining Chemicals

^[2] From the Greek word chroma or chromos, meaning “colour”, because of the many colours of its minerals and compounds

^[3] Melting below the optimum reaction temperatures of 1150°C

^[4] Direct current

^[5] Units are converted from original source (mol/dm³) to allow a comparison of discharge limits

^[6] The lethal concentration for 50% of animals exposed (respiration)

^[7] The lethal dose of a compound for 50% of animals exposed (swallowing, ingestion)

^[8] Body Weight

^[9] Occupational Safety & Health Administration

^[10] Permissible Exposure Limit

^[11] Uses low applied pressures; pore size 0.02 – 10 microns

^[12] Passes ions and rejects macromolecules (0.005 – 0.1 micron) and removes organics from process solutions

^[13] means occupancy rate = 1

^[14] The International Union of Pure and Applied Chemistry (IUPAC) recommend the use of the term liquid-liquid distribution. However, more traditionally the term solvent extraction (SX) is used in this paper.

^[15] Depending on temperature