cyanide in the atmosphere is approximately 267 days. .... 5−. The metallic gold is oxidized to Au+ by effect of the copper complex Cu(NH3)4 .... 2.3.1 THIOSULFATE CONCENTRATION ..... Their loading capacity can be up to 10-25kg/ton.
CHAPTER 1 1.0 INTRODUCTION The importance of gold recovery from both high and low grade ores is endlessly growing due to rising gold demand and thereby high gold prices. Presently cyanide is the most commercially used leaching reagent in the mine industry to extract gold from different ores such as sulphide ores and concentrates. However, the use of cyanide to leach gold in the mining industry has become the center of extreme attack by numerous groups throughout the world (Ferron et al., 2003). This is due to raising social awareness of the ecological impacts of cyanidation process and environmental specifications (Gokelma et al., 2016). Remarkably a renewed effort has been tossed to find suitable alternative reagents for gold leaching process (Dreisinger et al., 2003). Many of the research work that has been done is focusing on less detrimental reagents for the process of leaching. In this paper one of the most important gold leaching capable reagent (thiosulfate) is examined so that a better and clear overview from the point of economic and ecological view can be brought out. To facilitate the reagent comparison, cyanide is also briefly described. 1.1 PROJECT AIMS The general aim of the project is to assess the feasibility of thiosulfate as an alternative reagent for the leaching process through outlining the physical and chemical properties of thiosulfate, detailing thiosulfate leaching process, analyze and detail the currently used solvent (cyanide) sighting also problems associated with the use of cyanide, assessing the economic viability of using thiosulfate as an alternative reagent and then finally choose the optimum reagents that give high recoveries of gold from the ore. 1.2 PROBLEM STATEMENT According to Shenyang Florrea Chemicals, cyanide leaching produces many hazardous waste products that must be disposed properly, some of these hazardous materials are acids and heavy metal, and the cyanide compounds. Despite of being used in 90% of gold production, Au cyanidation is controversial due to the toxic nature of cyanide (Shanyang Florrea Chemicals). Although aqueous solutions of cyanide degrade rapidly in sunlight, the less toxic products such as cyanates, thiocyanates may persist for some years (Shanyang Florrea Chemicals). According to Korte and Coulston’s estimation in 1998, the amount of hydrogen cyanide that escapes into the
atmosphere from gold mining operations is 20000 tonnes annually and the halftime persistence of cyanide in the atmosphere is approximately 267 days. HCN is produced when the leaching process is done at low pH values and it is a very toxic gas. Cyanide is readily absorbed through inhalation and ingestion of skin contact and it can cause problems like increased thyroid weight and depressed thyroid functioning (Chaves and Verga, 2006). They are many other problems that can be imposed by using cyanide hence the need to find alternatives that are cheap, effective, ecologically, environmentally safe and efficient. 1.3 JUSTIFICATION Environmental compliance is an important factor that must be taken into consideration in flow sheet selection and economic evaluation of gold project. Cyanide is extremely toxic in low concentrations to aquatic life, wild life and people according to the International Cyanide management Code (ICMC, 2014). The cyanide discharged from mines, especially when cyanide is combined with mercury during cyanidation processes, is considered the greatest threat to aquatic life and aquatic environments (Davies, 2014). In most jurisdictions, cyanide must be destroyed before being discharge to the tailings dams and also into the natural environment. This can result in saving lives of both humans and any other living organisms. Compared to cyanide, thiosulfate is very environmentally friendly and very cheap, hence optimizing thiosulfate gold leaching process to give high recoveries and little thiosulfate consumption can improve the safety aspects of the gold industry and also help to reduce capital and operating costs. The problem that was being faced that challenged the commercial application ammonium thiosulfate in the industry was its high reagent consumption. In most of the test that were carried out, Cu catalyst was used, however, studies have shown that using nickel catalyst can significantly reduce thiosulfate consumption. Optimization of a gold thiosulfate system is the most challenging section of the analyses of the system, this is because the optimum reagents concentration vary with ore type hence the researcher is going to focus on the ore supplied from Golden Valley Mine located in Kadoma Zimbabwe. The researcher will then find the optimum reagent that will give maximum recovery of gold from the ore.
1.4 SCOPE This project covers the use of thiosulfate as an alternative reagent for gold leaching, development of some necessary procedures and equipment will be theoretical and then experiments will be done to determine optimum conditions for thiosulfate leaching process. 1.5 THEORETICAL BACKGROUND There are several basic metallurgical flow sheets that can be used to treat gold ore bodies with many permutations (SGS Minerals Services, 2014). The optimum flow sheet from an economical perspective is always that which achieves the greatest Au recovery at the lowest cost. Cyanidation was invented in late 1800’s to mid 1900’s and at this time alluvial gold was depleting (Chaves and Verga, 2006). In metallurgical applications, gold must be concentrated and smelted into a bar for the reason that most of the gold in different ores is generally present in low grades (Rogans, 2012). Since practically most gold reserves are very low, mostly the concentration method that are used rely on phase changes. According to John Rogans the managing director of Kemix (pty) ltd, the dissolution of gold from the ore results in gold present in water as the gold cyanide ion approximated as follows; 𝑂2 + 2𝐻2 𝑂 + 4𝐴𝑢 + 8𝐶𝑁 − → 4𝑁𝐴+ + 4𝐴𝑢(𝐶𝑁)2 − + 4𝑂𝐻 − The
as Ca2+ , Mg 2+ , H + , Na+ , K + , this is because the gold cyanide complex in water prefers to be a neutral species. The stability of the ion pair between the Metal ion and gold cyanide governs the selectivity for calcium as follows: 2Au(CN− )2 − + Ca2+ ↔ Ca (Au(CN− 2 ))2 (ion pair) In the cyanide system at any given time, an equilibrium is established, thus a certain percentage exists as gold cyanide ion and the rest of the gold exist as the ion pair (Rogans, 2012). Even if 99 % of the gold exists as an ion pair and 1 % as the gold cyanide ion, the equilibrium will still be present but the conditions favor ion pair formation (Rogans, 2012).
Studies have shown that since 1900, thiosulfate has been used in many cases to leach gold and silver ores (Aylmore and Muir, 2001). Major interest did not take place until around 1980 with emphasis on Cu-Au and carbonaceous ores that usually give poor gold recovery using cyanide (Aylmore and Muir, 2001). In early studies, high concentrations of reagents were being used and resulted in reasonable gold extraction, however consumed thiosulfate up to 40kg/t (Adams, 2005). It was then noted that there is need to limit the concentration of Cu2+ ions as this degrades thiosulfate thus resulting in very high reagent losses (Adams, 2005). Ammonium thiosulfate is under study to replace sodium cyanide hence minimizing the toxic risk to the environment including the public and occupational health. Ammonium thiosulfate does not cause noticeable damage using conservative quantities in addition it has the benefit of being an aqueous solvent for leaching gold and silver, which selectively leach gold with great certainty of replacing cyanide at low cost. One of the bottleneck of the wide industrialization of thiosulfate gold dissolution is it high thiosulfate consumption (Xu et al., 2017). Below are equations showing gold dissolution using thiosulfate. It is worth noting that Cu(NH3)4] 2+ as a catalyzer boosts gold dissolution dramatically, but it also markedly accelerates thiosulfate decomposition due to its relatively strong oxidizing ability (Xu et al., 2017). 𝐴𝑢 + 5𝑆2 𝑂3 2− + 𝐶𝑢(𝑁𝐻3 )4 2+ ↔ 𝐴𝑢(𝑆2 𝑂3 )2 3− + 4𝑁𝐻3 + 𝐶𝑢(𝑆2 𝑂3 )2 5− The metallic gold is oxidized to Au+ by effect of the copper complex Cu(NH3)42+, then the thiosulfate ions complexed the Au+ formed to integrate the complex Au(S2O3)23-. The Cu(NH3)42+(Cu2+) complex in presence of S2O32- form the Cu(S2O3)25- complex according to the reaction; 𝐶𝑢(𝑁𝐻3 )4 2+ +3𝑆2 𝑂3 2− + 1𝑒 ↔ 𝐶𝑢(𝑆2 𝑂3 )2 5− + 4𝑁𝐻3 To reduce thiosulfate consumption during gold dissolution process, nickel catalyst can be used instead of copper catalyst. The information below is based on the study by (Arima et al., 2004). In the gold leaching by nickel catalyst, nickelous oxide (𝑁𝑖3 𝑂4) formed from nickel amine complex, act as an oxidant for gold. The oxidation reaction of nickel ammine complex is shown below: − − 3𝑁𝑖(𝑁𝐻3 )2+ 6 + 8𝑂𝐻 → 𝑁𝑖3 𝑂4 + 18𝑁𝐻3 + 4𝐻2 𝑂 + 2𝑒 ……….. (1)
𝑂2 + 𝐻2 𝑂 + 2𝑒 − →2𝑂𝐻 − ………………………………………… (2)
The nickelous oxide in eqn 1 is reduced back to a nickel ammine complex with the oxidation reaction of the gold as shown below. − 𝑁𝑖3 𝑂4 + 18𝑁𝐻3 + 4𝐻2 𝑂 + 2𝑒 − → 3𝑁𝑖(𝑁𝐻3 )2+ 6 + 8𝑂𝐻 ………... (3)
2𝐴𝑢 + 4𝑁𝐻3 → 2[𝐴𝑢(𝑁𝐻3 )2 ]+ + 2𝑒 − …………………………….. (4) The overall reaction from eqn 1 to 4 is expressed as the following reaction Catalyzed by [𝑁𝑖(𝑁𝐻3 )6 ]2+ 4𝐴𝑢 + 8𝑁𝐻3 + 𝑂2 + 2𝐻2 𝑂 → 4[𝐴𝑢(𝑁𝐻3 )2 ]+ + 4𝑂𝐻 − …………… (5) Since the aurous ammine complex is not stable, the aurous ammine complex in eqn 5 is immediately converted to a stable aurous thiosulfate complex according to the following reaction; [𝐴𝑢(𝑁𝐻3 )2 ]+ + 2𝑆2 𝑂3 2− → [𝐴𝑢(𝑆2 𝑂3 )2 ]3− + 2𝑁𝐻3 ………………. (6) The sum of reactions 5 and 6 gives reaction 7, which is the overall gold oxidation reaction in a nickel catalyzed system. 4𝐴𝑢 + 8𝑆2 𝑂3 2− + 𝑂2 + 2𝐻2 𝑂 →4[𝐴𝑢(𝑆2 𝑂3 )2 ]3− + 4𝑂𝐻 − ………… (7) Although thiosulfate is classified as non-toxic, it is also metastable and can decompose either to polythionates and sulfate while consuming oxygen hence uncontrolled release of thiosulfate into the environment poses problems of de-oxygenation of waterways or the potential of formation of toxic species however this can be controlled prior to release by oxidation of thiosulfate to sulfate or minimize the release of thiosulfate and increase the recycle stream of thiosulfate (Adams, 2005). Ammonia also poses environmental and toxic problems both as a soluble solution and gas and unfortunately it is difficult to breakdown and may eventually metabolize to nitrate which have the potential to promote algae growth and to pollute ground water. Hence there is need to monitor, control ammonia emissions and also prevent release of ammonia into the environment.
CHAPTER 2 (LITERATURE REVIEW) 2.0 INTRODUCTION In this chapter the chemical properties, physical properties, and the benefits of using thiosulfate as an alternative dissolution solvent are outlined. The thiosulfate dissolution process is also detailed main focus being on the mechanism of gold dissolution, factors affecting gold dissolution, thiosulfate consumption, and lastly but not recovery of gold from the pregnant thiosulfate solution. Under this section main focus will be on thiosulfate gold dissolution using copper as a catalyst. Cyanidation process is also outlined for easier comparisons. Mechanism of gold dissolution, factors affecting gold dissolution, and recovery of dissolved gold from the pregnant cyanide solution is also looked at theoretically. 2.1 PROPERTIES OF THIOSULFATE 2.1.1 PHYSICAL PROPERTIES Ammonium thiosulfate is very much soluble in water, an inorganic compound and a white crystalline solid. Below is a table which shows the properties of thiosulfate with the corresponding values. Table 1: Physical Properties of Ammonium Thiosulfate PROPERTY
clear colorless solution to yellow
very soluble in water
hydrogen bond donor count
density at 25℃
2.1.2 CHEMICAL PROPERTIES When ammonium thiosulfate reacts with strong oxidizers, it releases toxic ammonia, it should be noted that thiosulfate is very much sensitive to heat and it is incompatible with magnesium and aluminum powder. A mixture of ammonium thiosulfate with sodium chlorate can result in an exothermic reaction. A table below shows some of the important chemical properties of thiosulfate. Table 2: Chemical Properties Of thiosulfate PROPERTY
conditions of stability
incompatible materials and water
stable under ordinary conditions of use and storage
incompatibility with various substances
highly reactive with metals, oxidizing agents, organic materials, water and alkalis
hydrogen bond acceptor count
rotatable bond count
specific gravity at 20℃
2.2 THIOSULFATE LEACHING According to studies that have been carried out, ammonium thiosulfate can substitute the cyanidation process without compromising efficiency, cost of the process and sensitivity (Hernandez et al., 2016). It is also one of the most kinetically and environmentally alternative to cyanide (Keskinen, 2013). Ammonium thiosulfate and sodium thiosulfate are the main commercial sources of thiosulfate in many process description. Thiosulfate gold leaching can be simply described as a process by which, gold is dissolved from a bearing ore to form a thiosulfate complex in which many recovery methods can be used to recover
the gold from the pregnant solution. As compared to many other alternative to cyanide, ammonium thiosulfate is less toxic, and offer greater efficiency preg-robbing ores (SGS minerals services, 2008). Ammonium thiosulfate is a better choice in replacing the cyanidation process in that in both agitated tanks and heaps, the size and type of equipment required for thiosulfate gold dissolution is similar to that of the cyanidation process (SGS minerals services, 2008). Below is a fig which shows a general overview of gold processing steps. Figure 1: General Overview of Gold Processing Steps (Eugene and Mujumdar, 2009)
Studies have shown that gold dissolution using thiosulfate as a solvent and copper sulfate as catalyst is a very complex system. This is because the dissolution rates are sensitive to thiosulfate concentration and Cu2+ concentration and also it is dependent on temperature. Gold dissolution process is given by the equation below (XU et al., 2017); 𝐴𝑢 + 5𝑆2 𝑂3 2− + 𝐶𝑢(𝑁𝐻3 )4 2+ ↔ 𝐴𝑢(𝑆2 𝑂3 )2 3− + 4𝑁𝐻3 + 𝐶𝑢(𝑆2 𝑂3 )2 5−
The Cu2+ in solution are stabilized in the solution as the Cu2+ tetra-amine complex with the copper(II)sulfate catalyst (Aylmore and Muir, 2001). The dissolution rate can be speed up to even 20 times due to the present of the Cu2+ ions in the solution (Aylmore and Muir, 2001). With a suitable catalyst, oxygen can also be used as an oxidant in an alkaline solution and it should be noted that in the present of oxygen, the dissolution rate is very slow without a suitable catalyst. 4𝐴𝑢 + 8𝑆2 𝑂3 2− + 𝑂2 + 2𝐻2 𝑂 ↔ 4𝐴𝑢(𝑆2 𝑂3 )2 3− + 4𝑂𝐻 − Though oxygen can be used as an oxidant, Cu2+ is a very efficient catalyst. Note that gold also form stable complexes in ammonia as shown below; 2− 𝐴𝑢(𝑆2 𝑂3 )2 3− + 2𝑁𝐻3 → 𝐴𝑢(𝑁𝐻3 )2+ 4 + 82𝑂3
Below is an overall ammonium thiosulfate gold dissolution reaction with copper(II)sulfate as a catalyst; 𝐴𝑢 + 5𝑆2 𝑂3 2− + 𝐶𝑢(𝑁𝐻3 )4 2+ ↔ 𝐴𝑢(𝑆2 𝑂3 )2 3− + 4𝑁𝐻3 + 𝐶𝑢(𝑆2 𝑂3 )2 5− It is imperative to note that the presence of ammonia is of paramount importance in the dissolution of gold using thiosulfate this is because it stabilize the Cu2+ ions in the solution as Cu2+ tetra amine ion. The 𝐶𝑢(𝑁𝐻3 )4 2+ complex oxidizes the metallic Au to Au+ which then forms a complex with thiosulfate ion (𝑆2 𝑂3 2−). Below is a diagram which shows a flow sheet for Barrick Gold Corporation that were using ammonium thiosulfate to treat their carbon bearing sulphide ore to recover gold.
Figure 2: flowsheet for Barrick Gold Corporation (thiosulfate gold leaching of a carbon bearing sulphide ore) (Dong et al., 2017)
2.3 FACTORS THAT AFFECT THIOSULFATE GOLD DISOLUTION Below is a brief outline of the major factors that affect gold dissolution rates when thiosulfate is used as a solvent. 2.3.1 THIOSULFATE CONCENTRATION Thiosulfate concentration has a direct proportion to gold dissolution that is increasing thiosulfate concentration increases gold dissolution. The dissolution rate can decrease due to the reduction of the Cu 2+ ions to Cu1+ and also as a result of the oxidation of thiosulfate when the concentration of thiosulfate is low. As in many process, there is need to analyze the system and come up with an optimum thiosulfate concentration that will give high gold recoveries at low costs. It is also of paramount importance to optimize the concentration of thiosulfate in oxygenated thiosulfate solution so as to reduce the effect of thiosulfate on the reduction of Cu2+ ions. This is because the resultant is the disturbance of catalytic power of the Cu2+ ions. Notedly, increasing thiosulfate
concentration can enhance gold dissolution up to 0.3 M and after that increasing the concentration will have a negligible effect on the gold dissolution (Navarro et al., 2002). 2.3.2 Cu2+ IONS Cu
ions act as a catalyst in the thiosulfate gold dissolution hence an increase in these ions in
solution can speed up the gold dissolution rate (Alymore and Muir, 2001). However, as much as the increase in Cu2+ ions increase gold dissolution rate it must be noted that this have also a negative effect on thiosulfate oxidation (Oraby, 2009). Based on studies carried by Arima in 2003, high gold dissolution rates can be observed when the copper sulfate concentration is 0.01 M and increasing the concentration to 0.05 M does not have an effect on the gold recovery. It should be noted that these values are dependent on the type of ore being studied. Increasing the concentration of the Cu2+ ion widens the stability region of the solid copper compounds such as CuO, Cu2O, CuS and Cu2S, while reducing the stability region of the cupric amine complex hence resultant the gold dissolution rate decrease significantly (Arslan et al ., 2008). 2.3.3 AMMONIA Ammonia is of paramount importance in the dissolution of gold using thiosulfate as it has two functions. Firstly, ammonia reacts with gold to form the following complex Au(NH3)2+, which then reacts with the sulfate ion to yield Au(S2O3)23- and the equation is shown below. Secondly, ammonia reacts with cupric ion to form cupric amine complex to stabilize the cupric ion since the cupric ion is a strong oxidizing agent. Au(𝑁𝐻3 )2 + + 2𝑆2 𝑂3 2− → 𝐴𝑢(𝑆2 𝑂3 )2 3− + 2𝑁𝐻3 It is also known that ammonia prevents gold passivation and also it prevents leaching of silicates, carbonates, iron oxides, and silica of which these are the most common gangue minerals in gold ores (Oraby, 2009). Conclusively, there is need to optimize ammonia concentration to ensure the stability of Cu2+ ions in solution and reduce the reaction between free Cu2+ ions and thiosulfate in the solution. 2.3.4 pH To ensure the stability and presence of necessary reagents and concentrations the pH of the system must be correct. Thiosulfate gold leaching system must be operated in alkaline conditions, it is worthy to note that the pKa of ammonia is 9.25 hence the system should have a pH which is greater
than 9.25 so that ammonia is maintained in the system. (Breuer and Jeffrey, 2003). At low values of pH, thiosulfate in solution will decompose to form polythionate and sulfur components. The sulfur formed from the decomposition of thiosulfate may coat the surface of gold hence inhibit the gold dissolution process. According to Arima in 2003, an optimum pH for the thiosulfate leaching process is 10 as decreasing the pH to below 9.2 or increasing it to above 12 will result in very poor dissolution of gold, it was discovered that the slight increase of pH to 10.7 decreases gold recovery and increase thiosulfate consumption. 2.3.5 AGITATION SPEED Agitation speed will only have an effect in diffusion controlled reactions not chemically controlled reactions and from studies carried out the thiosulfate system is chemically controlled hence agitation speed does not improve the gold dissolution rates (Oraby, 2009). 2.3.6 OXYGEN Oxygen is also a vital reagent in the leaching of gold with thiosulfate, this is because it convert the Cu2+ to Cu1+ for further gold dissolution (Fiuza et al., 2016). However, in the absence of oxygen, the cu1+ ions cannot return to the Cu2+ state as a result copper sulfate will precipitate from the solution (a result of decomposition of thiosulfate) (Fiuza et al., 2016). Notedly if there is inadequate solubility, there will be low dissolution rates of gold based on the slow catalytic reaction of Cu (Fiuza et al., 2016). 2.3.7 TEMPERATURE Temperature has got a negative effect on the evaporation of ammonia and thiosulfate decomposition, however it has also a positive effect on gold dissolution (Oraby, 2009). According to the proportion of the reactant species which have sufficient energy to overcome the chemical activation energy, all the reactions which are chemically controlled are affected by a temperature change (Oraby, 2009). It was noted that within the first hour, 90% of the gold would have been leached and this means that the initial rate of extraction is enhanced by increasing dissolution temperature. After one hour, a dramatic decrease in the dissolution rate is observed. Based on several studies, it is indeed a requirement to maintain the dissolution temperature in the range (3040) ℃ so as to achieve best gold extraction hence reduces ammonia losses and Cu2+ reduction (Oraby, 2009).
2.3.8 PULP DENSITY According to studies, increase in pulp density result in increase in gold dissolution rate but it should be noted that there must be an optimum pulp density as continuing increasing the density will have a little effect on the leaching rate. When the pulp density was increased from 28.6% to 61.5%, gold extraction increased by 1.9% which is very small, however this result in the thiosulfate consumption being reduced significantly from 77.9 kg/t to 17.4 kg/t within 24 hours of the leaching test (Xia et al., 2003). 2.3.9 IMPURITIES A vast of minerals that are associated with gold normally acts as impurities in the dissolution of gold using thiosulfate. Most of these minerals enhances thiosulfate oxidation thus reducing gold dissolution rate, however, some of these minerals can increase gold leaching (Oraby, 2009). The presence of sulfur can result in reduced thiosulfate decomposition and increased dissolution rate and the presence of manganese dioxide in sulfide ore enhances both the kinetics and gold extraction (Keskinen, 2013). Hematite and pyrite are known to form a coat on gold surfaces thus prevents the leach solution from diffusing to the gold surface hence result in a decrease of gold dissolution rate and also they can catalyze the decomposition of thiosulfate (Oraby, 2009). 2.4 THIOSULFATE CONSUMPTION According to studies that has been carried out on thiosulfate, the major stumbling block for the commercial use of thiosulfate is its high consumption during gold dissolution process when using copper sulfate as a catalyst. Thiosulfate consumption is one of the bottle necks of it being used in many gold industries. A study showed that for thiosulfate to compete effectively with thiosulfate, the consumption must be reduced at most to 10kg/ton (Yen et al., 1998). Research showed that consumptions rates of about 25kg/ton and even up to 100kg/ton (Bin et al., 2017). Thiosulfate can be easily oxidized by copper(II) and other minerals thus results in its excessive consumption and also thiosulfate is metastable. Significant losses of thiosulfate through the tailings are also reported and according to Alymore and Muir in 2001, 50 % of thiosulfate in ammoniacal thiosulfate solutions which contain copper have been reported. It was noted that increasing pulp density from 28.6% to 61.5%, the thiosulfate consumption for 24 hours was reduced from 77.9 to 17.4kg/ton (Keskinen, 2013).
Other minerals such as pyrite, pyrohotite and arsenopyrite have unfavorable effects on gold extraction, pyrite and pyrohotite noticeably increases the depletion of thiosulfate.However thiosulfate depletion during gold dissolution can be controlled by choosing favorable conditions and at the same time promote gold dissolution at the same time. It is important to note that Cu(II) increases gold dissolution rates but also increase the rate of thiosulfate depletion, ammonia reduces thiosulfate depletion but it hinders gold dissolution as it stabilizes Cu(II) thus decreasing its oxidizing ability.in concentrated solutions of thiosulfate, excess dissolved oxygen and high temperatures results in high dissolution rates but also results in thiosulfate consumption rates (Bin et al., 2017). Comparing to concentrated thiosulfate solutions, dilute systems, low dissolved oxygen concentration, low temperatures results in low thiosulfate consumption and sadly cause very low gold dissolution rates (Bin et al., 2017). Inorganic additives (phosphate, sulfate, chloride, nitrate, sulfite and sulfide) are sometimes added to reduce thiosulfate consumption and their order of performance is in the sequence phosphate > sulfate > chloride > nitrate > sulfite > sulfide, this is because their ability to stabilize Cu(II) are exactly in this order based on the analysis of hard-soft and Lewis acid-base concept (Bin et al., 2017). ). Hexametaphosphate and orthophosphate both can diminution thiosulfate consumption and boost gold dissolution, and hexametaphosphate has a superior influence because it does not only stabilizes Cu(II) via complexing with the cupric ion at the axial coordination site, but also disperses leach slurry by improving its rheology (Feng and Deventer, 2002). Other studies have proved that to use nickel as a catalyst can have a positive effect on the consumption of thiosulfate that is it reduces the consumption to economic values which is the purpose of this study to find the optimum reagent concentrations that will give the best recovery using nickel as a catalyst. Below is a fig which shows the schematic of gold dissolution mechanism in ammonium thiosulfate solution on gold surface using Nickel catalyst.
Figure 3: Schematic of Gold Dissolution in Ammonium Thiosulfate Solution on Gold Surface
2.5 RECOVERY OF GOLD FROM PREGNANT THIOSULFATE SOLUTION CIP tanks have limited application in the recovery of gold from pregnant thiosulfate solution, this is because gold thiosulfate complex is very poorly adsorbed by activated carbon. A brief description of solvent extraction, precipitation and resins is given in this section as methods of recovering gold from pregnant thiosulfate solution and these method have been proved to give better recoveries. Below is a table which show the technique and then a brief of its characteristics and then after that each technique is briefly described. Table 3: Brief Characteristics of Techniques used to Recover Gold from Pregnant Thiosulfate Solutions (Dong et al., 2017) Recovery Technique Characteristics Activated Carbon
Low requirements on the clarity of solutions.
Weak affinity for [Au(S2O3)2]3- anion
Modification is necessary to improve its gold loading capacity
Technique is simple.
High consumption of precipitation agents.
Low-purity gold product.
Difficulty in cyclic utilization of pregnant thiosulfate solution.
Technique is simple
Low current efficiency and high energy consumption due to the undesirable reactions.
Difficulty in cyclic utilization of pregnant thiosulfate solution
Pregnant thiosulfate solutions with high gold concentration is needed
High equipment and operating costs due to complete solid-liquid separation of pulp.
Dissolution and accumulation of organic extractant is inevitable
Fast adsorption speed and high gold loading capacity.
Low requirements on the clarity of solution
Simultaneous elution and regeneration at ambient temperature through the elaborate choice of eluent
Difficulty in gold elution from gold-loaded resins.
High gold loading capacity.
Separation difficulty between pulp and adsorbent due to its fine particle size.
High temperature requirement on the solution Ph values.
2.5.1 SOLVENT EXTRACTION A water immiscible organic solvent is used in which the leach liquor is contacted with a solution of extractant. This can be done by employing the extractants of primary, secondary, tertiary amine oxide, benzene and phosphate esters (Xu et al., 2017). The gold complex is then apportioned into the organic phase and the other metal stay in the aqueous phase and after the end of the organic phase separation from gold, the solvent is then returned to the extraction circuit. However, when Au(1) concentration is not enough, an enriching operation is required because the residual extraction and the organic phase in the solution is detrimental to its cycle use. Furthermore, the
clarification process is required to prevent formation of a third phase which is immiscible in the second phase (between aqueous phase and organic phase). Additional equipment, processing time, capital costs, and operational costs are required in order to attain a complete solid-liquid separation in the clarifying stage. 2.5.2 ACTIVATED CARBON Activated carbon is widely used in the recovery of gold from leach solution when cyanide is used as a solvent. However, its application in adsorption of gold thiosulfate solution is still under study as it has shown low affinity for [Au(S2O3)2]3- anion. Below is a block diagram which show the carbon adsorption process stages. Figure 4: Carbon Adsorption Process of Gold from Leach Solution
2.5.3 PRECIPITATION It is also called the cementation process or Merrill-Crowe process. In this process a pulverized metal such as Zn, Fe and Al is added to recover gold from pregnant solution through a displacement reaction. It is known that increasing the surface area increase the rate of reaction, hence excess metal powder is required to reduce gold in a reasonable time as the metal surface is easily passivated in the solution which then reduce the gold content that will precipitate. A reaction occurs between the zero valent base metal and gold as the primary mechanism of precipitation. A
filter press is used to separate the solution from the precipitant (Keskinen, 2013). It should be known that unwanted cations that are introduced usually complicates the recycle of the solution. To avoid copper ions from precipitating with gold, copper powder cementation can be done. This is quite a good choice as the copper ions in the solution can be recycled as leach solution. However, copper has a negative effect on the precipitated gold as it raises the redox potential of solution and at the same time resulting in the dissolution of precipitated gold and also heavy oxidation of thiosulfate. 2.5.4 RESIN ADSORPTION Considering most studies that have been carried, this is the only method which is considered appropriate in commercially recovering dissolved gold from pregnant thiosulfate solution. Strongbase anion resin are used in this case to directly recover gold from the pregnant solution. Notedly, this process does not pay much attention on water quality and resins in pulp or in leach can be easily be applied. Studies are still being carried out on the weak and strong base anion resins focusing mainly in their application of recovering gold from gold thiosulfate solution. The gold thiosulfate complex is a negative ion hence the resins are gold anion resins. Fig 4 below shows the circuit diagram for adsorption resins. Figure 5: Circuit for Adsorption Resins used for the Recovery of Au from Thiosulfate solution
18.104.22.168 WEAK-BASE RESINS In this type of resin, primary, secondary, and tertiary amine functional groups even mixture of these are used. Mostly the ion exchange properties of these amine functional groups is controlled by the pH of the solution. Weak-based resin have loading capacities that are less than 2kg/ton and furthermore the loading capacities decreases markedly with increase of pH which is in the range 8-11 (Dreisinger and Zang, 2002). This is because under strong alkaline conditions, the amine groups (linked to the polymeric matrix) usually tend to stay in the form of free base (Xu et al., 2017). It should be noted that the loading capacity of the resin depend on the number of amine groups that are protonated at a specific pH, the greater the number, the greater the potential for high loading capacity of gold. The pKa values in the range 6-8 are needed for protonation which is defined as the value at which 50% of amine groups are protonated, this means that most weakbase resins will not adequately be protonated in the pH range 9-11 of which this range is common range for thiosulfate solutions (Fleming and Cromberge, 1984). With reference to this analysis it is not economical viable and also efficient to use weak-base resins to recover gold from pregnant thiosulfate 22.214.171.124 STRONG-BASE RESIN Strong-base resins are not restricted by the protonation to adsorb the objective ions from thiosulfate solution because they contain ammonium functional groups, this means they can be applied in the recovery of gold over a broad range of pH values. Their loading capacity can be up to 10-25kg/ton and are independent of the pH of the solution (Kong et al., 2017). Strong-base resins can adsorb very low concentration of dissolved Au in thiosulfate solution efficiently and because they have high capacities, it means they are forbearing to competing anions hence a preferred option for the recovery of gold. However, due to the poor selectivity of the resins, undesirable anions such as [Cu(S2 O3 )3 ]5− and polythionates will strongly compete with [Au(S2 O3 )2 ]3− for the active sites on
the resin surface and this remarkably reduce the loading capacity 126.96.36.199 ELUTION OF GOLD FROM LOADED RESINS Due to the co-adsorption problem of copper(I) on the resins, a two stage elution process can be used. A solution of oxygenated ammonia-ammonium sulfate is first used to selectively elute the copper then gold is eluted using a single component solution (thiocyanate or nitrate) or a two component solution (thiourea+sulphuric acid or sulfite + chloride) (Jeffrey et al., 2010). Additional
regeneration procedure is needed for resins to recover their loading capacities and to avoid the accumulation of elution reagents in leach solution when elution is done with single component solutions. The additional procedure means additional capital and operational costs. With two component solutions, the synergistic reagents of thiourea and sulfite can form the mixed ligand complexes of [𝐴𝑢(𝑆2 𝑂3 )(𝑆𝑂3 )] 3− and [𝐴𝑢(𝑆2 𝑂3 )𝑇𝑈] – through reacting with [𝐴𝑢(𝑆2 𝑂3 )2 ]3− and the two complexes have low affinity for the resin, hence they are easily eluted by chloride and sulfate ions respectively (Kong et al., 2017). Notedly when the eluted resins are returned into pregnant leach solution, the 𝑆𝑂42− and 𝐶𝑙 − are easily substituted by [Au(S2 O3 )2 ]3− and these ions have no obvious harmful impact on thiosulfate leaching as they can be used as additives to stabilize Cu(II) during the leaching process. (Xu et al., 2017). In conclusion using two component eluent solutions eliminates the additional regeneration procedure and the resins gain their loading capacities during the gold elution process. Other elution process can be applied to recover gold from the resins that is elution of gold by chemical reaction and also by displacement. When gold is eluted by chemical reaction, [𝐴𝑢(𝑆2 𝑂3 )2 ]3− anions are transformed to cationic gold complexes through the replacement of thiosulfate ligand (O’Malley, 2002). Gold is then readily eluted from the resins because the resins have noticeably reduced affinity for the new-formed cationic complex than gold(I) thiosulfate complex. However, this process is difficult to apply since only a few eluents can be used to strip gold on the resins using this principle. Other than thiourea the alternative ligand that can replace thiosulfate and complex with gold (I) to form cationic complex is scarce. The only reported eluent is acidic thiourea that can be used however, this is not a viable option because thiosulfate and polythionates are not stable under acidic conditions and their breakdown can cause sulfur precipitation which can poison the resins (O’Malley, 2002). When gold is eluted through displacement which uses an anion with high affinity for the resin to exchange adsorbed[𝐴𝑢(𝑆2 𝑂3 )2 ]3− , equilibrium is altered to increase the concentration of the anion (O’Malley, 2002). However this elution process requires high concentration of eluent because [𝐴𝑢(𝑆2 𝑂3 )2 ]3− has a strong affinity for the resin. This directly translate to high reagent costs. The sustained accumulation of eluent anions on the resin cause a dramatic decrease in the loading capacity of the resins hence an additional regeneration step is required to change the resin into its original form after gold elution.
2.6 GOLD CYANIDATION PROCESS They are now many leaching reagents that are being used in mining industry though some of them are still under study. These reagents have got their own advantages and disadvantages and the choice depend on economic viability of the reagent, efficiency and effectiveness, human safety and environmental factors. Mostly cyanide is used to leach gold in many mines around the world but changes are being done because of cyanide safety issues. In this chapter an outline of cyanide as a leaching reagent with the associated problems is given. Gold cyanidation process resulted in an improved gold recovery greater than 90% compared to 50% which was before the cyanidation era. The use of cyanide in gold dissolution processes was developed in late 19th century and recoveries of up to 95-99% was observed, this is because cyanide is able to leach and recover all unlocked Au from the coarsest to the finest particle sizes (SGS minerals services, 2014). Many mines around the world use cyanide because of its simplicity, its chemical and physical durability, its relative cost and generally very high recoveries of gold. Below is a flowsheet for Golden Valley Mine located in Kadoma which use cyanide to leach gold from their sulphide ore.
Figure 6: Complete Flowsheet for Golden Valley Mine (Kadoma, Zimbabwe)
Many investigations and studies have been carried out on the chemistry and leaching kinetics of the cyanidation process. De Andrade Lima and Hodouin in 2005 stated that, the process of gold leaching using cyanide is very complex because gold exist as compounds or alloys which are embedded in a minerals matrix and between the phases galvanic interaction take place. The conclusion of a number of studies concerning the mechanism of gold dissolution using cyanide remain contradictory. Some studies suggest the rate of reaction is controlled by the diffusion of reactants to the gold surface and others have concluded that the chemical reaction is slow (Crundwell et al., 1997). Below is fig which shows some type of leach tanks that can be used for the cyanidation process.
Figure 7: Agitated Cyanide Leaching Tanks at a Gold Mine (Grewal and Lum, 2017)
2.6.1 CYANIDE SOLUTION CHEMISTRY Sodium and potassium cyanide salts have been widely used as sources of cyanide leaching. The salts ionize in water to form respective metal cations and free cyanide ions. It should be noted that the CN- hydrolyze in water to form HCN and OH- which increases pH. Side reactions sometimes occur during gold dissolution, this because HCN and free cyanide can be oxidized with oxygen to form CNO- and this does not dissolve gold but reduces the free cyanide concentration. 𝑁𝑎𝐶𝑁 ↔ 𝑁𝑎 + + 𝑂𝐻 − 𝐶𝑁 − + 𝐻2 𝑂 ↔ 𝐻𝐶𝑁 + 𝑂𝐻 − 4𝐻𝐶𝑁 + 3𝑂2 ↔ 4𝐶𝑁𝑂− + 2𝐻2 𝑂 3𝐶𝑁 − + 2𝑂2 + 𝐻2 𝑂 ↔ 3𝐶𝑁𝑂− + 2𝑂𝐻 −
2.6.2 LEACHING OF GOLD USING CYANIDE Cyanide is used to dissolve the gold that is associated with the gangue. There are many factors which affect gold dissolution rates for examples pH, dissolved oxygen and free cyanide concentration etc. the effects of these factors on the gold leaching rates will be discussed below. Optimizing these effects is very important for it can result in industrial scale improvements such as lowering of operational cost, increased leaching rates. Oxygen is used as an oxidizing agent for the reaction that is oxygen is adsorbed onto the surface of gold followed by reaction of this surface with cyanide to first give AuCN and then the complex [Au(CN)2] which passes into solution. The balance in salts and acids creates a medium that is conducive for the reaction of gold-cyanide complex formation being favored. Oxygen is reduced and hydrogen peroxide is formed as an intermediate product in the first step and becomes the oxidizing agent in the second step leading to the following chemical reactions (Srithammavut, 2008); 2𝐴𝑢 + 4𝐶𝑁 − + 𝑂2 + 2𝐻2 𝑂 → 2𝐴𝑢(𝐶𝑁)2 − + 𝐻2 𝑂2 + 2𝑂𝐻 − 2𝐴𝑢 + 4𝐶𝑁 − + 𝐻2 𝑂2 → 2𝐴𝑢(𝐶𝑁)2 − + +2𝑂𝐻 − And the summation of the two partial reactions is given by 4𝐴𝑢 + 8𝐶𝑁 − + 𝑂2 + 2𝐻2 𝑂 → 4𝐴𝑢(𝐶𝑁)2 − + 4𝑂𝐻 − As it is critical to optimize every process, they are factors that affect gold dissolution that is there concentration in the system may have positive or negative effect on the system as a whole hence need for optimization. Below is brief description of some of the factors that affect gold dissolution. 2.6.3 FACTORS THAT AFFECT CYANIDE GOLD LEACHING 188.8.131.52 CYANIDE CONCENTRATION Increasing cyanide concentration results in an increase in gold dissolution thus increasing gold recovery however, when maximum is achieved the excess cyanide has no effect. At Golden Valley mine the cyanide concentration is in the range of 0.045 % to 0.060 %.
184.108.40.206 pH Increasing pH result in decrease of gold dissolution rates because if the OH- ion adsorptions on the surface of gold it will reduce the surface available for cyanide leaching. Lime is added to maintain the required values of pH and at Golden Valley the required pH is in the range 10.5 to 11.50. pH below 10.5 cause he formation of a toxic gas HCN and this also result in cyanide losses and reduce dissolution rates hence the pH values must not be less than 10.5 and also greater than 11.5 for high dissolution rates. 220.127.116.11 OXYGEN Oxygen is added as pure oxygen and also in the case of Golden Valley mine it can be supplement using hydrogen peroxide. It should be noted that optimizing the amount of dissolved oxygen in the solution, the amount of cyanide consumed will be greatly reduced, the retention time will also be reduce and higher gold recoveries can be achieved. This means increasing dissolved oxygen concentration results in increased gold dissolution rates. At Golden Valley mine the required concentration of dissolved oxygen is in the range 15 -20 ppm. 18.104.22.168 PARTICLE SIZE Small particles provide larger contacting surface area between solid and liquid hence reducing particle size gives high gold dissolution rates. However, decreasing particle size results in competing side reactions and increased reagent consumption thus decrease the rate of gold dissolution. At Golden Valley, the required particle size is 75𝜇m. 22.214.171.124 AGITATION It is well known that for diffusion-controlled processes, the rate of reaction is increases with agitation, hence for gold leaching process, increasing agitation results in high rates of gold dissolution since gold leaching is a diffusion-controlled process (Habashi, 1999). 126.96.36.199 TEMPERATURE Increasing temperature increases the rate of gold leaching however, this can result in decreased oxygen content on the solution hence above optimum temperature the dissolution rate will decrease.
2.6.4 CYANIDE CONSUMPTION Cyanide consumption for gold leaching is less than that of thiosulfate for the same gold recovery. The consumption of cyanide depends on other conditions such as pH and particle size that is if the pH is less than 10.5 there is formation of HCN thus leading to increased cyanide consumption and also if the particles becomes very fine this result in increased cyanide consumption. At Golden Valley mine the cyanide consumption rate is approximately 0.31 kg/ton. Lime consumption rate is in the range 2.0 to 2.1 kg/ton and the optimum particle size is 75 𝜇m. 2.6.5 ADVANTAGES OF CYANIDE AS A LEACHING REAGENT
Great effectiveness for gold dissolution
Highly selectivity for gold and silver over other minerals
Relatively low cost
The process cyanidation is simple
It is chemically and physically durable
High recoveries of gold and silver
It degrades into non-toxic substances fairly easily and is quick dispersed and diluted in the natural environment hence the damage can be significantly be short term.
2.6.6 PROBLEMS ASSOCIATED WITH USING CYANIDE Cyanide must be destroyed prior to discharge to a tailings pond in most jurisdictions, and must always be destroyed prior to discharge to the natural environment but this is not the case in many mining operations. Due to the toxic nature of cyanide its use in the mining operation is facing opposition from the public and this has resulted in it being banned completely in some countries. In some parts of countries like Argentina, America and Czech Republic the utilization of cyanide in gold processing has been banned. This is due to the toxicity nature of cyanide to human and animal life and the negative perception it has created in the public domain. When a toxic amount of cyanide is present in the blood of mammals and fish it becomes a deadly poison as it can persist for a long time in underground water. If cyanide is misused for example cyanidation of Hg rich tailings forms Hg(CN)42+ which is very stable and persistent compound and it can be transformed into a highly toxic compounds like methlymerurycyanide. If a person is exposed to sufficiently high dose of cyanide, the person can die within minutes as it is a fast acting
poison. Humans may be exposed to cyanide by inhalation, ingestion or absorption through the skin. The most toxic form of cyanide is HCN gas. The American Conference of Governmental Industrial Hygienists (ACGIH) lists the ceiling threshold limit of HCN at 4.7 ppm. In respiratory exposure to HCN which is normally produced when cyanidation is done below pH of 10.5, death occurs at 0.1 to 0.3 g/cm3 (Chaves and Veiga). Cyanide prevents oxygen from being used by the cells, causing tissue hypoxia and cyanosis (a bluish discoloration of the skin). The respiratory system fails to nourish the cells with oxygen, a condition which if untreated causes rapid, deep breathing followed by convulsions, loss of consciousness and suffocation. In some cases cyanide is detoxified by rhodanase enzyme forming thiocyanate which is a less toxic substance but chronically elevated levels of thiocyanate in blood an inhibit the uptake of iodine by thyroid gland thereby reducing the formation of thyroxine. The cumulative exposure to thiocyanate results in thyroid toxicity including goiter and cretinism (neurological impairment, thickened skin, enlarged tongue and protruding abdomen). Chronic intoxication with cyanide affect organs such as the central nervous system (headaches, loss of appetite and equilibrium disturbances), cardiovascular and or respiratory system, gastrointestinal tract (nausea and gastritis), and thyroid (enlarged thyroid). It can also affect reproduction and development as there is a risk of giving birth to low body weight infants and of perinatal death.
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