The Leaching Gold and Silver from E-waste by LSSS Method

Li Jing-Ying

College of Environment and safety Engineering Qingdao University of Science & Technology

Qingdao, China e-mail: lijy741018@qust.edu.cn

Huang Lu College of Environment and safety Engineering Qingdao University of Science & Technology

Qingdao, China e-mail: huanglu84@163.com

Abstract—A new hydrometallurgical recovery technology of leaching gold and silver from E-waste has been presented by Lime Sulfur Synthetic Solution (LSSS) method. The influences of sodium sulfite concentration, divalent copper ions concentration, aqueous ammonia concentration, reaction temperature and leaching time were investigated experimentally. The results indicated that it was favorable for gold and silver leaching when solid-liquid ratio was 1:3 , leaching time 2.5h, at temperature of 318K, under the condition of Na2SO3 0.1mol/l, Cu2+ 0.03 mol/l, NH4

+ 0.5 mol/l solution, pH=10, for 5g e-waste power. The best leaching rate of gold and silver reached about 92% and 90% in this favorably experimental condition, which suggested this technological viability for gold and silver recovery. On the basis of practice, LSSS method has the advantages of non-toxic, low-cost, simple process, easy to operate and was expected to be a great potential method in recycling gold and silver from e-waste.

Keywords- Lime Sulfur Synthetic Solution; Leaching; Gold; Silver; E-waste

INTRODUCTION E-waste refers to discarded appliances, such as TVs, PCs,

air conditioners, washing machines, and refrigerators, as well as a variety of associated waste products. E-waste is one of the fast growing waste fractions. The total volume of e-waste is generated domestically in China. In addition, more than 70% of the e-waste of the United States goes to China[1]. E-waste contains five categories of materials: ferrous metals, non-ferrous metals, glass, plastics and others. Over 60% of e-waste is composed of metals such as iron, copper, gold and other metals[2]. So the e-waste recycling is becoming a profitable business opportunity, in which valuable materials can be recovered and reused as a favorable resource for the economic development. However, 2.7% of the e-waste are pollutants including hazardous materials such as cadmium, mercury and lead[3]. In the current e-waste recycling system, the valuable components and materials of the appliances are extracted by manual disassembly and open incineration, while the remainder is dumped. Such unregulated and risky processing of e-waste has resulted in health problems and deterioration of air, water and land quality.

Silver and gold are the most commonly used, with a demand of over 5,000 and 250 tonnes per year, mainly for switches and contacts in electric appliances. Considering the amount of e-waste produced in China, the recovery process of gold and silver from e-waste seems economically sustainable. For instance, the concentration of gold in gold ores is

commonly between 0.5 and 15g of metal per ton of mineral (0.5-15ppm), while in e-waste circuit boards its concentration is over 10 times higher (150ppm in expansion cards, over 10,000 ppm in central processing units, CPUs). Due to the large amount of precious and nonprecious metals utilized for electronic components, the disposal of such e-waste represents both an environmental and an economical concern.

At present, the recovery of metals from e-waste is generally accomplished by two methods: by oxidative thermal treatment followed by metallurgical or chemical processes or by electrostatic separation of shredded boards. Neither techniques represent the optimum one as the first one deserves a great amount of energy and non-combustible pollutants lag and fumes are produced while the second procedure is not able to separate small amounts of metals from nonmetallic supports, thus it is absolutely not suitable for the recovery of precious metals found in electronic devices. In addition, gold is commonly extracted from mines by cyanidization, a process which raised many concerns regarding its impact on the environment. For this reason in the last decades many efforts were made in order to design alternative environmentally acceptable processing routes.

In this work，a new environmentally friendly process-LSSS method was developed which can overcome such environmental and efficiency issues. The developed procedure was based on the leaching process from e-waste, improved in order to operate with such chemicals and tested to be safer for the environment.

Ⅰ.EXPERIMENTAL PRINCIPLE AND METHODS

A. Experimental principle The leaching gold by use of Lime-Sulphur-Synthetic -

Solution (LSSS) method was put forward by Zhangjian[4], it belongs to a new leaching technology. The chief constituents of LSSS were CaSx and CaS2O3. Its leaching function came from polysulfide and thiosulfate and had superior leaching efficiency[5]. The chemical process of LSSS describing the dissolution of gold and silver was given in equation (1)-(4):

2Au + 2S2- + H2O + (1/2)O2 = 2AuS- + 2OH- (1) 2Ag+4S2O3

2- + H2O + (1/2)O2 = 2Au(S2O3)23-+ 2OH- (2)

2Au + 2S2- + H2O + (1/2)O2 =2AgS- + 2OH- (3) 2Ag+ 4S2O3

2-+ H2O + (1/2)O2 =2Ag(S2O3)23- + 2OH- (4)

B. Experimental methods 1) Pretreatment

978-1-4244-4713-8/10/$25.00 ©2010 IEEE

Different types of the waste printed circuit board were used as samples after being eliminated the electronic components in the surface, then cut into small pieces and crushed by a mechanical crushing machine and sorted by size using a screen that allowed only pieces smaller than 250μm to pass through. The metal contents were determined by dissolving 0.5g of the sample in aqua regia-HF-HClO4, at high temperature of 200℃ or about 20h until the power was dissolved completely, measuring the metal concentration in the resulting solution by atomic absorption spectrophotometer (TAS-986, Beijing Purkinje general Instrument Co., Ltd), after appropriate dilution. Through calculation, determine the content of gold is 47g/t and silver is 215g/t in e-waste. 2)Leaching experiment

Place 5.00g e-waste in the beaker and 15ml de-ionized water was added. The leachant which consists of 140ml 6mol/l nitric acid was injected into the beaker. Then sucked filtration and put the residues into 250ml round bottom flask with three opening serving for a thermometer, an aeration machine and a mechanical stirrer. In the case of leaching at higher temperature, heating was performed in a digital thermostatic. The leachant which consists of 10ml LSSS, 5ml copper sulfates, 5ml aqueous ammonia and 5ml sodium sulfite were then injected into the bottom flask. Adjusting pH to 10 with sodium hydroxide. The mechanical stirrer was set at rotation speed of 400 rpm. Then sucked filtration and put the residue into 250ml volumetric flask. The metal concentrations were analyzed by AAS.

Ⅱ.RESULTSAND DISCUSSION In leaching experiment, the sodium sulfite concentration,

copper sulfate concentration, aqueous ammonia concentration, leaching time and temperature had the greatest impact on the leaching efficiency of gold and silver through orthogonal experiment. So these parameters were investigated in the following experiments.

(1)Effect of sodium sulfite concentration Reaction conditions: 5g e-waste, the ratio of liquid/ solid

L/S=3:1, leaching experiments were performed using 10ml LSSS as leaching solvent, with 5ml 0.03mol/l copper sulfate and 5ml 0.5mol/l aqueous ammonia as catalyst. The reaction was carried out for 2.5h at 40℃. The ranges of sodium sulfite concentration were investigated from 0.04mol/l to 0.12mol/l to determine the effect of this reactant on the gold and silver dissolution efficiency from e-waste.

Fig. 1 showed that the ratio of gold and silver dissolution generally increased with increasing sodium sulfite concentration. At a concentration of 0.1 mol/l, the leaching rate of gold and silver was rather high, nearly 90%.Yet, after the concentration of 0.1mol/l, the leaching efficiency began to decline significantly. Experimental results showed that the leaching effect was best at the sodium sulfite concentration of 0.1mol/l. According to the hydrolysis equilibrium of Na2S2O3, Na2S2O3 decomposition release SO3

2-, so the existence of SO32- could restrain the

decomposition of S2O32- and Sx

2-. Additionally, SO32- could react

with sulfur which comes from LSSS, this react could avoid

emerging layers of sulfur and sulfide. However, high concentration lead to the decline of the leaching efficiency. This maybe imply that excessive Na2SO3 damaged oxidizing environment due to high SO3

2- concentration, its reducibility were not conductive to stablization of Cu(NH3)4

2+[6].

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100

0 0.05 0.1 0.15 sodium sulfite concentrations(mol/l)

met

al le

achi

ng ra

te/%

Ag

Au

Fig.1 Effect of different sodium sulfite concentrations on gold and silver

leaching percents

(2) Effect of copper sulfate concentration 5g e-waste, L/S=3:1, leaching experiments were

performed using 10ml LSSS as leaching solvent, with 5ml copper sulfate and 5ml 0.5mol/l aqueous ammonia as catalyst as well as 5ml 0.1mol/l sodium sulfite as stabilizing agent.The reaction was carried out for 2.5h at 40℃. The ranges of copper sulfate concentration were investigated from 0.01mol/l to 0.05mol/l to determine the effect by this reactant on the gold and silver leaching percents from e-waste.

Fig. 2 showed that the ratio of gold and silver dissolution generally increased with increasing copper sulfate concentration.

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0 0.02 0.04 0.06 copper sulfate concentrations(mol/l)

met

al le

achi

ng ra

te (%

)

Ag

Au

Fig.2 Effect of different copper sulfate concentrations on gold and silver

leaching percents

This may be because of copper ammonia complex ion catalyze the reaction. At a concentration of 0.1mol/l, the leaching efficiency of gold and silver reached the highest, the leaching efficiency of gold was 87%, the leaching efficiency of silver was 89%. One reason was that Cu2+ can oxidize Na2S2O3 to S4O6

2-, high copper sulfate concentration consumed excessive Na2S2O3, resulting in low recovery efficiency of gold and silver[7]. Another was that with pH decline, the reaction generated Cu2S sediment, Cu2O-CuO passive film, which covered the surface of gold and silver to hinder the reaction. Therefore, the best copper sulfate concentration was 0.03mol/l.

(3)Effect of aqueous ammonia concentration

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0 0.2 0.4 0.6 0.8aqueous ammonia concentrations(mol/l)

met

al le

achi

ng ra

te(%

)

Ag

Au

Fig.3 Effect of different aqueous ammonia concentrations on gold and silver

leaching percents

The experimental condition was similar to the above experiment (2). The ranges of aqueous ammonia concentration were investigated from 0.2mol/l to 0.6mol/l to determine the effect of this reactant on the gold and silver leaching efficiency from e-waste.

Fig. 3 showed that the ratio of gold and silver dissolution generally increased with increase in aqueous ammonia concentration. At a concentration of 0.5mol/l, the leaching efficiency of gold and silver reached the maximum level, the leaching efficiency of gold was 90%, the leaching efficiency of silver was 88%. After the concentration of 0.5mol/l, the leaching efficiency began to decrease. The effect of aqueous ammonia was to maintain alkaline environment of the leaching system, ensure the stability of S2O3

2- and Sx2-. This

was because S2O32- and Sx

2- were prone to decompose in acidic environment, the main chemical reaction formula was followed:

Sx2- + 2H+ → H2S + (x-1) S (5)

S2O32- + 2H+ → H2O + S + SO2 (6)

So with the aqueous ammonia adding in, pH increasing, they restrained the reaction of S2O3

2- and Sx2- with H+, and

guaranteed the stability of S2O32- and Sx

2-,which was beneficial to the reaction[6]. Meanwhile, the stability of copper ammonia complex ion needed alkaline environment, and NH4

+ which comes from NH4OH releasing also played a part in adding

ligand to solution. When the concentration exceeded 0.5mol/l, the leaching efficiency begin to decline, this may be overly high pH would block dissolution reaction of precious metal and balance of complex compound. Therefore, 0.5mol/l concentration of aqueous ammonia should be an ideal option.

（4）Effects of leaching time 5g e-waste, L/S = 3:1, leaching experiments were

performed using 10ml LSSS as leaching solvent, with 5ml 0.03mol/l copper sulfate and 5ml 0.5mol/l aqueous ammonia as catalyzer as well as 5ml 0.1mol/l sodium sulfite as stabilizing agent. The reaction was carried out at 40℃ for five times (1h, 1.5h, 2h, 2.5h, 3h) to determine the effect of reaction time on the metal dissolution efficiency from e-waste.

As shown in Fig. 4, as time increased, precious metal dissolution efficiency was on the rise. For the time of 2h, the silver dissolution efficiency reached the maximum level 89%, and for the time of 2.5h, the gold dissolution efficiency reached the maximum level 92%. Longer reaction time resulted in better reaction degree. But too long reaction would cause complex ion instability, on the other hand, aqueous ammonia releasing and subsidiary reaction of Cu2+ all made reactant reduction, which brought about the decrease of precious metal dissolution efficiency. Given that gold is more valuable than silver, 2.5h was an optimal leaching time.

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0 1 2 3 4Reaction time(min)

met

al le

achi

ng ra

te(%

)

Ag

Au

Fig.4 Effect of different leaching times on gold and silver leaching percents

（5）Effects of leaching temperature 5g e-waste, L/S=3:1, leaching experiments were

performed using 10ml LSSS as leaching solvent, with 5ml 0.03mol/l copper sulfate and 5ml 0.5mol/l aqueous ammonia as catalyst as well as 5ml 0.1mol/l sodium sulfite as stabilizing agent. The reaction was carried out for 2.5h at five temperatures to determine the effect of reaction temperature on the precious metal dissolution efficiency from e-waste.

As shown in Fig. 5, as time increased, precious metal dissolution efficiency was on the rise in a short time. At the temperature of 40℃ , the gold leaching efficiency reached maximum level 89%, and the silver dissolution efficiency reached the maximum level 90%. After 40℃, the dissolution efficiency began to decrease. This maybe resulted from two

factors: one was that the following chemical reaction appeared in the reaction process: Cu2+ + S2- = CuS. With temperature rising, more CuS emerged, consumed more Cu2+ and CuS, covered precious metal surface which stopped precious metal from dissolution. Secondly, with temperature rising, the decomposition of S2O3

2- and Sx2- accelerated, and the complex

ion became instability in higher temperature. Finally, with temperature rising, NH4OH volatilized accelerate which would destroy the alkaline environment of solution. Therefore, 40℃ should be an ideal experimental temperature.

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0 20 40 60 80 Reaction temperature(℃)

met

al le

achi

ng ra

te(%

)

Ag

Au

Fig.5 Effect of different leaching temperatures on gold and silver leaching

percents

(6) compare with other leaching reagents Document 8 research the way of putting Cu2+ into

Na2S2O3, when S:L = 1:5, pH =10, T = 50℃, t =3h, [Na2S2O3] = 0.4mol/l, [Cu2+] = 0.03 mol/l, [NH4OH] = 0.45mol/L, [SO3

2-

] = 0.2%, the leaching efficiency of gold was above 85%[8]. The question in the way of Na2S2O3 was that it consume more reagent. Yet, LSSS is more cheap and easy to gain. So it is superior to Na2S2O3 in the economic view of point. Document 9 research the experiment of leaching gold in abandoned printed wiring board with thiourea. The suitable leaching conditions are as follows: the solid-liquid ratio is 1: 5, leaching temperature is 35℃, the thiourea concentration is 10g/l, the concentration of Fe3+ is 0.3%, the leaching period is 1h and the concentration of H2SO4 is 5%, the leaching efficiency is above 90%[9].The flow of the leaching environment way is prone to corrode equipment, and thiourea have the nature of potential carcinogenicity, yet LESS was non-toxic and the reaction was in alkalinity environment. So it is superior to thiourea in the environment view of point.

Ⅲ.CONCLUSION E-waste is a promising source for precious metal due to

its high metal content and the recovery of precious metal from

e-waste suggests the significance from economical and environmental point of view. (1)Gold and silver was successfully recovered by hydrometallurgical treatment using LSSS as leaching solvent, with copper sulfate and aqueous ammonia as catalyst, as well as sodium sulfite as stabilizing agent. The maximum leaching rate of gold reach 92% and silver reach 90%. (2) The favorably experimental conditions were the ratio of solid/Liquid=1:3, the addition of 5ml 0.1mol/l sodium sulfite and 5ml 0.03mol/l copper sulfate, 0.5mol/l aqueous ammonia, 2.5h leaching time and 40℃ leaching temperature. (3) From the experiment results mentioned above, the precious metal leaching procedure presented in this study was feasible and its merits could be expressed below: a) lower leaching agent cost; b) non-toxic and cheap; c) facilitation of leaching process; d) safe operation.

Compared with other technology (such as sodium thiosulphate), LSSS method had advantages in economic and environment point of view and had the possibility of taking the place of cyanidation.

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