طراحی مدلی یکپارچه جهت بازیابی فلزات گرانبها از زباله های الکترونیکی – روشی جدید در مدیریت پسماندهای الکترونیکی Development of an integrated model to recover precious metals from electronic scrap – A novel strategy for e-waste management

Abstract The authors in this paper makes an attempt to propose a novel integrated model for the recovery of precious metals (gold – Au and silver – Ag) for the management of e-waste using a combination of hydrometallurgical (chemical) and biometallurgical (low cost biomass) processes. Chemical process involves leaching of gold/silver from electronic scrap using cyanide. The leachate containing gold/silver complex thus obtained is then subjected to biosorption process using low-cost biomass. The model proposed by us will help to strengthen the ‘processing’ part of the functional elements employed for proficient e-waste management. Feasibility study was also conducted to explore the possibility of removal/recovery of silver-cyanide using low-cost biosorbents. Results showed that Eicchornia root biomass and Waste tea powder were efficient biosorbents for leached silver-cyanide from electronic scrap. The concentrated silver-cyanide recovered could further be used as an input material for electroplating industry. ۱٫ Introduction Waste Electrical and Electronic Equipment (WEEE) or e-waste, like municipal solid waste (MSW), is one of the fastest growing advanced type of solid waste streams in the urban environment worldwide. E- wa0ste is a generic term encompassing various forms of electrical and electronic equipments (EEE) that are old, end-of-life electronic appliances and have ceased to be of any value to their owners (UNEP, 2007). E-waste/WEEE in general comprises of all old, end-of-life appliances like computers, laptops, audio and video products, refrigerators, freezers, mobiles phones, etc. along with their peripherals. The sources of e-waste are relatively expensive and essentially durable products used for data processing, telecommunications, or entertainment in private households and businesses. Globally, WEEE is growing by about 40 million tons a year (Wath et al., 2010). In developed countries, e-waste constitutes 1-2% of the total solid waste generation. In US, it accounts for 1-3% of the total municipal waste generation. In European Union (EU), total amount of e-waste generation ranges from 5-7 million tons per annum or about 14-15 kg per capita and is expected to grow at the rate of 3-5% per year. Developing countries are also in the challenging phase as they are already facing the continuum of hazardous e-waste mountains (UNEP, 2007). In South Africa and China for example, the report predicts that by 2020, e-waste from old computers will jumped by 200 to 400 percent from 2007 levels and by 500% in India (UNEP, 2007). The growth rate of discarded electronic waste is high in India since it has emerged as an Information Technology giant and due to modernization of lifestyle. The total ewaste generated in India amounts to > 1,46,180 tons per year as on 2010 (Wath et al., 2011). The physical composition of e-waste is very diverse and contains over 1000 different substances, which falls under organic and inorganic fractions. Heavy metals form a significant part of inorganic fraction accounting for 20-50%. E-waste consists of hazardous metallic elements like lead, cadmium, chromium, mercury, arsenic, selenium and precious metals like silver, gold, copper and platinum. Overview indicates that manufacturing of mobile phones and personal computers consumes 3% of gold and silver mined worldwide each year; 13% of the palladium and 15% of cobalt. Whether hazardous or precious, heavy metals are non-renewable and finite resource and therefore eventually become very valuable. Moreover, managing e-waste is a confounding task due to the various challenges like technical, financial, strategic, information failures, etc. It is, therefore an urgent need to manage e-waste in a formal, systematic and eco-friendly manner by way of removing/recycling the precious metals from waste streams. In emerging economy like India, current practices of e-waste management are followed completely in disorganized manner which may cause deleterious impacts on human health and ecology. It was thought that if an efficient system for removal/recovery could be proposed and developed, precious metals could be conserved, which in authors opinion would be a novel approach of resource recovery. In the present paper, the authors attempt to propose a novel integrated model for the recovery of precious metals (viz. gold and silver) for the management of e-waste using a combination of hydrometallurgical (using chemicals) and biometallurgical (using low cost biomass) processes. The integrated model proposed by us would help to strengthen the ‘processing’ part of the functional elements employed for proficient e-waste management. Feasibility study was also carried out to explore the possibility of removal/recovery of silver-cyanide from leachate solution using low-cost biosorbents. ۲٫ Gold and silver leaching from electronic scrap by hydrometallurgical (chemical) process As mentioned earlier, heavy metals form a significant part of inorganic fraction accounting for 20- 50% of the total e-waste being generated. Apart from hazardous metals (viz. lead, cadmium, chromium, mercury, arsenic, selenium) many other economically important metals like gold, silver, copper and platinum are also present in e-waste. Whether hazardous or precious, heavy metals are non-renewable and finite resource and therefore contribute the most value in electronic scrap. And from the economic point of view, recovery of precious metals from e-waste is one of the most attractive options. Significant literature has been cited on the recovery of precious metals using hydrometallurgical techniques and has been one of the most active research areas since 1980s (Macaskie et al., 2007; Ogata & Nakano, 2005). Hydrometallurgical methods when compared with conventional pyrometallurgical methods are more accurate, predictable, and can be controlled easily (Cui & Zhang, 2008). Main steps in hydrometallurgical processing consist of leaching, purification, precipitation, solvent extraction, adsorption and ion-exchange to isolate and concentrate the metals of interest. The solutions consequently are treated by electro-refining process, chemical reduction, or crystallization for metal recovery (Ritcey, 2006). The first step in hydrometallurgical processing is the leaching of metals using suitable solvent. The most commonly adopted leaching agents in recovery of precious metals is cyanide. Halide, thiourea and thiosulfate are also used (Sheng & Etsell, 2007). Cyanide is being employed for extraction of gold and silver in mining industry for more than a century and is a low-cost option (Hilson & Monhemius, 2006). The mechanism of gold/silver dissolution in cyanide solution is an electrochemical process. The process involves reaction between gold, oxygen, cyanide and water (Rajeshwari, 2008). The overall reaction is shown below.Maximum dissolution of gold, silver and other precious metals in cyanide solution occurs at pH above 10. Cyanide, although a very useful chemical from industrial point of view, also enjoys the higher status due to its evil reputation in the past. Bhopal Gas Tragedy, Bhopal, India (1984); Green Spring Gold Operation, Ely, Nevada, USA (1992); and Szamos-Tisza Cyanide Pollution, Romania and Hungary (2000) are some of the examples, wherein either organic or inorganic cyanides have played the key role. These accidents have killed and affected large number of people, birds, other flora and fauna, severe contamination of surface and groundwater has precipitated the widespread concern over the use of cyanide as a leach reagent (Hilson & Monhemius, 2006). Therefore, many non-cyanide substitutes have been reported of which thiourea and thiosulfate is regarded as being the most realistic substitutes. Several research investigations have also been undertaken to determine the effectiveness of chlorine as noncyanide substitutes. However, it is difficult to employ it (and therefore not in use) for two main reasons: (i) special steel and rubber lined equipment is required to resist oxidizing conditions and highly corrosive acidic nature; (ii) Involves health risk due to poisonous nature of chlorine gas. Thiourea, although an effective reagent for gold leaching, very few full-scale operations are in existence and are only restricted to developed nations. Commercial applications of thiourea for gold leaching has been hindered for the following reasons (Cui & Zhang, 2008): (1) thiourea is more expensive than cyanide; (2) since thiourea readily gets oxidized in solution its consumption in gold processing is very high; and (3) steps involved in gold recovery process requires more development. Because of these reasons, the thiourea process is still in its infant stage. Another non-cyanide substitute chemical widely researched out is thiosulfate (S2O3 2− ) (۸۱-۹۷). It has large number of applications in photography units and pharmaceutical industries. Although thiosulfate has the potential environmental benefits it has several disadvantages: (i) high thiosulfate consumption during extraction (ii) the leaching process (reaction rate) is very slow (iii) requires copper (II) as an oxidant, which is again a non-renewable and precious metal resource and, (iv) the process uneconomical. Currently, no simple and affordable method for recovering gold/silver from thiosulfate leach solutions exists. The authors further argues that high consumption of thiourea/thiosulphate during the leaching process would probably consume more energy and other supplementary resources for its formulation and further use during it entire life cycle assessment (LCA) when compared with cyanide. In the light of above account, use of cyanide has several advantages over various non-cyanide substitutes like thiourea, thiosulphate and chloride ions. These could be classified into primary, secondary and tertiary (ultimate) advantages and has been depicted in Table 1. Although cyanide has several advantages, using it as a leaching agent has some disadvantages. Cyanide is deadly poisonous chemical and therefore requires careful handling, storage and transportation. Nevertheless, improper management of cyanide may cause deleterious impact on human health and ecology. Moreover, the low-tenor cyanide effluents emanated by the industry require stringent control and statutory compliance. Inspite of the fact that cyanide is potential dangerous, it could be managed effectively by adopting sound management practices by way of using personal protective equipments (PPEs) at workplace, appropriate containers (avoid metal containers) for storage, proper transportation facilities and cost-effective treatment of cyanide effluents. Considering the above mentioned merits and demerits of cyanide, the authors strongly suggest the use cyanide for the leaching of gold and silver and therefore recommend the same in the proposed integrated model. In a developing and emerging economy like India, waste management has to be a low cost proposition for its wider acceptance and utilisation.