MARIA DOMINIQUE RUSTIA MIT LGO (MBA & SM Mechanical Engineering) April 1, 2017

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Developments in the Extraction of Gold from Disposed Consumer Electronics

I. INTRODUCTION

Gold is most commonly recognized for its monetary value, traded heavily in financial

markets and preserved in the form of jewelry. One of the other popular applications of gold is in

consumer electronics. Because of its high conductivity, corrosion resistance, malleability and low

contact resistance, gold has been widely used in electroplating, bonding wires, sputter targets,

inks and solders [1]. The unique color and luster of gold also facilitate inspection of electronics

[1]. Alternatives are limited since few elements have the accessibility and properties of gold.

Gold can be found in varying quantities in mobile phones, printed circuit boards (PCBs),

wiring and other applications. The actual gold content in consumer electronics is not significant:

approximately 0.023% of PCB metal content and 0.21% of metal content for mobile phones.

Despite this, gold can contribute between 63.7% to 77.2% of the total value of the metals in the

product [2][3]. The importance of gold is further emphasized by several trends: (i) rising prices of

gold, especially after the 2008 global financial crisis, (ii) a depleting supply of gold and (iii) an

increasing demand for gold for a variety of purposes.

Figures 1 and 2 illustrate some trends in gold supply and demand which is showing an

average annual increase of between 10-12%, with the demand for gold in consumer electronics

growing at 1.2 to 2% annually [4]. The demand for gold for use in consumer electronics is now

estimated at 254.5 tons for 2016, valued at $10.2 billion (only a fraction less than the total

amount of gold in the US federal gold reserve) [5]. Ironically, e-waste is also projected to grow

exponentially by 33% [6], rising to 93.5 million tons of e-waste in 2016, only 29% of which is

recycled [7]. This translates to a significant amount of gold that is not recycled. For perspective,

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approximately 5.17 tons of gold are thrown annually in mobile phones alone (assuming 0.034

grams of gold in each phone). The total value of gold retired annually from mobile phones alone

is estimated at $52 million [2]. Table 1 from Cui and Zhang provides an estimate of the gold that

can be derived from various types of e-waste as of the year 2000.

The depletion of gold supply increases the need to recover and recycle gold. Syed [8]

estimates that all natural sources of gold will be depleted by the year 2030. Aside from a

depleting supply, the amount of gold that can be recovered from these secondary sources is 35

to 50 times more than what can be obtained from ores and concentrates [9]. Primary production

of gold also has a more negative environmental impact, generating around 17,000 tons of CO2

for every ton of gold [10]. Although there have been efforts to address this including reducing

the gold content in electronics [9] and enforcing regulations on recycling gold in the EU, there is

a need to develop more efficient methods of gold extraction that will encourage industries to

invest in this process. Currently, the overall costs and environmental impact are the primary

challenges that prevent widespread extraction of gold. As Alzate, et al. [11] discuss, “developing

suitable and optimal recycling methodologies capable of avoiding pollution, reducing reagents

toxicity and time-consuming reactions has become one of the most important topics in WEEE

management research.”

This paper compares the most popular methods used to day to extract gold from

consumer electronics, the challenges that are hindering efficient extraction of gold from e-waste

and the latest developments in gold extraction processes which seek to address these challenges.

The paper will conclude by highlighting areas for future study in this field, based on the

shortcomings of existing methods and based on recent studies on gold extraction.

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II. COMMON GOLD EXTRACTION METHODS

The extraction and recycling of gold has been performed for centuries, dating as far back

as the melting of jewelry to recover and form new gold products. Figure 4 shows the breakdown

of all the basic steps involved in gold extraction. As for the extraction itself, there are three

methods that are prevalent today: (a) pyro-metallurgy, (b) hydro-metallurgy and (c) bio-

hydrometallurgy. All methods have different advantages and varying efficiencies in extracting

gold from e-waste. There is no single best option, given the variety of consumer electronics that

go into e-waste and a combination of all three methods is used across the industry [9].

A. Pyro-metallurgy

Pyro-metallurgy is the simplest but most environmentally damaging method of extracting

gold. In this method, e-waste is melted, oftentimes with another substance like NaOH or copper

(Ronnskar method) and gold is extracted through electro-refining, electro-wining (Norandra,

Umidore methods) or a chemical reaction [12] and methods are known to achieve up to 99.9%

recovery of gold [10], making it one of the most effective and common methods used today.

With growing restrictions and environmental policies, interest in pyro-metallurgy has

decreased over time. This method has the greatest environmental impact among all of the known

gold extraction methods due to its release of hazardous gases and carbon [10][13]. The overall

process requires significant energy, leading to high operating costs. It often requires high-grade

feeds or pre-treatment to improve the recovery of metals [13]. These limitations make pyro-

metallurgy a less attractive option in the long run.

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B. Hydrometallurgy

Hydrometallurgy is the most common method of recycling gold from secondary sources

used today due to its high recovery of gold [12]. Figure 5 illustrates the process of

hydrometallurgy. E-waste parts are physically sorted, separated (i.e. grinding into powder,

shredding, magnetic separation), pre-treated, then dissolved in an acid or alkaline, producing a

“pregnant solution” that can then be subjected to various purification processes. This process of

dissolving the e-waste in a solution is frequently referred to as leaching. Studies on efficient

leaching have used a variety of solutions such as HCl, HNO3, H2SO4, H2O2, cyanide, halide, nitride,

iodide, thiourea, thiocyanate, thiosulfate and aqua regia [11].

Given the heterogeneity and constantly evolving nature of e-waste, the composition,

metal quality and complexity can influence the type of purification process required [10][14].

Some examples of purification processes include precipitation of impurities, solvent extraction,

adsorption (i.e. through carbon) or ion-exchange [12]. After purification, gold is recovered from

the solution through any of the following methods:

a) Cementation – “the electrochemical precipitation of a noble metal from solutions of its

salts on a more electropositive metal” [15]

b) Electro-wining – uses the elution of activated carbon

c) Chemical reduction

d) Crystallization

Mining plants and recycling facilities, especially in developing countries, have favored

hydrometallurgy over other methods for several reasons. Hydrometallurgy requires a relatively

lower capital cost [13] and despite a more labor-intensive process, countries have managed costs

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by exporting e-waste to developing countries where labor costs are lower. Hydrometallurgy also

has a relatively higher gold recovery performance [13], up to 99% based on prior studies [12].

The process itself is more exact, easier to operate, more flexible and controllable, which is

needed to manage variability in the e-waste inputted [12][9]. Because of its use of acidic solutions

and reagents, energy consumption is also lower versus other methods, though this is

compensated for by a longer processing time. Although hydrometallurgy has fewer to no toxic

substances released into the land (soil) and air (vapor, dust) [11][9], its widespread use in gold

recovery is still limited by the environmental threat it poses, which includes large quantities of

secondary waste (sludge, used solutions) [10].

C. Bio-hydrometallurgy

The environmental waste concerns in hydrometallurgy led to innovations that have used

more natural and biological means to extract gold from used electronics. Limited studies have

been conducted in the past two decades and most have focused on two main methods: bio-

oxidation and bio-sorption [12]. Bio-oxidation uses chemical reactions assisted by bacteria to

recover gold from metallic sulfides. Bio-oxidation may be performed using dump leaching or

stirred tank leaching, and is recommended for smaller deposits and companies that do not have

sufficient capital for a full extraction facility. Bio-sorption (or bioleaching) is an adsorption process

where microorganisms like algae, fungi, bacteria or biomasses (i.e. lignin, polysaccharides, chitin,

persimmon tannin) are used to produce sludge that the e-waste is then immersed in. Plants and

microorganisms contain anionic charged groups that react with metal ions [15]. These organisms

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can selectively adsorb specific metals, making it easier to extract gold. Figure 7, taken from a

patent on cyanide-free bioleaching, exhibits one example of a bio-hydrometallurgy process.

Studies from the 1980’s list the following microorganisms as capable of adsorbing gold:

Penicillium spinulosum, Aspergillius niger and sewage sludge [15]. Advanced Mineral

Technologies Inc. (AMT) also developed the AMT-Bioclaim, a process that uses bacterial biomass

for removal of various metals, including gold [15]. Other notable biomasses used include tannin

gel, magnetotactic bacteria, persimmon gel, desulfovibrio desulfricans, C. violaceum and other

cyanide-producing bacteria [12].

Although these methods are not widely used, there are several benefits to be gained.

Biomasses are cheaper and more accessible than the powerful leaching agents used in regular

hydrometallurgy. The sludge produced is also biodegradable and can therefore reduce secondary

waste generation and toxic impacts to the environment [12][13]. Biomasses also have greater

selectivity and load capacity for gold [12][9][8], leading to a recovery of more than 90%. They can

also be more efficient in batch operations than chemical processes [9]. However, much is still to

be learned on using this method on a more industrial scale. A 2015 study by Jadhav and Hocheng

[13] also demonstrated that bio-hydrometallurgy can have poor lead dissolution compared to

HCl, which may be challenging when extracting gold from electronics that have high lead content.

D. Comparing Gold Extraction Methods

Although all three methods are applicable to consumer electronics and e-waste, each

method should be evaluated carefully based on the net value they can deliver. As Syed [12]

stated, the value of the metals extracted must be able to justify the total cost of the process.

Table 2 compares the different methods available in literature and assigns scores from a scale of

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1 to 5 to each criterion. Weights were also arbitrarily assigned based to give each criterion the

right weight. In this case, environmental impact and cost were given equal weights due to

regulations on e-waste recycling. It should be noted that there are different iterations of each

method, some of which may yield more favorable extraction results than traditional methods.

These specific cases are not included in this table. Bio-hydrometallurgy appears to be the most

promising method, though least applied at present due to its very specialized nature and slow

processing time. Overall, despite the high recovery performance of other methods, these are

limited by safety, environmental impact and cost, which are now opportunities that future

studies can address.

III. CHALLENGES AND LATEST DEVELOPMENTS IN GOLD EXTRACTION

Despite the high gold recovery rates from existing processes, very little gold is recycled

from consumer electronics. An excerpt from US Patent US8663584 best summarizes the key

challenges driving this:

“There exists an unmet need for a more effective and economical system employing a

more practical reagent in the extraction of precious metals... Such a method should yield

the highest amount of recovered precious metal with a significantly lower utilization of

extractant than current systems. Finally, thus a system and extractant should significantly

reduce the time needed for the process by the elimination of time-consuming scrubbing

stages so that production may speed up and further lower costs by increasing valuable

recovered precious metals using less labor and extractant material.” [16]

Based on past studies and research reviews, gold extraction methods can be evaluated

based on a few criteria that may also be treated as challenges to overcome for a more frequent

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and wider use of gold extraction. As the patent suggests, new developments in gold extraction

must be able to tackle at least one of these challenges, summarized in Table 1.

Cost is the foremost challenge that has yet to be addressed by new methods. It is often

more expensive to extract and recycle gold from e-waste than to harvest new gold and build new

electronics. Every other challenge enumerated has some influence on cost.

1. Increasing the speed of extraction: Increasing acid concentration and agitation.

Aside from capital costs for setting up the recovery facility, the overall costs of gold

extraction are rooted in operating expenses to run the facility. These variable costs depend on

how much time is required to perform one round of gold recovery for a given volume. For labor-

intensive processes like hydro-metallurgy, improvements can be made to reduce processing

time. In 2015, Jadhav & Hocheng performed experiments on PCBs and determined that

increasing acid concentration and agitating or shaking the e-waste in the solution up to 150 rpm

would. Based on their study, the size of the electronic component also influenced the speed of

extraction, where a smaller piece would be processed faster [13].

2. Improving recovery efficiency: Phased treatment with copper.

Efficient extraction of gold can also ensure that the most value is recovered from an

extraction process. Improving efficiency can range from using a more powerful reagent or

applying a multi-step approach. In their 2015 study, Torres and Lapidus [17] suggested pre-

treating the electronics using copper leaching in HCl or in citrate and peroxide, followed by gold

leaching with thiourea. This technique resulted in greater than 90% removal of gold within one

hour. Copper has also been validated by Cayumil et al. [10] to have a high affinity with precious

metals like gold, making copper leaching an effective pre-treatment.

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Improving recovery also reduces the need for additional grinding and purification steps,

thus saving total processing time. For example, ammonium persulfate and cupric chloride have

been used to oxidize and leach e-waste, resulting in 98% recovery of gold [11]. A similar study

that was conducted by Madenoglu in 2005 [18] found that the highest recovery was from using

nitrate and chloride solutions. Petter et al. [14] have also verified that thiosulfates are still not as

effective as cyanide-based reagents in terms of gold recovery.

3. Reduce environmental impact and safety risks.

Despite the high gold recovery rates from pyro-metallurgy and hydro-metallurgy, there

has been limited use of these methods due to their environmental impact. Two approaches to

address this would be to (i) find a leaching solution that would be cheaper, more recyclable or

reusable, and (ii) find alternative methods of extracting gold. In this field, bio-hydrometallurgy

emerges as a viable option. Syed [12] lists 41 different technologies that can be used with a

lower environmental impact. Notable among these technologies is the use of orange waste gel

biomass, tannin gel, lignin and potassium persulfate. In 2016, the University of Saskatchewan

developed a 5% solution from acetic acid and an oxidant to recycle gold with minimal impact to

the environment.

Although these are the main issues and approaches that the industry can take, there are

other external factors that should be considered beyond the bounds of gold extraction. Reducing

the overall reliance on gold by finding an alternative substance to use in consumer electronics

would help reduce the amount of gold wasted annually. Similarly, establishing standards for

electronics manufacturing would pave the way for a more uniform recycling process that can

therefore gain efficiency over large batches.

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IV. CONCLUSION AND FUTURE RESEARCH OPPORTUNITIES

Methods for the extraction of gold from consumer electronics and other secondary

sources have existed for many years, but challenges in cost, recovery rate, speed, safety and

environmental impact have discouraged the industry from recycling larger quantities of gold,

especially from consumer electronics. Currently, there is no single gold extraction process that

achieves gold recovery efficiency without sacrificing safety and the environment. To make gold

extraction and recycling more attractive, future studies must develop new methods that combine

the best features of each while maintaining low costs.

Based on the literature and evaluation, bio-hydrometallurgy appears to have the most

promise due to the availability of the leaching agents and the low environmental impact. Future

research can also explore multi-stage gold extraction processes that use some form of pre-

treatment or initial dissolution at the beginning. This paper has briefly detailed the importance

of extracting and recycling gold from the fast-growing deposits of e-waste which, when paired

with a decreasing supply of gold worldwide, will present serious issues and rising gold prices in

the future. This is exacerbated by human behavior such as the improper disposal of used

electronics, planned obsolescence, lack of software support for older models, insufficient

commitment to repair electronics and low manufacturing costs [19]. Ultimately, the success of

gold extraction from consumer electronics will depend on the cooperation of many stakeholders.

First, the government to set regulations, second third-party refineries to use more efficient

methods of extraction based on academic research, third consumer electronics companies to

design products that rely less on gold, and the customers who must learn to use their electronics

for a longer period of time rather than disposing them and building up more e-waste.

REFERENCES:

[1] P. Goodman, “Current and future uses of gold in electronics,” Gold Bull., vol. 35, no. 2, pp. 21–26, 2002.

[2] D. E. Sullivan, “Recycled Cell Phones — A Treasure Trove of Valuable Metals Metal Contents in Cell Phones,” Surv. U S Geol., vol. 2004, no. July, p. 4, 2006.

[3] M. Yu, J., Williams, E., Ju, “Review and prospects of recycling methods for waste printed circuit boards,” in Sustainable systems and technology, 2009, pp. 1–5.

[4] World Gold Council, “Gold Demand Trends Full Year 2016,” 2017. [Online]. Available: http://www.gold.org/supply-and-demand/gold-demand-trends/back-issues/gold-demand-trends-full-year-2016/technology. [Accessed: 01-Mar-2017].

[5] Bureau of the Fiscal Service, “Status Report of U.S. Government Gold Reserve,” 2017. [6] T. Lewis, “World’s E-Waste to Grow 33% by 2017, Says Global Report,” Live Science, 2013. . [7] Electronics TakeBack Coalition, “Facts and Figures on E-Waste and Recycling,” 2016. . [8] S. Syed, The Recovery of Gold from Secondary Sources, First. World Scientific. [9] A. Akcil, C. Erust, C. S. Gahan, M. Ozgun, M. Sahin, and A. Tuncuk, “Precious metal recovery from

waste printed circuit boards using cyanide and non-cyanide lixiviants - A review,” Waste Manag., vol. 45, pp. 258–271, 2015.

[10] R. Cayumil, R. Khanna, R. Rajarao, P. S. Mukherjee, and V. Sahajwalla, “Concentration of precious metals during their recovery from electronic waste,” Waste Management, vol. 57, pp. 121–130, 2016.

[11] A. Alzate, M. E. Lopez, and C. Serna, “Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate,” Waste Management, vol. 57, no. February, pp. 113–120, 2016.

[12] S. Syed, “Recovery of gold from secondary sources-A review,” Hydrometallurgy, vol. 115–116, pp. 30–51, 2012.

[13] U. Jadhav and H. Hocheng, “Hydrometallurgical recovery of metals from large printed circuit board pieces,” Sci. Rep., vol. 5, no. 101, p. 14574, 2015.

[14] P. M. H. Petter, H. M. Veit, and A. M. Bernardes, “Leaching of gold and silver from printed circuit board of mobile phones,” Rem Rev. Esc. Minas, vol. 68, no. 1, pp. 61–68, 2015.

[15] M. Dinardo, O., Kondos, P.D., MacKinnon, D.J., McCready, R., Riveros, P., Skaff, “Study on metals recovery/recycling from acid mine drainage,” J. Chem. Inf. Model., no. July, 1991.

[16] M. Loghman, “Method and Technique Employing a Novel Extractant to Enhance Recovery of Gold and Palladium from Hydrochloric Acid Media,” US8663584 B2, 2014.

[17] R. Torres and G. T. Lapidus, “Copper leaching from electronic waste for the improvement of gold recycling,” Waste Manag., vol. 57, pp. 131–139, 2015.

[18] H. Madenoglu, “Recovery of some metals from electronic scrap,” Ege University, 2005. [19] S. F. Ahmed, “The Global Cost of Electronic Waste,” The Atlantic, 2016. [Online]. Available:

https://www.theatlantic.com/technology/archive/2016/09/the-global-cost-of-electronic-waste/502019/. [Accessed: 25-Mar-2017].

[20] Y. V. et al. Nancharaiah, “Biological and Bioelectrochemical Recovery of Critical and Scarce Metals,” Trends in Biotechnology, vol. 34, no. 2, pp. 137–155, 2016.

[21] E. Zinke, Gabor, “Green Mining: Process of Cyanide-Free Bioleaching and Bioadsorption of Precious Metals,” EP 2271781B1, 2012.

Figure 1: Gold Fabrication Demand Trends from 1979 to 2015.

http://www.kitco.com/charts/CPM_charts.html

Figure 2: Gold Supply Trends from 1973 to 2015. http://www.kitco.com/charts/CPM_charts.html

Figure 3: Economic Trends in Gold. Graph based on data from World Gold Council (2017).

Table 1: Estimated Gold Content for Various Types of E-Waste (Cui and Zhang, 2008)

Figure 4: Flowchart of Possible Methods that can explored for gold extraction. Diagram taken directly from Nanchariah et al. [20]

Figure 5: Sample Hydrometallurgy Process Flow for Gold Extraction.

Taken directly from Syed [12]

Figure 6: Process Flow for Bio-hydrometallurgical Extraction from E-Wastes [20]

Figure 7: Diagram of Bio-hydrometallurgical Processes [21]

Table 2: Comparison of Three Main Gold Extraction Methodologies

Cost Efficiency SafetyEnvironmental

Impact

Technology

RequiredTime Difficulty Score

20% 15% 15% 20% 10% 15% 5% 100%

Total extraction

cost per kg of

gold

What % of gold is

successfully

recovered?

Risk of injury or

death during

process

Additional waste

produced and/or

other

environmental

damages

Are special and

uncommon

equipment

required to fulfill

this process?

Amount of time

required to

extract gold

Ease of extracting

gold based on

special

knowledge and

skills required

1Pyro-

metallurgy

High; Large

capital cost99.90%

High; Hazardous

emissions, high

temperatures,

special handling

required

High; Carbon and

toxic emissions

Requires special,

high-grade feeds,

large capital cost

Requires pre-

treatment to

facil itate

reaction

Some expertise

required to

understand the

complex

materials and

thermodynamics

required.

1 5 1 1 2 3 5 2.2

2Hydro-

metallurgyMedium 95 to 99%

Medium Risk;

Some leachants

l ike cyanide and

aqua regia are

highly toxic and

dangerous to

humans

Medium;

Reusable /

recyclable

leaching

solution, though

stil l toxic upon

disposal

Does not require

too many special

technologies, but

can experience

some loss of

metals during

dissolution

Slow dissolution

process

Standard process

with little

complexity

3 4 3 4 3 2 4 3.3

3 Bio-metallurgy Low 90 - 100% Low Risk

Low;

Biodegradable,

easy disposal,

non-toxic

Does not require

any special

technology; ideal

for smaller sizes

of e-waste

Slower than all

other methods

due to absorption

time required by

bacteria

Easy operation,

though bacteria

and biomasses

can be variable

4 4 5 5 4 2 4 4.1

Methodology#

Table 3: Evaluation Criteria for Gold Extraction Methods

Cost Total extraction cost per kg of gold

Efficiency What % of gold is successfully recovered?

Safety Risk of injury or death during process

Environmental Impact

Additional waste produced and/or other environmental damages

Technology Required

Are special and uncommon equipment required to fulfill this process?

Time Amount of time required to extract gold

Difficulty Ease of extracting gold based on special knowledge and skills required