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ASM Conference 2018
Johannesburg, 10–11 September 2018
The Southern African Institute of Mining and Metallurgy
95
Strategies for sustainable gold processing in the artisanal and
small-Scale mining sector in Zimbabwe
T. Mukono1, G.G. Dembetembe1, L.S. Mapamba1, T.O Ndoro1, N.Z Dzimunya1, and T. Mabikire2
1University of Zimbabwe
2Ministry of Mines and Mining Development, Zimbabwe
The artisanal and small-scale gold mining (ASGM) sector is considered one of the most
important sources of income for both rural and urban communities in Zimbabwe,
particularly with diminished access to alternative opportunities. However, gold
processing in the ASGM sector is characterised by a lack of capital and limited use of
appropriate technology in the processing of gold ores. This paper examines gold
processing in the ASGM sector in Zimbabwe and identifies processing gaps that negatively
affect the sustainability of the small-scale mining sector in Zimbabwe. The focus was to
identify improvement opportunities to make processing more efficient, less destructive to
the environment and more meaningful to the operators. Preliminary analysis suggests that
the deployment and use of appropriate technology for small-scale gold processing will
significantly improve productivity and reduce impact of operations on the environment.
This paper further proposes ways of deploying such integrated and pragmatic
interventions to the ASGM sector.
Keywords: ASM, gold processing, appropriate technology, environmental impact,;
productivity, sustainability.
INTRODUCTION
Gold remains one of the key minerals in Zimbabwe’s mining industry, accounting for some 47% of the
mineral exports as of 2016 (See Figure 1) (Pact & The Institute for Sustainability Africa, 2017).
Figure 1. Mineral share in Zimbabwe's total minerals output value, 2016 (Pact & the Institute for
Sustainability Africa, 2017).
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The Zimbabwean mining sector employs in excess of 25% of those in formal employment and over
500 000 are believed to be involved in artisanal gold mining (AGM), (The Chamber of Mines of
Zimbabwe, 2017) ). Zimbabwe has a gold resource estimated at 84 million tonnes at an average grade
of 4.9 g per tonne (Mlambo, 2015 ). Gold is one mineral among an estimated 60 other minerals, 40 of
which have been exploited at some point. However, according to Pact and the Institute for Sustainability
Africa (2017), only a handful of these minerals contribute significantly to the economy of Zimbabwe:
gold, diamonds, platinum group metals, coal, and nickel (See Figure 1). From Figure 1, it is clear that
the gold sector is certainly important to the country’s economy. It is no surprise therefore, that the sector
is considered key in poverty alleviation efforts. The structure of the gold sector as reflected by the
distribution of contribution by artisanal, small-scale and large-scale gold miners (LSGMs), shows that
ASGM has become an important sector that requires attention. In view of this, it is worthwhile taking
some time to understand the different players in the sector.
There are two important sources of definitions for the players in the mining sector, the Ministry of Mines
and Mining Development (MMMD) and the Environmental Management Agency (EMA). The Mines
and Minerals Act Amendment Bill, 2015, defines a small-scale miner as: an indigenous person
employing not more than 50 people including contractors, on a registered mining location of not more
than 40 hectares in extent, who produces and or processes not more than 1 200 tonnes of ore per year.
Zimbabwe’s Environmental Management Regulations (2014) define an artisanal miner as: a miner who
carries out mining activities using simple tools and employs up to 50 people; these include Government-
registered groups or syndicates or co-operatives.
It is generally accepted in Zimbabwe that the distinction between ASGM and LSGM (and further,
between artisanal gold miners and small-scale gold miners) is based on the scale of operation and degree
of mechanization. On the other hand, artisanal miners are understood to be unregistered and hence
possess no property rights. While the sector used to be dominated by LSGM, ASGM contribution has
increased significantly in recent years as shown in Figure 1. Gold mining contributes a significant share
of 47% of the entire mining sector. The 47% is divided broadly into 26% LSGM and 21% ASGM. ASGM
therefore makes significant contribution to the growth and development of the Zimbabwean economy
(Pact, 2017).
The significant share of ASGM on the total gold output is attributed to an increase in the number of
players over the past two decades. The economic instability over the past two decades left limited
employment opportunities in the formal sector; consequently, informal, artisanal and small-scale
mining becoming increasingly important as an income source (Spiegel, 2015). Table II shows a
comparison between the contributions of ASGM and LSGM to Zimbabwe’s economy as of 2016.
Table II. A comparison of the contribution between ASGM and LSGM in Zimbabwe (Pact, 2017).
Artisanal and Small-
scale Gold Mining
Large-Scale gold
mining
Total gold mining
Production 311,000 troy ounces 428,000 troy ounces 739,000 troy ounces
% of GDP 1.2 % 1.4 % 2.6 %
Royalty
(% of government
revenue)
US$ 3.8 million (0.24%
of government
revenue)
US$ 11.8 million (0.76%
of Government revenue)
US$ 15.6 million (1% of
government revenue)
Employment 500,000 (7 % of labour
force)
11,000 (0.1% of labour
force)
7.1 % of labour force
Direct and indirect
beneficiaries
1 million 81 000 1.1 million
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In a study by Pact (2017) ,in 2016, the gold mining sector as a whole (both ASGM and LSGM) contributed
2.6 % of GDP, 18 % of exports, 28 % of mining output, 1% of Government revenues (royalties only) and
employed 7.1 % of the labour force. The ASGM share of the total gross domestic product (GDP) in
Zimbabwe was 1.2 % against a share of 1.4 % by LSGM; an indication of ASGM’s profound contribution
to the economy.
The sector does not only bring about economic benefits but numerous challenges for the environment,
as well as society as a whole. ASGM is associated with environmental degradation, social ills, and poor
health and safety records (Pact & The Institute for Sustainability Africa, 2017). Environmental pollution
through mercury and cyanide, land degradation, river siltation, mine accidents and high prevalence of
occupational diseases are among the common negative impacts of ASGM. A number of these problems
can be connected to the use of inappropriate/unsustainable processing methods/practices.
Generally, in Zimbabwe many ASGM operators use basic tools for mining, ore handling and mineral
processing, leading to low levels of productivity. Hentschel, et al., (2003) argued that most problems
associated with ASGM are technical and require technical solutions implemented in an intergral
approach. Appropriate technological interventions on ASGM operators can enhance the economic
sustainability of such operations without compromising on their environmental sustainability. Closing
this technological gap is one way of transforming ASGM operations into vibrant mining entities. The
technical solutions have to be commensurate with the economic potential of the target group and need
to be accompanied by education and training and be affordably replicable.
METHODOLOGY
The study was guided by the following research questions:
1.What are the typical gold processing techniques used by ASGM operators in Zimbabwe?
2. What are the inadequacies of these technologies?
3.Are there any sustainable processing alternatives suitable for the ASGM sector?
4.What strategies can be implemented to ensure sustainable gold processing for the ASGM
sector?
Qualitative methods, inclusive of desktop research were used to establish sustainable processing
methods for the ASGM sector. Data used in the research was obtained from secondary sources which
includes reports published by Pact Zimbabwe, Ministry of Mines and Mining Development (MMMD),
Zimbabwe Miners Federation (ZMF), relevant academic publications, reports, articles and government
publications. A field survey of the existing gold processing techniques in Gwanda Mining district of
Matebeleland South Province was conducted. Processing methods and attention policies to rendering
the ASGM sector more sustainable were proposed.
FINDINGS
An overview of common gold processing methods in the ASGM sector in Zimbabwe
Most small-scale miners do not have ore processing facilities and rely on custom millers for their ore
processing. Custom milling plant in Zimbabwe is defined in the Statutory Instrument 329 of 2002 as any
plant for processing or extraction of gold or gold bullion that is not operated exclusively for the benefit
of an operator who mines the gold. In this set up, artisanal and small scale miners bring ores for milling
and recovery of free gold for a fee and leave gravity concentration tailings for the mill operator to leach.
Most custom millers use stamp mills, three or five stamps, in the recent years there has been a noticeable
increase of other milling and grinding machinery. Figure 2 shows a typical stamp mill used in the ASGM
sector.
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Figure 2. Stamp mill in a Custom Milling operation
Ores from ASGM operators are typically a few wheel barrows, averaging seven tonnes, and after every
milling cycle the milling machinery has to be washed off any residual ore. The change-overs’ frequency
and procedure favours machinery that is easy and fast to clean up, this made stamp mills preferred
machinery for custom millers. Stamp mills are however operated with a mesh size 16 for faster
throughput and retention of gold in gravity concentration tailing for the benefit of the miller on leaching
stage. There has been an advent of new equipment for custom milling in the past three years, these
include hammer mills, Chilean roller mills, and amalgam barrels (used for milling). These machinery
not only offer the fast and easy cleaning change-overs but are less capital intensive on purchase. On
average a stamp mill cost around US$35 000 to purchase while hammer mill is US$2500, Chilean roller
mill US$8 000 and amalgam barrel US$2 000. This equipment offer a finer product at a faster milling
rate than stamp mill and artisanal miners are preferring them. The gravity concentration process is
similar to those used on stamp mills. Figure 3 shows the setup of a typical diesel powered hammer mill
connected to a gravity concentrator.
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Figure 3. Setup of a typical diesel-powered hammer mill connected to a gravity concentrator for ASGM
operations.
At custom milling plants artisanal miners recover free gold using either cloth lined sluices, centrifugal
concentration bowls, or mercury smeared copper plates. For the later, the gold-mercury amalgam is
removed by rubber plates from the copper plate. For sluices and centrifugal concentrator a gold
concentrate, with a concentration ratio averaging 25 is obtained. The concentrate is hand mixed with
mercury for amalgamation in small plastic dishes as shown in Figure 4. The amalgamation process
normally takes 5 to 6 hours for every 7 tonnes of ore processed. These processes expose artisanal miners
to skin contact with mercury which is a health risk to them directly. Disposal of the water used on
amalgamation releases mercury to the environment.
Figure 4. Hand mixing of gold concentrate with mercury for amalgamation.
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The produced amalgam is heated to evaporate the mercury. This process is done in open air, releasing
the mercury vapour into the atmosphere. The obtained sponge gold is smelted and sold with purity
estimated from its specific gravity. Statutory Instrument (S.I.) 329 of 2002 has set conditions for use of a
retort to recycle mercury, however very few plants have the retorts. Generally, a great majority of miners
in the region do not use retorts, as they claim that the process is time consuming as they use low-
temperature bonfires.
Most of the gold is left in the primary tailings and the custom miller leaches these tailings from the
gravity concentration to recover the remaining gold. Most custom millers use NaCN vat leach tanks,
with a subsequent carbon adsorption circuit. However, miners receive no compensation for the extra
gold extracted by cyanidation. Figure 5 shows the set-up of a vat leach tank typical of ASGM operations.
Figure 5. Vat leach tank for gold recovery from tailings in ASGM operations.
The leaching cycles averages three days. The pregnant solution is drained and passed through activated
carbons. The batch cycle is repeated until the activated carbon is fully loaded. The leaching tanks used
are open posing a danger of possible poisoning of the fauna. It is very common to find dead frogs and
birds in and around the cyanide return tanks. During the rainy season there is possible overflow into
natural water channels of the cyanide solution in leach tanks. In 2015 the Zimbabwean Environmental
Management Authority made it mandatory for millers to line tailings dumps to prevent seepage of
heavy metals into the underground water. As such, tests are conducted regularly through boreholes to
monitor possible contamination of underground water.
Activated carbon is used for the adsorption process because it has the following properties: highly
developed internal pore structure; narrow mean size particle size distribution; high adsorptive capacity
and adsorption rate; good mechanical strength; wear resistance; reactivation characteristics and low cost
and readily available. Figure 6 shows an adsorption circuit which consists of drums loaded with
activated carbon and pregnant leach solution in ASGM operations.
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Figure 6. Adsorption circuit consisting of drums loaded with activated carbon and pregnant leach solution.
After completion of adsorption process, the loaded carbon is then eluted at toll elution plants, which
uses the Zadra process to recover gold from the activated carbon.
Key issues associated with common gold processing methods in the ASGM sector
ASGM makes noteworthy contributions to both economic growth and economic development,
however, the gold processing techniques exhibit the following key issues:
•Poor recoveries: Lack of gold liberation is an evident problem when using stamp mills and this
is the main reason why miners recover less than 30 % of the total gold by gravity separation
followed by amalgamation.
•Poor economic returns for miners: Due to low efficiencies, most of the gold is left in the primary
tailings and the millers apply vat-cyanidation to extract the remaining gold. Miners receive no
compensation for the extra gold extracted through cyanidation and this is a clear indication that
miners are disadvantaged of their extra potential revenue.
•Environmental degradation: The use of inefficient technology and limited processing
techniques in ASGM operations cause severe environmental impacts. For example, the disposal
of water used for amalgamation releases mercury to the environment and happens to be
problematic in ASGM processing operations.
•Occupational health and safety: The ASGM operators are exposed to skin contact with mercury
which is a direct health risk.
Field data reflects that most of the issues associated with gold processing in ASGM sector can be
attributed to lack of technical and environmental knowledge, education and training. For instance, in
ASGM operations ore head grades are not known to ascertain recoveries. Further, some miners believe
that adding more cyanide increases the gold dissolution kinetics. However, the chemistry of gold
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extraction reveals that more cyanide is only effective when there is an excess oxidising agent that results
in the formation of cyanate, which is not as efficient in leaching gold as cyanide (Santos, 2013).
Proposed processing methods for ASGM in Zimbabwe
ASGM in Zimbabwe has great potential for growth and the technical solutions proffered to the sector
are commensurate with the economic potential of the sector. In order to organize the rationale behind
the decision on selection of processing methods, some criteria were followed:
Must be easily accessible
Must not be very complex
Must be in-expensive and use locally available raw materials.
The proposed methods are methods which have been developed with a view to improving ore recovery
in gold processing as well as to eliminate the environmental hazards associated with the use of mercury.
A few processing methods meet that criteria, i.e., the direct smelting method and the Igoli processing
technology and as such these were recommended for deployment.
The Direct smelting method
The direct smelting method is a safer process which was developed to eliminate the negative
environmental/health-related effects of mercury which has been used extensively in small-scale mining
communities to amalgamate gold particles facilitating their separation from sands. Figure 7 and Figure
8 show the processing flow sheets for the direct smelting method for free (alluvial) gold and hard rock
deposits.
Figure 7. Direct smelting process for alluvial gold deposits.
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Figure 8. Direct smelting process for hard rock deposits.
In direct smelting method, a mixture of black sand containing gold, borax and soda ash, is melted by
heating the mixture in a charcoal or gas fired smelter. Borax and soda serve the purposes of respectively
lowering the melting point of the solution and helping the material to flow easily after heating (Salati,
et al., 2016). A study by Amankwah, et al., (2010) on the application of direct smelting of gold
concentrates as an alternative to mercury amalgamation showed that the direct smelting methods yields
99.8 % recoveries against 97 % recoveries for amalgamation.
The Igoli processing method
Figure 9 shows the process flowsheet of the proposed Igoli method, which was developed by Mintek
South Africa.
Figure 9. Processing Flowsheet of Igoli Technology (Salati, et al., 2016).
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The Igoli process consist of leaching the gold concentrates with dilute hydrochloric acid and bleach (aka
sodium hypochlorite ()) to obtain a pregnant leach solution. The solution is filtered and sodium
meta-bisulphite () added to the filtrate to precipitate gold as gold powder. The Igoli process has
the following advantages:
The process has high gold recoveries and produces very pure gold up to 99% which can be
easily valued and sold;
Most of the residue to be discarded contain a very high percentage of silica which is
environmentally friendly;
The liquid effluent is neutralised and gases released from the process can be minimised or
controlled.
The Igoli process is highly suitable for groups of artisanal miners and is proposed for use in the ASGM
sector in Zimbabwe.
Most operators in the ASGM sector in Zimbabwe operate under marginal economic conditions,
providing no more than a daily living and as such, the proposed processing methods have better
recoveries compared to the common processing techniques, thus offering better economic returns for
the operators. They are also environmentally friendly as they eliminate various environmental hazards
and health risks associated with the use of mercury. However, deployment of the processing methods
remains a challenge in the ASGM sector and as such, ways of deploying such integrated and pragmatic
interventions to the sector are proposed.
Ways of deploying the proposed methods: Strategies for sustainability
The proposed methods are meant to ensure sustainable and environmentally friendly ASGM operations
in Zimbabwe. Sustainable development has three overarching goals (Shields & Solar, 2005):
•Economic prosperity
•Environmental health
•Social equity for present and future generations
ASGM operations in Zimbabwe have relatively low recoveries and productivity per unit operation, low
safety, environmental and health risks and as such the application of proposed processing methods
requires efficient and effective instruments to ensure sustainability in the sector. Sustainable processing
will go a long way in promoting income growth and poverty alleviation for the ASGM sector and the
following strategies are recommended:
1.Education and training are key strategies to improve processing sustainability in the ASGM
sector. Implementing technical change requires detailed knowledge of the proffered
solutions/methods. Zimbabwe have quite a number of universities and institutions offering
mining and processing programmes, with staff capable of finding practical solutions for the
ASGM operations. These institutions should take a leading role in the education, training, co-
ordination and research for appropriate technology in the processing and handling of
environmental problems concerning ASGM operations.
2.Large Scale Gold Mining (LSGM) companies are highly encouraged to support ASGM
operations in training and offer required services as part of their corporate social responsibility.
Most ASGM operators are located closer to LSGM operations, and as such LSGM are
encouraged to improve communication and assistance in educating and training ASGMs to
foster good relationships in cohabitation. In ASGM operations, as with any other conventional
ore processing, is subject to design problems, variability of ores and engineering problems. As
a conventional processing plant relies on studies and characteristics of their ore, an ASGM
operation would also benefit from these services.
3.ASGM operators face serious challenges in obtaining access to credit and formal banking, as
they are considered to be too risky. As such, the ASGM operators lack capital to acquire
improved tools and equipment for processing their ores. The government of Zimbabwe is
encouraged to create a special fund to assist the operators in acquiring improved processing
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facilities. Non-governmental agencies (NGOs) supporting the sector should create funds
available for prospective operators and are encouraged to dialogue and offer pragmatic
solutions to ensure sustainable development in the ASGM sector of the Zimbabwean economy.
CONCLUSIONS
The deployment and use of the proposed methods for the ASGM sector in Zimbabwe will significantly
improve productivity and reduce impact of operations to the environment and enable operators to
standardize their operations to levels in tandem with best global gold processing practices. ASGM
operations in Zimbabwe have relatively low recoveries and productivity per unit operation, low safety,
environmental and health risks and as such the application of proposed Direct smelting method and
the Igoli processing method requires efficient and effective instruments to ensure sustainability in the
sector. In view of this, technical solutions proffered are commensurate with the economic potential of
the ASGM sector in Zimbabwe and need to be accompanied by education and training and be affordably
replicable. ASGM oprators are highly encouraged to partner LSGM and benefit from technology
transfer as part of their capacity building. Fundamentally, the ASGM sector in Zimbabwe has the
potential to alleviate poverty and be a tool for sustainable development and as such the strategies for
sustainable processing will go a long way in promoting income growth and poverty alleviation for the
sector.
REFERENCES
Amankwah, R. K., Styles, M. T., Nartey, R. S. & Al-Hassam, S., (2010). The application of direct
smelting of gold concentrates as an alternative to mercury amalgamation in small-scale gold
mining operations in Ghana. International Journal on Environment and pollution, pp. 1-12.
Environmental Management Regulations, 2014. Statutory Instrument 92 of 2014, s.l.: Goverment of
Zimbabwe.
Government of Zimbabwe, 2002. Mines and Minerals (Custom Milling Plants) Regulations, s.l.: Statutory
Instrument 329 of 2002.
Hentschel, T., Hruschka, F. and Priensten, M., (2003). Artisinal and Small-Scale Mining: Challenges and
Opportunities. London: World Business Council for Sustainable Development.
Ministry of Mines, 2015. MINES AND MINERALS AMENDMENT BILL, Harare, Zimbabwe:
Government of Zimbabwe.
Mlambo, L., (2015), . Key areas where Zimbabwean Mining Policy is aligned to the African Mining Vision
(AMV). s.l.:Workshop hosted by Open Society Initiative for Southern Africa (OSISA) / Southern
Africa Resource .
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D.C USA: Pact.
Pact and The Institute for Sustainability Africa, 2017. The contribution of Artisanal and Small-Scale Gold
Mining to Zimbabwe's Economic Growth and Development, Washington, D.C.: Pact.
Salati, L. K., Mireku-Gyimah, D. & Eshun, P. A., (2016). Proposed Mining and Processing methods For
Effective Management Of Artisinal and Small-Scale Gold Mining in Nigeria. International Journal
of Scientific and Engineering Research, 7(12), pp. 952-970.
Santos, G. A., (2013). Myths and realities in artisinal gold mining mercury contamination, s.l.: The
University of British Columbia.
Shields, D. J. and Solar, S. V., (2005). Sustainable development and minerals: measuring mining's
contribution to society. Geological Society Special Publication , Issue 250, pp. 195 - 212.
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Spiegel, S. J., (2015). Shifting Formalization Policies and Recentralizing Power: The Case of
Zimbabwe's Artisinal Gold Mining Sector. Society and Natural Resources: An International Journal,
pp. 543-558.
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Chamber of Mines of Zimbabwe.
Tichaona Mukono
Lecturer
University of Zimbabwe
Tichaona holds a BSc Hons in Metallurgical Engineering (UZ) and an M.Min.Sc in Metallurgy and
Mineral Processing (UNZA). He currently lectures in the Department of Metallurgical Engineering at
the University of Zimbabwe. Before which he has also worked in the Ministry of Mines and Mining
Development in Zimbabwe as a Metallurgist. Tichaona has research interests in the areas of Extractive
Metallurgy, Metallurgy of Iron & Steel, and Small-Scale Mining & the Environment.
