`

Green Mining Case Study

`

1. Gold mining

1.1 Reserves Reserves data are dynamic because they may be reduced as ores are mined and/or the

extraction feasibility decreases, or more commonly, they may continue to increase as

further deposits (known or recently discovered) are developed, or currently exploited

deposits are more completely explored and/or new technology or economic variables

enhance their economic feasibility. Hence, reserves data are a major issue because they

betray where the largest resources are, allowing us to be aware of the countries that must

improve its mining methods in order to extract in the best possible way. [1]

1.2 Production

Details of world gold production over the past 150 years appear in Fig. 1. The effects of the

California gold rush in 1849, followed by the gold rushes in Australia in 1851 and South

Africa in 1884 are evident. The development of new cyanide milling technology (carbon-in-

pulp) and the major rise in the real price of gold in the 1970s led to expanded production

from 1980 onwards. [2]

Figure 1. World gold production [2]

`

The gold mining boom since the late 1970s has been facilitated by the combination of a real

price rise, the development of carbon-in-pulp (‘CIP’) milling technology, and to a lesser

extent the evolution in large-scale bulk earth-moving vehicles and mining techniques. These

factors led to more exploration, focused initially on previous gold producing provinces,

along with the development of many new gold mines around the world. These mines have

often been based on open cut mining techniques, which allow more complete extraction

and processing of all gold-mineralised ore. The economics of gold mining were radically re-

defined during this period. It led to an extra-ordinary renaissance in some countries such as

Australia, the United States and Canada for about 20 years. In South Africa, by contrast, this

pattern did not emerge, due to the deep underground nature of their gold mines as well as

political and social issues. From a global view, based on gold resources data presented,

there is only sufficient known economic resources to sustain existing levels of newly mined

production for less than 20 years. The future extent of economic resources and production

is, of course, difficult to predict but will continue to depend on exploration effort,

economics, social and environmental issues, technology as well as the recycling of the world

gold stockpile. [2]

1.3 Gold mine wastewater production

Gold is generally extracted from ores or concentrates by the alkaline cyanidation (Elsner)

process. The gold-bearing ore is crushed and ground to approximately 100 microns. Next, it

is transported to a leaching plant where lime, cyanide and oxygen are added to the ground

and slurried ore. The lime raises the pH, while the oxygen and cyanide oxidize and complex

the gold. [3]

The cyanide solution thus dissolves the gold from the crushed ore. Next, the gold-bearing

solution is collected. Finally, the gold is precipitated out of the solution. [3]

The common processes for recovery of the dissolved gold from solution are carbon-in-pulp,

the Merrill-Crowe process, electrowinning and resin-in-pulp. In the carbon-in-pulp (CIP)

technique, the gold cyanide complex is adsorbed onto activated carbon until it comes to

equilibrium with the gold in solution. Because the carbon particles are much larger than the

ore particles, the coarse carbon can be separated from the slurry by screening using a wire

`

mesh. The gold-loaded carbon is then removed and washed before undergoing ‘elution’ or

desorption of gold cyanide at high temperature and pH. The rich eluate solution that

emerges from the elution process is passed through electrowinning cells where gold and

other metals are precipitated onto the cathodes. Smelting of the cathode material further

refines the gold and produces gold ingots suitable for transport to a refinery. [3]

Mercury amalgamation and gravity concentration are the other processes for obtaining gold

concentrate from gold ore. The tailings, contaminated with metal and cyanide ions, are

usually stored in tailings ponds, with the potential for groundwater contamination and high

risk of failure, which can lead to spillage of the toxic metals and cyanide-bearing solution

into the environment. The types of heavy metals present depend on the nature of the gold

ore. Acid mine drainage is another type of mine effluent, which is produced when sulphide

ores are exposed to the atmosphere as a result of mining and milling processes where

oxidation reactions are initiated. Mining increases the exposed surface area of sulphur-

bearing rocks allowing for excess acid generation beyond the natural buffering capabilities

found in host rock and water resources. Collectively, the generation of acidity from sulphide

weathering is termed acid mine drainage (AMD). Concentrations of common elements such

as Cu, Zn, Al, Fe, As and Mn all dramatically increase in waters with low pH. [3]

Gold mining operations result in contamination of soils and water with tailings that release

toxic metals such as Cu, As, Pb, Mo, Fe, Ni and Zn. Various regulatory bodies have set the

maximum prescribed limit for discharge of toxic heavy metals into the aquatic ecosystem.

Table 1 shows the maximum contaminants level (MCL) values set by the US EPA and the

position in the Comprehensive Compliance Environmental Response, Compensation and

Liability Act (CERCLA), 2005 list of priority chemicals of some of the toxic heavy metals.

Nevertheless, metal ions are discharged into water bodies at much higher concentrations

than the prescribed limit by industrial activities such as gold mining, thus leading to health

hazards and environmental degradation.

`

Table 1. Maximum permissible concentrations for heavy metals in gold mine’s wastewaters

Heavy metal Concentration, mg/L Arsenic (As) 0.01 Lead (Pb) 0.015 Mercury (Hg) 0.002 Cadmium (Cd) 0.005 Chromium (Cr(VI)) 0.01 Zinc (Zn) 5.0 Manganese (Mn) 0.05 Copper (Cu) 1.3 Selenium (Se) 0.05 Silver (Ag) 0.05 Antimony (Sb) 0.006 Iron (Fe) 0.3

1.4 Health and environmental risks of heavy metals and cyanide

Heavy metal pollution is one of the important environmental problems today. These heavy

metals are of special concern due to their toxicity, bioaccumulation tendency and

persistency in nature. The removal of heavy metals from water and wastewater is important

in terms of protection of public health and environment due to their accumulation in living

tissues through the food chain as a non-biodegradable pollutant. Heavy metals (such as

lead, copper and arsenic) are toxic to aquatic flora and fauna even in relatively low

concentrations. [3]

The excessive intake of copper by man leads to severe mucosal irritation, widespread

capillary damage, hepatic and renal damage, central nervous system problems followed by

depression, gastrointestinal irritation and possible necrotic changes in the liver and kidney.

Arsenic dissolved in water is acutely toxic and leads to a number of health problems,

including disturbances to the cardiovascular and nervous system functions and eventually

death. Other heavy metals (such as Hg, Cd, Se, Pb, Ni, Zn, etc.) produce similar health effects

when injected in significant quantities. [3]

Cyanide is acutely toxic to humans. Toxicological studies have indicated that short-term

exposure to high levels of cyanide causes rapid breathing, tremors and other neurological

`

effects; while long-term exposure causes weight loss, thyroid effects, nerve damage and

death. Skin contact with liquids containing cyanide may produce irritation and sores. Clearly,

heavy metal and cyanide pollution of the environment is of paramount concern due to their

health risk to humans and threats to the ecosystem. [3]

2. Treatment methods of gold mine wastewater

The conventional methods for removing heavy metal ions from wastewater include

chemical precipitation, coagulation–flocculation, flotation, filtration, ion-exchange, reverse

osmosis, membrane-filtration, evaporation recovery and electrochemical technologies.

Advantages and drawbacks of these methods are presented in Table 2. [3]

Table 2. Advantages and limitations of physicochemical treatments of industrial wastewater

Type of treatment

Target of removal Advantages Disadvantages References

Reverse osmosis

Organic and inorganic Compounds

High rejection rate, able to withstand high temperature

High energy consumption due to high pressure required (20–100 bar), susceptible to membrane fouling

Potts et al.32, Kurniawan et al.33

Electrodialysis Heavy metals Suitable for metal concentration less than 20 mg L−1

Formation of metal hydroxide, high energy cost, can not treat a metal concentration higher than 1000 mg/l

Bruggen and Vandecasteele34, Ahluwalia and Goyal28

Ultrafiltration

High molecular weight compounds (1000–10 000 Da)

Smaller space requirement

High operational cost, prone to membrane fouling, generation of sludge that has to be disposed off

Vigneswaran35

Ion exchange Dissolved compounds, cations/anions

No sludge generation, less time consuming

Not all ion exchange resins are suitable for

Vigneswaran35, Ahluwalia and Goyal28,

`

Table 2. Advantages and limitations of physicochemical treatments of industrial wastewater

Type of treatment

Target of removal Advantages Disadvantages References

metal removal, high capital cost

Kurniawan et al.33

Chemical precipitation

Heavy metals, divalent metals

Low capital cost, simple Operation

Sludge generation, extra operational cost for sludge disposal

Ahluwalia and Goyal28, Bose et al.36, Wingenfelder et al.37

Coagulation–flocculation

Heavy metals and suspended Solids

Shorter time to settle out suspended solids, improvedsludge settling

Sludge production, extra operational cost for sludge disposal

Shammas38, Semerjian and Ayoub39, Ayoub et al.40

Dissolved air flotation

Heavy metals and suspended Solids

Low cost, shorter hydraulic retention time

Subsequent treatments are required to improve the removal efficiency of heavy metal

Lazaridis et al.41

Nanofiltration

Sulphate salts and hardness ions such as Ca(II) and Mg(II)

Lower pressure than RO (7–30 bar)

Costly, prone to membrane fouling

Ahn et al.42

Electrochemical precipitation Heavy metals

Can work under both acidic and basic conditions, can treat effluent with a metal concentration higher than 2000 mg/l

High capital and operational costs Subbaiah et al.43

Membrane electrolysis

Metal impurities

Can treat wastewater with metal concentration of less than 10 mg L−1 or higher than 2000 mg L−1

High energy consumption

Kurniawan et al.33 (2006)

`

Precipitation is most applicable among these techniques and considered to be the most

economical. However, this technique produces a large amount of sludge precipitate that

requires further treatment. Reverse osmosis and ion-exchange can effectively reduce the

metal ions, but their use is limited due to a number of disadvantages such as high materials

and operational cost, in addition to the limited pH range for the ion-exchange resin. Table 3

gives performance characteristics of some conventional heavy metal removal and recovery

technologies while Table 4 summarizes research work on heavy metal removal using

physicochemical techniques. [3]

Table 3. Performance characteristics of some conventional heavy metal removal and recovery technologies

Performance characteristics

Technology pH change Metal selectivity

Influence of suspended

solids

Tolerance to organic

molecules

Metal working level (mg

L−1) Adsorption Limited

tolerance Moderate Fouled Can be poisoned < 10

Electrochemical Tolerant Moderate Can be engineered to tolerate

Can be accommodated > 10

Ion exchange Limited tolerance

Some selectivity (e.g. chelating resin)

Fouled Can be poisoned < 100

Precipitation as hydroxide Tolerant Non-selective Tolerant Tolerant > 10

Solvent extraction

Some tolerant systems

Metal-selective extractants available

Fouled

Table 4. Summary of heavy metal removal data by physicochemical treatment methods

Treatment method Metal

Initial metal conc. (mg L−1)

pH Removal efficiency

(%)

Power consumption

(kWh m−3) References

Chemical precipitation Zn(II) 450 11.0 99.77 NA Cherernnyavak45 Cd(II) 150 11.0 99.76 NA

`

Table 4. Summary of heavy metal removal data by physicochemical treatment methods

Treatment method Metal

Initial metal conc. (mg L−1)

pH Removal efficiency

(%)

Power consumption

(kWh m−3) References

Mn(II) 1085 11.0 99.30 NA

Cu(II) 16 9.5 80 NA Tünay and Kabdalsi46

Electrochemicalcoagulation/ As 442 99.90 NA Mavrov et al.31 Membrane Filtration Se 2.32 98.70 NA

Ultrafiltration Cu(II) 78.74 8.5–9.5 100 NA Juang et al.47

Zn(II) 81.10 8.5–9.5 95 NA

Cr(III 200 6.0 95 NA Aliane et al.48 Nanofiltrattion Ni(II) 2000 3–7 94 NA Ahn et al.43

Reverse osmosis Cu(II) 200 4-11 99 NA Mohammad et al.49

Cd(II) 200 4-11 98 NA Froatation Cu(II) 3.5 5.5 98.26 NA Rubio et al.50 Ni(II) 2.0 5.5 98.6 NA Zn(II) 2.0 5.5 98.6 NA Zn(II) 50 7-9 100 NA Matis et al.51

Electrodialysis Ni(II) 11.72 NA 69 NA Tzanetakis et al.52

Co(II) 0.84 NA 90 NA Membrane electrolysis Cr(VI) 130 8.5 99.6 7.9 × 103 Martınez et al.53 Ni(II) 2000 5.5 90 4.2 × 103 Orhan et al.54 Electrochemical precitipation Cr(VI) 570-

2100 4.5 99 20 Kongsricharoern and Polprasert55

Cr(VI) 215-3860 1.5 99.99 14.7–20 Kongsricharoern

and Polprasert56 Ni(II) 40 000 NA 85 3.43 × 103 Subbaiah et al.44 Ion exchange Ni(II) 100 90 NA Cr(III) 100 3-5 100 NA Rengaraj et al.57 Ni(II) 100 90 NA Sapari et al.58 Cu(II) 100 100 NA Cr(VI) 9.77 NA 100 NA Kabay et al.5

`

NA – not available

3. Gold Mine in South Africa

Gold Mine located in the Far West Rand Gold field is the object of current case study. It is

situated in the geologically unique and world renowned Witwatersrand Basin, one of the

world’s premier gold regions. The Witwatersrand Basin has made significant contributions

to South Africa’s economy and remains the most important gold depository in the history of

mining. Since the establishment of the first shaft in 1934, mine has produced more than 70

Moz of gold. Gold Mine operates two gold plants and the underground workings are

accessed from surface through five shaft systems to a depth of 3,347 m below surface. It is

estimated that the current Mineral Reserves will be depleted in 2030. [4]

3.1 Geological Settings and Mineralization

All of Gold Fields’ South African operations are located in the Witwatersrand Basin and are

intermediate to deep level underground mines exploiting gold bearing, shallowly dipping

tabular ore bodies. The gold mineralisation in the Witwatersrand Basin occurs within quartz

pebble conglomerates termed “reefs”. Considered gold mine is located in the West Wits

Line Goldfi eld of the Witwatersrand Basin. This gold field is geographically divided into the

Far West Rand and the West Rand areas. The mining area is underlain by outliers of Karoo

Supergroup shales and sandstones, followed by Pretoria Group sediments and the

Chuniespoort Group dolomites. The Dolomites overlie the Klipriviersberg Group volcanic

rocks, which in turn cap the Ventersdorp Contact Reef and sediments of the Central Rand

Group that hosts the other gold-bearing reefs exploited by gold mine. [4]

3.2. Mining method The predominant mining layout at considered gold mine is breast stoping with dip pillars,

with a minor contribution from scattered mining. Breast stoping with dip pillars has been

selected for the below infrastructure projects. Mining spans and pillar widths depend on the

location, the reef being mined and the depth of working. [4]

`

3.3. Mineral processing

Company has two operational metallurgical facilities using proven metallurgical processes,

with a central elution and smelting facility. The third metallurgical facility that processed

surface material was closed in April 2005. The facility was demolished and all gold

recovered. 1 Plant was commissioned in 1968 to treat underground ore. This plant

comprises three stage crushing, utilising open circuit rod mills for primary milling and closed

circuit pebble mills for secondary milling. After milling, the pulp is thickened and then

processed through air agitated leaching, drum filtration, zinc precipitation and smelting to

doré. In June 2001 an AAC Pump Cell CIP circuit was installed to replace the less efficient

drum filtration and Zinc precipitation. Smelting was also discontinued, with loaded carbon

being transported to 2 Plant for elution and thermal regeneration. The current operational

capacity of 1 Plant is 180 ktpm. 2 Plant was commissioned in November 1990. This Plant

receives underground Run-of-Mine ore (RoM), which is crushed and delivered to a stacker

reclaimer system, where the ore is stored and blended prior to reclamation and delivery to

the mills. Surface material is also delivered to the stacker pad to utilise plant capacity. There

are two Semi-Autogenous Grinding (SAG) mills, which are equipped with variable-speed ring

motor drives, and can be operated as fully autogenous units or as semiautogenous units by

adding steel grinding balls. Milled ore is thickened ahead of cyanide leaching in air-agitated

tanks and adsorption onto activated carbon in a conventional CIP circuit. Loaded carbon is

eluted in an AARL elution circuit, which was upgraded in June 2001 and further in October

2003. It now serves as the central elution facility. The upgrade included the installation of

Continuous Electrowinning Sludge Reactors, which are working very efficiently. Cathode

sludge is filtered and smelted to produce doré. The current operational capacity of 2 Plant is

150 ktpm. [4]

3.4. Gold Mine Tailings

Since gold mining started more than a century ago, South Africa has been the largest

producer of gold in the world. In 1996 alone, 377 million tons of mine waste was produced,

accounting for 81% of the total in South Africa. These mine wastes contain large amounts

(between 10 and 30 kg/ton) of sulphide minerals, such as pyrite, which are prone to

generate acid mine drainage (AMD). AMD is a global pollution problem and is generally

`

reflected by high salt loads and acidification of the affected environment. In addition, AMD

is often associated with significant concentrations of toxic trace elements and radionuclides.

These contaminants remobilise under acidic conditions and migrate into the vadose zone

and groundwater system. More than 270 tailings dams related to gold mining and covering a

total area of about 180 km2 have been identified in South Africa. Most of the tailings dams

are situated either in highly urbanised areas or close to valuable agricultural land. A

conceptual model of the various pollution pathways from gold mining tailings is presented

in Figure 2. [6]

Fig.2. Conceptual model of tailing dam and affected subsurface [6]

The vadose (unsaturated) zone is considered to be a geochemical and physical barrier

between the primary source of contamination (i.e. tailings dam) and the recipient

groundwater system. Moisture movement and attenuation processes such as adsorption in

the vadose zone have the potential to mitigate the contamination of the groundwater.

`

However, once this barrier has become contaminated, it can also act as a continuous source

of pollution. Furthermore, it must be stressed that gold mine tailings from the

Witwatersrand can contain significant amounts of radionuclides such as uranium and

radium. As a result, this material is classified as low level radioactive waste.

Composition of wastewater from considered gold mine is presented in Table 5. [3] Amount

of wastewater stored in tailings is estimated to be 234.9 Mt. [4]

Table 5. Composition gold mine wastewater

Parameter Gold mine wastewater

PH 7.40

Conductivity (µS cm−1) 5600

TDS (mg L−1) 2900

TSS (mg L−1) 22

Temperature ( °C) 31.3

Cyanide (mg L−1) 9

As (mg L−1) 7.350

Fe (mg L−1) 0.114

Pb (mg L−1) 0.140

Cu (mg L−1) 5.063

Zn (mg L−1) 0.042

3.5. Task

Due to environmental legislation restrictions the problem of gold mine tailings should be

solved by the company as soon as possible. Currently wastewaters are stored in tailings

without any treatment system.

Recently two mining companies in South Africa suggested to test their waste materials

(“iron hydroxide” and “fly ash”) for the purification of wastewaters. These companies are

ready to provide their wastes to gold mine for free. Only transportation costs should be

`

covered by gold mine. “Steel sand” is a waste from considered gold mine and it was tested

together with “iron hydroxide” and “fly ash”. Results of conducted research are presented

below.

Laboratory has received three different materials from two different companies, such as “iron hydroxide, steel sand and fly ash”. These materials are waste products from some industries.

The composition of these materials has been studied by XRF (Table.6), surface area by BET (Table.7) and morphology by SEM (Fig.3), and adsorption properties of these materials by mining wastewaters purification from As (III), As (V), Ni (Table.8), Fe and Mn.

Different amount of adsorbents such as 0.5, 1, and 2 g/l were studied for 26 hours. Also adsorption of different elements was studied (0 - 300 ppm). It was found out that the optimum amount of adsorbent is 2 g/l. At a concentration of elements 200 ppm removal was almost 100%. In most of solutions with concentration 100 ppm adsorption occurs within three hours (Fig.4). After this, the system is became balanced. In the course of the experiment it was found that iron and manganese are precipitated on the sulfate forms. However, adsorption of arsenic and nickel on different materials was observed and these results are presented in Table 8.

Table 6. Composition of materials by XRF method

Elements “Iron hydroxide”, % “Steel sand”, % “Fly ash”, % Al 1,7 3,1 4,2 Si 0,2 23,0 12,4 S 17,6 0,03 0,19 K 0,3 0,3 0,33

Ca 14,4 0,84 1,45 Ti 2,3 0,13 0,28 Cr 0,031 0,145 0,014

Mn 0,27 0,022 0,006 Fe 7,2 1,1 3,7 Ni + + - Cu 0,002 0,001 0,007 Zn 0,018 0,006 0,002 Rb 0,001 0,003 0,004 Sr 0,021 0,007 0,033 Zr 0,010 0,012 0,02

`

Table 7. Characterization of materials by BET-method

a. b. c.

Fig.3. SEM pictures of waste materials, a. iron hydroxide, b. steel sand, c. fly ash

Table 8. Adsorption of As (III), As (V) and Ni from wastewater in batch system

Material Adsorption of As III from wastewater, %

Adsorption of As V from wastewater, %

Adsorption of Ni II from wastewater, %

“Iron hydroxide” 98 93 93 “Steel sand” 17 23 83

“Fly ash” 16 17 65 Activates carbon (for

comparison) 98 ~100 ~100

Material

Specific surface area of product

BET, m2/g

Adsorption capacity, cm3/g

”Iron hydroxide” 62,5 0,130 ”Steel sand” 0,7 0,002

”Fly ash” 1,7 0,009 Activated carbon (for comparison)

500 0,970

`

Fig.4.

Wastewater treatment by iron hydroxide (1b) and steel sand 2(b), concentration of adsorbents 2g/L, initial concentration of As (III), As (V) and Ni were 200 ppm

Based on presented data please do following:

1. Characterize each material as probable adsorbent for treatment of wastewaters a. material composition b. surface area c. morphology d. adsorption capacity

2. Choose the best treatments method for purification of gold mine wastewater from

the environmental, economic and technological point of view (estimation of

technological solutions and costs can be found in following document: “Technologies

and Costs for removal of Arsenic from drinking Water”).

0

20

40

60

80

100

0,5 10,5 20,5 30,5

1b_As III 2b_As III 1b_As V 2b_As V 1b_Ni 2b_Ni

`

References [1] Adriana Dominguez and Alicia Valero, Global Gold Mining: Is technological learning

overcoming the declining in ore grades? Proceedings of ECOS 2012 – The 25th International

Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of

Energy Systems, June 26-29, 2012, Perugia, Italy

[2] Gavin M. Mudd, Global trends in gold mining: Towards quantifying environmental and

resource sustainability? Resources Policy 32 (2007), p. 42-56

[3] Mike A. Acheampong, Roel J.W. Meulepas and Piet N.L. Lens, Removal of heavy metals

and cyanide from gold mine wastewater, J Chem Technol Biotechnol 2010; 85; 590-613

[4] Kloof gold mine, Technical Short Form Report

[5] Frank Winde, Peter Wade and Izak Jacobus van der Walt, Gold tailings as a source of

waterborne uranium contamination of streams – The Koekemoerspruit (Klerksdorp

goldfield, South Africa) as a case study. Part I of III: Uranium migration along the aqueous

pathway, Water S A Vol.30 No.2, 2004, p.219-225

[6] T. Rösner, A van Schalkwyk, The environmental impact of gold mine tailings footprints in

the Johannesburg region, South Africa, Bull Eng Geol Env (2000) 59: 137-148