THE IGOLI MERCURY - FREE GOLD EXTRACTION PROCESSERROR: REFERENCE

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Sidney Mahlatsi,Senior Engineering Technician: Mintek - Small Scale Mining Division

ABSTRACT;

Mintek has developed a process called iGoli Mercury-free Gold Extraction Process that can be used to extract gold from 0.1 % gold concentrate to produce 99.90 percent gold product. The process uses a mixture of pool acid (dilute hydrochloric acid), bleach (sodium hypochlorite) and sodium metabisulphate to leach and recover gold.

Artisanal and small-scale miners concentrate their gold using sluice boxes followed by panning as a final physical separating method. The gold produced in this manner is extracted from the concentrate by a chlorine solution produced from a mixture of pool acid and bleach. The leach liquor contains gold and other metals that are soluble in chlorine media such as base metals and ferrous iron. During leaching excess gas that is produced is redirected to a separate vessel where it is scrubbed using caustic soda to form water and sodium chloride (salt).The slurry from the leaching process is filtered to separate the gold bearing leach liquor and the solid residue. This gold pregnant solution is treated with sulphur dioxide, introduced in the form of sodium metabisulphate, to reduce gold ions in solution to a metallic gold powder. The solid waste produced from iGoli process is rich in silicate with effluent and is neutralized using lime/ limestone with an addition of apatite where necessary to destroy or precipitate all base metals and ferrometals available. The development of this process was mainly driven by the need to ensure reduction in mercury use and subsequent alleviation on the entire biophysical environment. There is an array of mitigating measures that could be used as treatment methods for the safe use of iGoli leaching process.

The process wastes which are significantly less toxic are characterized as follows: → Waste solids

Feed → → Waste solution (effluent) → Waste heat → Waste gasesAny potentially hazardous gases released from the gold leaching process are monitored and controlled by neutralization below the legislative requirements. The tailings produced have a high percentage of silica and iron with low levels of base metals. The process is very important to artisanal and small-scale miners as it lowers the risk of mercury poisoning on the miners themselves and mercury pollution into the environment. The process also offers economic benefits because of its high recoveries and product purity coupled with its simple way of waste management. Lastly the process recovers refractory gold which is very difficult to recover using other conventional ASM extraction methods.

1. INTRODUCTION

The environment is a very important part of our lives. It is the complex totality of circumstances

surrounding an organism or a group of organisms, especially the combination of external physical

conditions that affect and influence the growth, development, and survival of organisms.

The natural environment consists of all the conditions affecting the nature of an individual or

community, which we shall never completely understand until we see it as a living organism.

Most of our chemical and mining activities occurring in our every daily life have a significant

negative impact to our environment. In addition, new processes that are developed daily still

impact on the environment either positively or negatively, depending on the type of waste that is

produced by the process.

A process technology has been developed in Mintek for small-scale miners, which uses pool acid,

bleach and metabisulphate to dissolve and recover gold from a gold concentrate. During recovery

the process produces chlorine gas emissions, effluent and solid waste as a generated waste.

2. STUDY OBJECTIVE

A study was conducted in South Africa to derive measures, which could eliminate or minimise

the environmental impacts that might be caused by the implementation of the iGoli process. The

method developed focussed on making the iGoli process environmentally friendly so that the

process can be operated within existing South African legislative standards.

Table 1 and 2 depicts the South African impurity standards within the underground water and the

solid waste generated. The objective of the study was to develop a method, which can be used to

treat iGoli effluent for an effective waste disposal.

Table 1.Limits of the elements in underground water

pH Cl P F As Cd Cr Cu Fe Pb Mn Hg Se Zn B CN

ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm

5.5-9.5 0.25 10 1 0.02 0.05 0.05 0.01 0.3 0.01 0.1 0.005 0.02 0.1 1 0.02

Table 2.Limits of the elements in waste solids disposal

Cd Co As Cr Cu F Mo Ni Pb Hg Se Zn B

g/kg g/kg g/kg g/kg g/kg g/kg g/kg g/kg g/kg g/kg g/kg g/kg g/kg

2 20 2 80 6.6 200 2.3 50 6.6 0.5 2 46.5 10

3. PROCESS DESCRIPTION

Mintek’s Small Scale Mining Division (SSMD) developed a process which recovers gold from

gold concentrates without using mercury. The miners obtain this concentrate by panning or

simple gravity concentrating methods.

The gold in the solids concentrate is dissolved in a solution containing a mixture of pool acid (33

- 34 % HCℓ) and bleach (15 - 16 % NaOCℓ). The solution mixture produces chlorine gas in the

ratio 1:1 (HCℓ-NaOCℓ) of mixture to Cℓ2, which dissolves gold in the concentrate to form an

aqueous media of gold and other elements. The solid material fed into this process should be

concentrated to a grade of at least 1000 g/t (0.1% gold) for the process to be viable.

The combined mixture of pool acid (HCℓ) and bleach (NaOCℓ) produces chlorine gas, where

some of it is lost as excess chlorine to the surrounding environment. To avoid losses of gas to the

environment the excess chlorine gas (Cℓ2) from the reaction chamber is redirected into an

attached scrubber/vessel where neutralisation using NaOH solution is carried out as shown in

Figure 1.

Figure 1. Leaching vessel with the attached scrubber

The leaching of gold from the solid concentrate occurs between a gas fluid and a solid gold

surface in a chloride solution media, it is modeled by heterogeneous reaction equation.

During the reaction process gold that dissolves goes into solution to form an ionic gold and the

remaining solids are separated from the solution using a vacuum filtration unit to separate the

pregnant solution and the solids residue. The solution formed is subjected to a solution treatment

using sodium metabisulphate (as a source of SO2) to reduce gold ion in solution to a metallic

gold.

The solution, at a low pH and low electro potential difference, precipitates gold out in the form of

a gold powder. The effluent which results after recovering gold is treated to produce an

environmentally friendly waste that will have either no or less environmental impact.

3.1Mitigation measures

During treatment of the effluent stream three reagents were investigated under laboratory

conditions namely lime, calcium hydroxide and sodium hydroxide. These neutralising agents

were studied based on their reactions with the effluent solution and also on the economical

advantage.

The effluent solution was neutralised using the latter reagents at a pH>7. Calcium chloride is very

soluble in water i.e. the solubility of calcium chloride is about 39 g per 100 ml of water. The

existence of arsenic in some of the gold bearing material introduced arsenic in our effluent stream

when iGoli process was implemented. Phosphorus in an apatite mineral reacts with arsenic

forming a apatite complex. The complex compound precipitate out in pH higher than 7 as a stable

(Ca10(AsxPyO4)6(OH)2 ) arsenic apatite complex.

3.2. EFFLUENT SOLUTION

The effluent solution produced from the leaching process contains different kinds of impurities,

which are not environmentally friendly and need to be removed from the effluent solution before

being disposed into the environment. Table 3 shows the levels of impurities, which are formed

when recovering gold.

Table 3. Effluent solution produced by the leaching process

Soln P Cl Cr Mn Fe Co Ni Cu Zn As Mo Hg Pb

ppm g/l ppm ppm g/l Ppm ppm ppm ppm g/l ppm ppm g/l

1 2860 145 330 569 42.8 70 141 7.00 600 0.02 9.00 23.8 4.8

2 2584 112 99 601 26.4 59 971 0.56 29 30.3 8.60 <8 380

3 2481 117 474 758 22.9 68 113 0.29 15 9.18 25.8 <4 10.6

3.3. THE CHLORINE EMMISION

During the gold leaching process a mixture of pool acid and bleach produces chlorine that is used

to dissolve gold as follows:

NaOCℓ + 2HCℓ NaCℓ + H2O + Cℓ2 …………………1

where 1 mole of sodium hypochlorite (stored in bleach ) and 2 moles of acid produces 1 mol of

chlorine gas in the balanced reaction equation, which is a consumption of approximately 62.7 ml

of pure bleach and 30 ml of pure HCl. Normaly 15% of sodium hyphochlorite and 33%

hydrochloric acid are used as bleach and pool acid, therefore a dosage of approximately more

than 391ml of sodium hypochlorite and 90.9 ml of acid will produce at least 68g of chlorine

which in turn is consumed by the reaction with gold.

This production of chlorine is understood to be an elementary homogenous reaction, where the

rate law equation is:

-rNaOCl = kNaOCl CNaOClC2HCl ………………………………2

where from the conversion equation the concentration of HCl is replaced by concentration of

sodium hypochlorite i.e.

CHCl = CHCl - 2CNaOClX = CNaOCl(Q – 2X) where Q=CHCl/CNaOCl

-rNaOCl = kNaOClCNaOCl[CNaOCl(Q-2X)]2 = KNaOClC3NaOCl(Q-2X)2

Initially we start by pouring the pool acid in the vat reactor, and then it is assumed that the acid is

in excess when hypochlorite is added in the reaction vessel therefore reducing our rate law

equation to:

-rNaOCl = kNaOClCNaOCl…………………………………………3

Let NaOCl = A, B = HCl and Cl2 = C, p = p0 = density. Therefore the mole balance on the

hypochlorite is:

Inflow – Outflow + generation of hypochlorine = accumulation of a NaOCl mole

Fa0 – Fa + rdV = dNa/dt

pv0 –0 + rAdV = d(pV)/dt = (pdV+Vdp)/dt……………………….4

The reactor V varies with time. The volume at any time t can be found from an overall mass

balance of all species:

p0v0 – 0 + 0 = pdV/dt

Therefore substituting this in 4 with p0= p i.e.

v0Cao + rAv0 = VdCa/dt + Cav0

v0 (Cao – Ca) + rAv0 =VdCa/dt

Cao – Ca + rA. = dCa/dt

Considering that dCa/dt = dCa/d and that the rate law reduces to a first order:

dCa/d = Cao/ - CA/ + kCA

Therefore:

dCa/d + Ca(1+ k)/ = Cao/

using the integral factor and the initial condition =0 when Ca=C1a i.e.

Ca = Cao/k – ( Ca0/0k – C1a) 0exp(k0 - k)/…………………5

In terms of real time i.e.

Ca = Cao/(t + 0)k – (Ca0/0k – C1a) 0/(0 + t) .exp(-tk)…………6

with the stoichiometry in a semibatch reactor written as follows:

CB= Cao(1-2X)/(V0 + v0t)…………………………………………7

CC = Nc/V = Ca0X/(1+X)(P/P0.T0/T)…………………………….8

According to the reactions above the chlorine (concentration) generation is or can be increased by

a presurised container where gas produced increases with pressure at constant temperature.

According to South African legislature on gas regulations, state that the maximum amount of

chlorine gas to be released into the atmosphere / air is 25-mg/ m3. If the chlorine gas liberated is

higher than the limits, chlorine being known as an air pollutant will deplete the ozone layer and

affect vegetation.

This will happen by changing the color of the trees and leaves and also by making some trees

leafless. A high concentration of chlorine in the air also affect humans, during inhalation, it will

cause sore throat, coughing, eyes and skin irritation.

3.4. SOLID WASTES

Most of the minerals available in the gold bearing material which normally get processed using

the iGoli process are shown in the table below:

Table 4. Crystalline minerals determined by X-ray diffraction and their chemical

formulae

Mineral nameMost concentrate

materialChemical formulae

Arsenopyrite 69.4 FeAsS

Pyrite 4.5 FeS2

Quartz and mica 1.6 SiO2

Fe-metal

22.4

Fe

Hematite Fe2O3

Goethite FeO.OH

Tungsten and carbide and

gold

2.1 WC

During the extraction of gold most of the minerals that are in different particle sizes and

quantities are also oxidized by chlorine to form chlorites of gold, silver, base metals, ferrometals

etc.

The recovery of gold also includes the recovery of ferro metals, base metals, etc., which when the

gold is selectively precipitated they all remain in the leach liquor as wastes or as a source of

valuable base metals etc. The unreacted mineral or inert solids are left and disposed as waste

solids, which are environmental friendly i.e. mostly silicate mineral.

The reactions governing the formation of wanted and unwanted products occur in a

heterogeneous mixture with the reaction equations written to show the reaction between the

minerals and chlorine gas being produced by a mixture of pool acid and bleach.:

Oxidation reactions of the different minerals are as follows:

FeS2 + Cℓ2 + H2O → FeCℓ2 + H2SO3 …………..………………..9

FeAsS + Cℓ2 + H2O → AsCℓ2+ FeCℓ2 + H2SO .………………..10

Fe + Cℓ2 +H2O → FeCℓ2+ H2SO ………………....…………….11

With the objective of the process being to oxidize gold and silver as follows:

Au + Cℓ2 + H2O → AuCℓ2 + H2O ……………………………….12

Ag + Cℓ2 + H2O → AgCℓ2 + H2O ..………………………………13

The gas chlorine is introduced into the vat reactor using sodium hypochlorite. The heterogeneous

reaction is assumed to be reaction rate controlled at the surface mixture of the available mineral

particles. Since different minerals have different particle sizes and different mole rate transfer, the

transfer of chlorine to the surface of different mineral particles is equated as follows:

Qtransfer = -bD(CCℓ2 – CoCℓ2) /(R-Ro)……………………….……….14

QCℓ2 = -kCℓ2bCCℓ2

where CoCℓ2= 0,

p = density of the mineral particle,

SAex= external surface area and

k = D/(R-Ro)

Therefore the reaction is the same for all particles of the different minerals with the difference in

the equation being the reaction rate or the rate at which different particles dissolve in the solution

or gas.

Qreaction = 1/SAex .dNFeS2 /dt = b/V.dNCℓ2/dt ………………………15

dNFeS2 = dNCℓ2 and SAex = 4r2 and N FeS2 =pV = p.4/3r3

dN FeS2 = p.4r2drFeS2

Therefore

1/SAex dNFeS2 /dt = 1/4r2 . p.4r2drFeS2 /dt = p (r2/r2)dr/dt

p dr/dt = kbC Cℓ2

intergrating the following with the limits as R=initial radius and r=final radius

p/kbC Cℓ2 (R-r)= t

The time taken to dissolve this entire particle will be

= p/(kbCCℓ2)

The conversion of a particle of size R to a size r will be

t/ = 1- r/R, ………………………………………………..……..16

If (1-Xsolid particle) = volume of unreacted core of size r/(Total volume of particle) = XCl2,

Therefore

t/ = 1-r/R =1-(1-XB)1/3 ………………………………………….17

where B = undissolved solid particle

Therefore the reaction equation if the system is reaction rate controlled is as follows:

tFeAsS/ = 1-rFeAsS /RFeAsS = 1-(1-XFeAsS)1/3………………………18

tFe / = 1- rFe /RFe = 1-(1 –XFe)1/3 ………………………………19

tAu / = 1- rAu /RAu = 1-(1 –XAu)1/3………………………………20

tAg/ = 1- rAg /RAg = 1-(1 –XAg)1/3 ………………………………21

The reaction equations 18, 19, 20 and 21 show that different minerals have different residence

times to completely dissolve in a certain chemical media. The model equation 18, 19, 20 and 21

above gives almost a straight-line graph when tested against experimental data. This means that

the reactions given by the equations 9, 10 and11 are actually reaction rate controlled with the time

of complete dissolution determined using equation 18, 19, 20, and 21. In essence chlorine gas is

actually manipulated using equation 5 and 6. When equations 5 and 6 are combined with 17, 18,

19, and 20 they can be manipulated to give data that could be used to effectively leach gold and

silver from the solid mineral. The reaction of both gold and silver can also be controlled using the

model equations to produce different mineral waste while optimizing the conversion of gold into

solution.

Figure 2. The behaviour of Arsenic, gold and iron predicted by the modeling equations.

The graph in Figure 2. was drawn using experimental conversion of As, Au and Fe kinetic data in

the reaction modeling equation, which shows that As is reaction rate controlled. The graph of

gold and Fe seems to be controlled by a combination of rate equations, where the mineral iron

seems to be controlled by diffusion and reaction rate, where gold is controlled by three model

equations which are ash, diffusion and reaction rate at the surface of the gold mineral particle.

4. EXPERIMENTAL PROCEDURES

4.1. NEUTRALISATION

Three representative solution produced from leaching gold in three different gold bearing material

were used to investigate the behavior against treatment.. The leach liquor was produced by

leaching different concentrates from Klipval (Mpumalanga Province), Ventersdorp (North West

Province) and Springs (Gauteng Province). The samples were known to have ore bodies, which

contained major impurities such as arsenic (Klipval and Mpumalanga), uranium (Witwatersrand -

Springs) and lead in (North West – Krugersdorp area). The effluent produced was divided into 3

sets of one liter effluent samples, each to be treated using the following procedure:

All solutions produced were neutralized to at least a pH greater than 7 to determine elements

not precipitating that are harmful to the environment.

The slurry formed was filtered to separate the solution from the solid cake, which was later to

be tested for washability of base metal etc.

The total cake was dried and 200 g sampled and pulverized for traditional fire assay to

determine which elements are easily washed during rainy season..

Solution containing arsenic as chloride:

The solution containing arsenic was co-precipitated using apatite of different stoichiometric

ration before being neutralised with hydrated lime (CaOH) or unhydrated lime (CaO).

The slurry formed followed the same procedure as shown above.

4.2. WASHABILITY TEST

A solubility test was carried out to test leachability of impurities into underground water table.

The cake formed using limestone, lime, caustic and apatite was divided into four parts where a

temperature distribution test was carried out on the cake using normal tap water (23-50) 0C.

5. RESULTS AND DISCUSSIONS

5.1. NEUTRALISATION SYSTEM CHEMISTRY

Reaction equations for three different neutralizing agents are simulated

with FeCl2 compound as follows:

2NaOH + FeCl2 + H2O 2NaCl + Fe(OH)2 + H2O……………..22

CaCO3 + FeCl2 + H2O CaCl2 + FeCO3 + H2O…….…………..23

Ca(OH)2 + FeCl2 + H2O CaCl2 + Fe(OH)2 + H2O………….…24

The reaction is similar with other elements such as base metals etc., in the effluent or waste

solution.. Most of the elements form hydroxides and carbonates with different neutralizing agents

such as lime,caustic and limestone. In the reaction with sodium hydroxide, the consumption is

twice the amount in comparison to other neutralizing agent as shown in Table 7.

Table 5 shows a summary of the results for a processed effluent solution obtained when different

neutralizing agents was used.

Table 5. Final effluent impurity levels with ordinary neutralising agent

Soln P Cl Cr Mn Fe Co Ni Cu Zn As Mo Hg Pb

ppm g/l ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm

NaOH 18.7 224 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2 <2

CaOH <10 256 <2 3.3 <2 <2 4.1 3.5 <2 <2 <2 <2 <2

CaO <10 256 <2 <2 <2 <2 <2 3.2 <2 <2 <2 <2 <2

A neutralization testwork was carried out at pH higher than 7 to encourage elements in solution to

drop to levels below the required legislative standards. The level of chlorine in the filtrate

produced was high due to the formation of calcium and sodium chloride which are highly soluble

in water. It is known that calcium chloride and sodium chloride have a solubility constant of 390

g/l and 350 g/l in water respectively.

Table 6. Final average effluent impurity levels with apatite-neutralising agent

Soln P Cl Cr Mn Fe Co Ni Cu Zn As Mo Hg Pb

Ppm g/l ppm Ppm ppm Ppm ppm ppm ppm ppm ppm Ppm Ppm

NaOH 9.75 161 <2 <2 <2 <2 <2 10 25 <2 <2 <2 <2

CaOH 7.01 184 <2 <2 <2 <2 <2 12 29 3.7 <2 <2 <2

CaO 5.00 184 <2 <2 <2 <2 <2 15 34 <2 <2 <2 <2

The solubility constant of chlorine in solution is decreased by the addition of apatite because of

the formation of a complex arsenic and chlor-apatite. A relative addition of apatite seems to

reduce the solubility of chlorine by almost 50%.

Table 7. Summary of the results for different neutralization agent

Activity Neutralising agent Consumption (g/l) Cost (R/l) pH

Normal

Netralisation

NaOH 179 11.46 8.8

Ca(OH)2 200 10.00 7.66

CaO 200 19.20 8

Normal

Netralisation

NaOH 91 5.82 7.21

Ca(OH)2 114 5.70 9.32

CaO 65 6.24 7.48

Normal

Netralisation

NaOH 89 5.70 7.43

Ca(OH)2 103 5.15 7.66

CaO 101 9.69 7.58

Apatite addition NaOH 150 9.6 9.2

Ca(OH)2 105 5.25 7.8

CaO 55 5.28 9

Table 6 and 7 indicate that to reduce the level of impurities in an effluent stream a pH greater

than 7 must be achieved using any of the above neutralizing agents. Table 7 shows that when

apatite was used to complex with arsenic, consumption of a neutralising agents is reduced by

almost 10 percent. The benefit of using apatite is relative to its advantage of complexing with

arsenic, decreasing solubility levels of other impurities such as chlorides etc. In all the testwork

carried out, calcium hydroxide had the lowest cost when used as a neutralising agent.

5.2. SOLUBILITY

Most of the elements precipitated during neutralisation process were tested to determine their

solubility during rainy seasons to indicate their washability into underground water. Water at

different temperatures was used to test distribution of elements with temperature. Hydrolysis of

elements controls their solubility into solution.

FeCl2 + H2O Fe 2+ + 2Cl- + H+ + OH- FeCl2.XH2O, …….……….25

In essence this equation could only be formed in an acidic medium where the cake formed was

also acidic. In our case most of the elements that could be washed will be elements that are

soluble at high pH such as group I and II elements which are not harmful to the environment.

Table 7. Summary of washability test using water at different temperatures

Test no Temp Solubility

0 C Reagent As

(ppm)

Cl

(g/l)

P

(ppm)

Fe

(ppm)

30 NaOH 75 20.6 15.6 113

30 CaO 3.7 22.4 <3 2

30 Ca(OH)2 32 20.9 9.72 <2

40 NaOH

40 CaO 73 25.6 28.2 60

40 Ca(OH)2 40 23.4 23.8 <2

1 50 NaOH 59 26 7.78 6.68

50 CaO <2 27.7 <5 3.2

50 Ca(OH)2 8.1 23.7 <5 2.4

Most of the chlorides in solution are precipitated during neutralisation testwork and are

precipitated with group I or II elements. The chlorides formed in the solid cake have a very high

solubility constant, therefore leaches out in water to form a chloride solution as shown in Table 7.

The chlorides of group I or II elements increases on the solubility with an increase in temperature.

The other elements that are washed out are totally independent on temperature.

Table 7. Summary of washability test using water on an apatite - base precipitated cake

S Temp Solubility

0 C Reagent As

(ppm)

Cl

(g/l)

P

(ppm)

Fe

(ppm)

1 30 CaO-apatite <2 13.7 5.52 <2

30 Ca(OH)2-apatite 4.2 9.7 6.0 4.6

2 40 CaO-apatite <2 12.1 3.22 <2

40 Ca(OH)2-apatite 4.5 13.1 5 5

3 50 CaO-apatite <2 12.9 5.22 <2

50 Ca(OH)2-apatite <2 11.2 5 <2

6. CONCLUSIONS

The waste produced can be made environmentally friendly by neutralising with

approximately 100g of either dehydrated lime (CaO) or hydrated lime (Ca(OH)2 ) per litre of

impurity concentrated iGoli effluent at a pH greater than 7.

It was observed and understood from washability testwork that all the chemicals used to

precipitate impurities in the waste effluent were not harmful to the environment; instead they

produce a stable solid cake with most formed salts remaining in solution being from group I

or II elements of the periodic table.

The consumption of both hydrated and dehydrated “lime” is reduced by 10 to 50 percent

when apatite mineral is used during neutralisation process, with an average neutralisation cost

of R10/l of effluent produced..

Most of the elements formed or precipitated using apatite as an additive are more resistant to

water wash because of an arsenic apatite complex formation.

7. RECOMMENDATIONS

The following recommendation was established:

A method must be derived to minimise or purify the formation of group I or II chlorides

during neutralisation of the iGoli effluent.

Due to resistance posed by miners who refuses to use other processes for the extraction of

their gold mineral etc., it is advisable that the mercury retorts which they are using currently

be discouraged at the same time encouraging them to adapt to new processes that are safer

and more environmentally friendly.