The text for Mining Expert Training

New Technology

for Gold Recovery

Contents

1. Forword・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・1

2. Introduction ・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・1

3. Conventional gold production methods・・・・・・・・・・・・・・・・・・・・・・・・・・2

3-1 Primitive gold production method・・・・・・・・・・・・・・・・・・・・・・・・・・2

3-2 Amalgamation・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・2

4. Cyanidation・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・3

5. New technology of gold production・・・・・・・・・・・・・・・・・・・・・・・・・・・4

5-1 The principle of gold recovery by carbon ・・・・・・・・・・・・・・・・・・・・・・・4

5-2 Leaching with thiourea・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・5

6. Recovery of gold and silver by CIP ・・・・・・・・・・・・・・・・・・・・・・・・・・・5

6-1 The practice of CIP・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・5

6-2 CIL・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・11

6-3 Combination with heap leaching ・・・・・・・・・・・・・・・・・・・・・・・・・・12

6-4 Characteristics of carbon granules ・・・・・・・・・・・・・・・・・・・・・・・・・13

7. Advantages and disadvantages of CIP・・・・・・・・・・・・・・・・・・・・・・・・・・13

8. Epilogue・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・ 13

Appendix

Slides of CIP・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・ 15

1

　　　　　　　　　　　　　　Text of Video “CIP”

New Technologyfor Gold Recovery

1. Foreword

The Ministry of International Trade and Industry (MITI) of the Government of Japan, decided to make these Mining Expert Training materials for technical cooperation program in the fi scal year 1994-95. And MITI entrusted this to the Metal Mining Agency of Japan (MMAJ). This text, in combination with pictures (video tape and slides), was planned by MMAJ. Without the permission of JOGMEC, any copying or other usage than that of Mining Expert Training are strictly prohibited.

Fig. 1 Gold consumption in the world(in 1986, excluding former communist bloc)

2. Introduction

Gold has been utilized and treasured for centuries. Of all metals known to man, it is gold than inspires dreams of immense wealth and visions of power. Gold has traditionally been used for the manufacture of jewelry and coins and for dental treatments. In more recent times, it has also found widespread use in the so-called high-tech industries as a material for electronic contacts. (Fig. 1)

3. Conventional gold production methods

3-1 Primitive gold production method

The production of gold began historically from placer gold. Heavy gold particles were recovered by washing the river sand which held the placer gold. This method, which utilizes the difference in the specifi c gravities of these two materials, is called panning or sluicing and is a kind of gravity concentration method.(Photo 1) In the 19th century, many people obtained placer gold by this method during the great Gold Rush in North America. These days, this method of gold production is only found in crude small scale developments in some developing countries.

3-2 Amalgamation

Mercury has also been used in gold recovery for a long time. Gold and silver alloy easily with mercury to form amalgam in a process known as amalgamation. Ground gold ore is mixed with mercury to form a gold and silver amalgam and then the waste sand is washed away with water. After fi ltering the mixture through cloth, amalgam is obtained. By heating the amalgam to vaporize the mercury, gold and silver are obtained. Many small scale gold mines produce gold and silver by this method. Unfortunately, the irresponsible use of mercury can lead to serious environmental pollution problems.

2

4Au+ 8NaCN + 2H2O + O2 = 4Na[Au(CN)2]＋４NaOH

The cyanidation process is carried out as follows. After adding a sodium cyanide solution to a ground ore, air is blown into the mixture for leaching the gold and silver. After leaching, the undissolved residue is fi ltered and washed from the gold and silver solution, and is deposited in a tailing dam. To minimize the loss of gold and silver, fi lteration and washing are repeated several times.

Photo1 Tools for recovery of placer gold

4. Cyanidation

A process in which gold and silver are recovered from ores by dissolution in a cyanide solution is now widely used. This process is known as cyanidation and was fi rst patented in 1888 in Great Britain. (Fig. 2)Cyanidation Process Gold and silver dissolve in a diluted cyanide solution in the presence of oxygen to form cyanide complexes in a process called “Leaching”.

Poto2 Gold recovery by amalgamation

3

Fig. 2 Cyanidation processFig. 2 Cyanidation process

The resulting fi ltrate which is rich in gold and silver is called “pregnant solution”. Zinc powder is commonly used to recover the gold and silver from the pregnant solution. This zinc powder method is known as the “Merrill Crowe Method” After very fi ne particles in the pregnant solution have been removed by fi ltration, zinc powder is added to the fi ltrate to precipitate the gold and silver. In this process, the excess oxygen dissolved in the pregnant solution must be removed under vacuum conditions before precipitation, because eny excess oxygen in the pregnant solution will increase the zinc consumption.

2Na [Au (CN)2]＋ Zn = Na2 Zn (CN)4 + 2Au

After dewatering this precipitate with a fi lter press, the gold and silver are melted into bullion.

5. New technology of gold production

5-1 The principle of gold recovery by carbon

In the 1970's, a new technology was developed for the recovery of gold and silver dissolved in cyanide

solutions by adsorption on carbon. The process is the same as the Merrill Crowe Method untill the gold and silver leaching step using a cyanide solution. The dissolved gold and silver are adsorbed onto carbon granules charged into the pulp. After taking the loaded carbon out of the pulp, the gold and silver are desorbed with a hot, alkaline cyanide solution to produce a highly concentrated solution of gold and silver. This process is called “Stripping”. Gold and silver are then obtained by electrowinning. This method is called “Carbon In Pulp”, or “CIP” for short. (Fig. 3)

5-2 Leaching with thiourea

Leaching processes that do not use toxic cyanide solutions are currently being researched. One such process utilizes dissolution with thiourea to produce stable compounds of gold and silver. Unfortunately this process is inferior to cyanidation both economically and technologically. The economic recovery of gold and silver by this method still requires research and further refi nement. The reaction is as follows;

Au + 2CS(NH2)2 = Au[CS(NH2)2]2 +＋ ｅ -

4

6. Recovery of gold and silver by CIP

6-1 The practice of CIP

Mined ore is transported by trucks to a plant where it is crushed, and ground in semi-autogenous mills or ball mills to produce a wit pulp. The pulp thickened between 45 to 50% solids to provide a longer leaching time which increases the effi ciencies of the leaching and carbon adsorption. Another reason for thickening the pulp is to allow a uniform dispersion of the carbon granules. If the pulp density is too low, the carbon granules will sink, and if the pulp density is too high, the carbon granules will fl oat.

Impurities such as wood chips are removed by a vibrating screen. The thickened pulp is sent to the fi rst leaching tank. Lime and sodium cyanide are added, and gold and silver are leached in a cyanide solution at a pH of 10 and a cyanide concentration of 0.05%, while air is blown into the pulp. The reaction time for leaching is from several hours to over 70 hours. The leached gold and silver are adsorbed onto carbon granules in the pulp while fl owing continuously through several adsorption tanks interconnected with screens. The pulp fl ows through screens located at the outlet of each tank and the carbon granules that cannot pass through the screen remain in the tank.

Fig. 3 CIP process

Photo 3 Leaching Tanks

5

Carbon is fed to the last adsorption tank after removing the fi ne-sized carbon by a screen and is transported from one tank to the next tank once a day by the air lift pipe. The carbon fl ow and the pulp fl ow are in opposite direction, so that the amount of carbon granules is kept constant in each tank. The gold concentration of the pulp in the adsorption tanks decreases, while the amount of gold adsorbed on the carbon granules increases compared to the last tank. Each ton of carbon in the fi rst tank will contain a few kilograms of gold. The loaded carbon, together with the pulp, is taken out of the fi rst adsorption tank, and is separated from the pulp by a screen before being sent to the stripping

process.(Fig. 4) Gold is stripped from the carbon with a hot solution of alkaline cyanide in the stripping process. Before stripping, the carbon is washed with acid to remove any impurities such as copper or calcium. The stripping solution usually consists of a 0.1% cyanide and 1% caustic soda, heated to about 100 ℃ . Sometimes stripping uses only caustic soda, or with a solution containing alcohol. In order to shorten the reaction time, the stripping operation is sometimes carried out at high pressure. Generally it will take from one to three days.

Photo 4 Adsorption tanks

\Photo 5 Cylindrical screen in a adsorption tank

6

Photo 7 Steel wool cathodes

Photo 6 Stripping facilityPhoto 6 Stripping facility

Fig. 4 Stripping process

7

Photo 8 Electrolytic cell

Photo 9 Melting furnace

The stripped solution containing dissolved gold and silver is electrolyzed in electrolytic cells. The gold and silver are precipitated onto the cathodes of steel wool, which improves electrowinning effi ciencies. The steel wool cathodes are placed in the perforated steel plate baskets which are placed in the electrolytic cell. In some plant, the electrolytic cell is covered in order to prevent

the dispersion of gases and to keep a high temperature and pressure. The cell voltage should range from 2.5 to 10 volts, and the current effi ciency is 5 to 10 %. The concentration of gold in the barren solution exiting from the electrolytic cell is controlled to stay at less than 20 ppm, and the operation is stopped when it reaches to less than 5 ppm.

The waste solution from electrowinning is returned to the stripping tank for reprocessing. This loaded steel wool is washed with acid and calcined. Then it is melted in a furnace together with fl uxes to remove any impurities resulting as a slag. The slag overfl ows and the heavy gold and silver remain in the mold. Gold and silver are then cast into bullion.

After stripping, the carbon is reactivated in a kiln at a temperature of 170 ℃ in an anaerobic condition and is recycled for later usage.(Fig. 5) Waste pulp from the adsorption tanks is disposed pf in a tailing dam. The overfl ow of the tailing dam is recycled and used as plant water.

8

Photo 11 Tailing dam

Photo 10 Gold bullion

Fig 5 Carbon reactivation kilnFig 5 Carbon reactivation kiln

9

Fig. 6 CIL process

Fig. 7 Heap leaching- CIC process

6-2 CIL

In the CIP process, carbon is charged in the adsorption stage. There is another method, where carbon is charged in the fi rst leaching tank. When the ore contains organic matter or clay, the gold and silver are adsorbed on these materials before they are adsorbed on the carbon granules. In such a case, carbon granules should be charged at the beginning of the leaching process in order to prevent this from happening. This method is called “Carbon In Leach”, or CIL for short. (Fig. 6)

6-3 Combination with heap leaching

In the United States of America and Australia, gold and silver from low grade ores are often recovered in heaps by leaching with a mixture of dilute cyanide solution and lime. In Canada, it is not so popular because of the problem of freezing in winter.Electrowinnig Melting Gold Bullion The cyanide solution is carried to the top of the heaps by pipes and sprinklers are used to spray it onto the ore. This method is called “Heap leaching”. The pregnant solution from the heap is collected and the gold and silver in the pregnant solution are recovered by adsorption in columns packed with carbon. This method is called “Carbon In Column”, or CIC for short.

10

6-4 Characteristics of carbon granules

In these processes, carbon plays a major role. Therefore, the characteristics and treatment of carbon infl uence the effi ciency of the CIP. Reduction in size of the carbon granules by abrasion or breakage leads to the loss of gold and silver by passing through the screens. Activated carbon from charred coconut shells is commonly used, as it has excellent adsorption characteristics and is resistant to abrasion.

7. Advantages and disadvantages of CIP

The CIP process has some advantages. In the conventional Merrill Crowe process, several large scale thickeners and fi lters are required to separate a large volume of pregnant solutions from tailings. Zinc powder used for the precipitation of gold and silver is expensive. When there are signifi cant quantities of impurities such as sulfur, arsenic, antimony or copper in the ore, the amount of zinc powder needed increases to the point where the Merrill Crowe Method becomes too costly. The removal of oxygen from the pregnant solution before adding zinc powder for precipitation is another complicating factor. But in the CIP process, the equipment is not as But in the CIP process, the equipment is not as expensive, because carbon is easily separated by screens. expensive, because carbon is easily separated by screens. Also, such highly concentrated solutions of gold and

silver can be obtained that it is possible to electrolyze the stripped solution directly. And after stripping, those carbon granules can be used again by reactivation. It follows that the operating cost of CIP is less than the “Merrill Crowe” process. When CIP is used in combination with heap leaching , the cost reductions are so dramatic that it is possible to recover gold and silver from ore with grades as low as one or two grams of gold per ton. One drawback of CIP is that the silver adsorption rate onto carbon granules is not good. The Merrill Crowe Method is consequently superior to CIP, when the ore value depends on silver rather than gold.

8. Epilogue

Now the CIP process plays an important role in the gold production in the world, and the gold production of the United States of America, Australia and Canada has increased greatly since the introduction of CIP in the last ten years.(Fig. 8) Figure 9 shows gold production classifi ed according to the processes used. In the United States of America, Canada and Australia, it is clear that a large amount of gold is now produced by the CIP process. It is now possible to extract gold from ores that was It is now possible to extract gold from ores that was previously considered economically unrecoverable previously considered economically unrecoverable by conventional methods. The CIP processes are by conventional methods. The CIP processes are particularly effective, when it is used in combination

Photo 12 Heap leaching

11

with large scale open pit mining and heap leaching. By this epoch-making CIP process, economically recoverable gold ore reserves have increased rapidly, and the future looks bright in the gold production.

Fig. 9 Gold production by process

Fig. 8 Gold production in the world

12

APPENDIX

List of slides

1. Forword2. Title “New Technology for Gold Recovery CIP”3. Gold consumption in the world (in 1986, excluding former communist bloc)4. Placer gold5. Amalgamation6. Reaction frmula of cyanidation7. Cyanidation leaching tanks8. CIP process9. Grinding in SUG mill10. A trash screen11. CIP leaching tanks12. CIP adsorption tanks13. A cylindrical screen in CIP adsorption tank14. Charging of carbon15. Transportation of carbon together with pulp16. A loaded carbon screen17. Stripping facility18. Stripping process19. Steel wool cathodes20. An electrolytic cell21. Cathodes with electrolyzed gold22. A melting furnace23. Carbon reactivation kiln24. Tailing dam25. Recycled plant water26. CIL process27. Heap leaching-CIC process28. Heap leaching29. Pregnant solution by heap leaching30. Comparison of CIP and Cyanidation31. Gold production of the world (in 1984-1993)32. Gold production by process (in 1986)

EXPLORATION TECHNOLOGY FOR DEEP SEABED MINERAL RESOURCES SEARCH FOR THE RESOURCES LYING DORMANT AT THE BOTTOM OF THE SEA

©Japan Oil, Gas and Metals National Corporation 2005

Produced by Lukio Co., Ltd.

Previous edition ©Metal Mining Agency of Japan 1998

Produced by DOWA Mining Co.,Ltd.