AU614420B2 - Process for leaching noble metals using a leaching solution containing cyanide, and hydrogen peroxide - Google Patents

Description

FORM 10 SPRUSON FERGUSON COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE US 6 e 1 FOR *I A Li.

Class Int. Class Complete Specification Lodged: Accepted: Published: Priority: Related Art: .i Name of Applicant: Address of Applicant: Degussa Aktiengesellschaft Weissfrauenstrasse 9 Frankfurt (Main) FEDERAL REPUBLIC OF GERMANY 0*441~ 0444 Address for Service: 0 4 4 o o I a04 Spruson Ferguson, Patent Attorneys, Level 33 St Martins Tower, 31 Marke.t Street, Sydney, New South Wales, 2000, Australia l 4 a04 4 Complete Specification for the invention entitled: Process for Leaching Noble Metals using a Leaching Solution Containing Cyanide, and Hydrogen Peroxide The following statementis a full description of this invention, including the best method of performing it known to me/us

ABSTRACT

The invention relates to a process for leaching gold and silver from ores and ore concentrates using a cyanide leaching solution and hydrogen peroxide, and maintaining an oxygen concentration of 2 to 20 mg 02/1. In accordance with the invention, leaching is carried out in the presence of decomposition catalysts, and preferably the cyano-complexes formed are separated from the leaching solution during leaching. Decomposition catalysts are manganese compounds 0.01 to 1 mg calculated as Mn/1 of barren solution or 1 to 50 mg calculated as Mn/kg of ore pulp or inorganic or organic polymers or carbon. Active carbon is preferably used in a amount of 1-20 g/l of leaching solution, as this H 2 0 2 decomposes and adsorbs noble metal cyano-complexes simultaneously. The process leads to reduced consumption of H202 and optionally cyanide, and to maximum gold yields at shortened leaching time.

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'd L1 Irr rC -2- Process for Leaching Noble Metals Using a Leaching Solution Containing Cyanide, and Hydrogen Peroxide

DESCRIPTION

The invention relates to a process for leaching gold and silver from ores, ore concentrates or noble metal wastes, in particular from oxide and sulphide ores or ore concentrates and dump material from previous incomplete leaching processes, using an aqueous leaching solution containing cyanide, at a pH value of 8-13, with addition of an aqueous hydrogen peroxide solution to the ore pulp of a stirred leaching process or to the barren solution of a column leaching process, and maintaining an oxygen concentration of 2 to 20 mg/l in the leaching solution. By adding a decomposition catalysts for hydrogen peroxide and optionally separating the Ooo o gold and silver cyano-complexes formed from the leaching solution during leaching, it is possible to reduce the chemical requirement and to achieve high noble metal yield in a shorter leaching time. The process of the "amo" invention enabled the economics of leaching to be considerably increased by 0o.0 using hydrogen peroxide as the oxidising agent.

It is known to convert gold and silver into soluble cyano-complexes using an aqueous solution containing cyanide in the presence of air oxygen. Leaching of these noble metals from ores or ore concentrates or o from noble metal wastes, for example electronic scrap, is based on this S principle. In the so-called stirred leaching process, the finely ground ore is made into a suspension in water and, after adjusting the pH value o and adding an aqueous cyanide solution, agitated in cascaded, cylindrical agitation vessels or Pachuca tanks with air gassing up to 48 hours and thereby leached. Hereafter the aqueous phase of the ore slurry shall be called the leach solution. In industry, besides the leach solution, use is °oa also made of so-called heap leaching, whereby a leach solution as a rule adjusted between a pH of 9 and 12 and containing cyanide is made to percolate heaps of ore, atmospheric oxygen being the oxidant.

Both in heap leaching and in agitation leaching, an oxygen deficiency may arise in the ore slurry, resulting in limited gold yield and lowered rate of leaching. Attempts already have been undertaken to leach previous metals ores in the presence of oxidants other than atmospheric air. Among these approaches is the use of hydrogen peroxide.

Illustratively US Patent No. 732,605 disclosed a procedure for /zA N leaching ores, wherein the ore is mixed with a metal oxide and this mixture r;l- F; i:; i: i i '3 :r iZ~i

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2a is treated in a cyanide solution containing the hydrogen peroxide that reduces the metal oxide. The quantity of metal oxide that can be reduced, such as Fe 2

O

3 MnO 2 Ag 2 0 and HgO, should be at least equivalent to the quantity of hydrogen peroxide. The more of the metal oxide present, the more advantageous the procedure in relation to forming nascent oxygen.

Prior to leaching, the ore must be so carefully mixed with such a quantity of metal oxide that there is a content of at least 0.05%, i.e. 500 ppm.

Nobody in the mining industry t I i| I KXN 123 has made use of this process known since 1903, which obviously relates only to the column leaching process, because it does not comply with practical requirements: The addition of a metal oxide in the amounts mentioned is not only uneconomical because of the costs involved and because of the required mixing process, but an increased use of cyanide must also be expected because the metals in question partly form cyano-complexes themselves. Furthermore, difficulties in the leaching itself, for example from passivating, and in the subsequent treatment of waste water to remove cyanide, cannot be avoided as a result of the presence of added metal oxides in the required amount.

The leaching process known from United States patent 3,826,723 using a cyanide leaching solution and hydrogen peroxide, in addition to the presence of an alkali cyanide and a stabilised hydrogen peroxide solution, also requires additionally a lignin sulphonate. The large amounts of chemicals added, in particular of NaCN and H 2 02, given in this document do not permit an economical process.

The process of Canadian patent 1 221 842 relates to leaching of, inter alia, gold and silver from iron pyrite concentrates, wherein the leaching solution contains an alkali cyanide, an alkali carbonate and hydrogen peroxide. According to the preferred embodiment, the leaching solution contains at least 2 wt.% sodium cyanide and at least 2 wt.% sodium carbonate and 0.6 sw.% hydrogen peroxide. Sodium cyanide is oxidised to a I considerable extent by hydrogen peroxide at these high 4 4- BJG/397P -3concentrations known cyanide detoxification using hydrogen peroxide is based on this additional use of these chemicals results from this. To recycle the leaching solution containing cyanide back to the leaching process after separation of noble metal cyano-complexes, a costly purification step is required. This process is not economical to operate because of the high chemical requirement alone.

As can be seen from German Patentschrift 36 37 082, the leaching process can take place economically using cyanide and hydrogen peroxide, if addition of aqueous H 212 solution is regulated by means of the concentration of the oxygen dissolved in the leaching solution in such a way that the leaching solution contains 2 to 20 mg, preferably 7 to 13 mg, of 02 per litre. In accordance with this process, hydrogen peroxide is thus not added in one portion, but is added continuously or periodically to the leaching solution or the ore pulp. The lowest amounts of hydrogen peroxide used are obtained when hydrogen peroxide is added in the form of a very dilute aqueous solution, preferably having a content of 0.5 to 5 wt.%, in particular I to 2 wt.%.

In the practical use of the process according to German Patentschrift 36 37 082 in gold mines, it has been shown that the sodium cyanide and hydrogen peroxide requirement does not always come up to expectations in spite of careful control of H 2 02 addition and use of a dilute H202 i solution. In some cases, for example leaching iron pyrite ore concentrates t and/or dump material from previous incomplete leaching processes, there was an 4 BJG/397P unsatisfactorily high chemical requirement. This increased requirement is obviously associated with the physical and chemical properties of the ores to be leached.

The object of the present invention is thus to improve the process according to German Patentschrift 36 37 082 in the abovementioned cases to the effect that the hydrogen peroxide requirement and, as far as possible sodium cyanide requirement also, can be reduced. The chemical requirement should also become more independent of the composition and the natural variations of the ores to be leached. In addition, there is considerable interest in the mining industry to obtain a maximum leachable amount of noble metal using cyanide in a shorter time than has been previously possible.

This object is achieved by means of a process for leaching gold and silver from ores, ore concentrates or noble metal wastes, in particular oxide and sulphide ores or ore concentrates and dump material from previous incomplete leaching processes, using an aqueous leaching solution containing cyanide, at a pH value of 8-13, with addition of an aqueous hydrogen peroxide solution to the ore pulp of a stirred leaching process or to the barren solution of a column leaching process, and maintaining an oxygen concentration of 2-20 mg/l in the leaching solution, which is characterised in that before or during addition of hydrogen peroxide, divalent to heptavalent manganese compounds in an amount of 1-50 mg, Scalculated as manganese, per kg of leaching solution, or organic or 5' inorganic polymers in an amount of 0.1-20 g per kg of leaching solution, or powdered or granulated carbon BJG/397P IIIII__L-l 1.IL.-~ in an amount of 0.1-50 g per kg of leaching solution, are added to the ore pulp of a stirred leaching process, and divalent to heptavalent manganese compounds in an amount of 0.01-1.0 mg, calculated as manganese, per kg of barren solution are added to the so-called barren solution of a column leaching process, as decomposition catalyst for hydrogen peroxide.

The sub-claims relate to preferred embodiments of the process of the invention, wherein the separation of the Au and Ag cyano-complexes formed during leaching is particularly advantageous.

The addition of divalent to heptavalent manganese compounds or of special inorganic or organic polymers or of powdered or granulated carbon effects decomposition of hydrogen peroxide to give oxygen and water which is controlled and adapted to the material to be leached. Both the choice of decomposition catalyst and hence its activity and the amount must be adapted to the system to achieve the lowest consumption of hydrogen S' peroxide and sodium cyanide at the highest yield of gold and the lowest residual gold content. The expert will determine the optimum conditions by pilot tests. The decomposition catalysts are added to the ore pulp in the so-called stirred leaching process, but to the so-called baren solution, o000 hence the leaching solution which is essentially free of ore and with which the column is sprayed, in the so-called column leaching process in which o oo manganese compounds are only used in very low concentration.

o a o The conventional stirred leaching process requires technically °o o a costly, and in many respects problematic gassing of the ore pulp. To i 00 0 o o0 a 0 00 increase the oxygen content in the ore pulp and to 0 BJG/397P -6i improve the leaching result, it has also been suggested to use molecular oxygen instead of air for gassing and/or to carry out leaching under pressure. However, these measures make the process more expensive.

Whereas all gassing methods require the often inhibited phase transition of oxygen from the gaseous to the dissolved state, the oxygen resulting from the decomposition of hydrogen peroxide is available in dissolved form directly for leaching. The oxygen concentration and the hydrogen peroxide concentration in the leaching solution can be adapted to the requirements by selecting decomposition catalysts for hydrogen peroxide and by the amount used.

When gold is dissolved in the presence of cyanide and hydrogen peroxide, various mechanisms take effect. To achieve optimum leaching results high gold yield, low residual gold content, high leaching rate with minimum use of chemicals and technically simpler and more reliable control of the process the reaction components, that is, in this case, inter alia, hydrogen peroxide and the oxygen resulting from decomposition of the same, must be present in amounts adapted to one another. This is effected by adding a substance which decomposes hydrogen peroxide in accordance with the invention. Whereas the presence of a low concentration of hydrogen peroxide leads to an acceleration of leaching, a higher concentration has the opposite effect, namely passivating.

It has been discovered that when some ores are leached, for BJG/397P example oxide gold ores and dump material from previous incomplete leaching processes, according to the process know from German Patentschrift 36 37 082, the leaching solution indeed has the desired oxygen concentration of preferably 7 to 13 mg 02/1, but at the same time the concentration of hydrogen peroxide is so high that cyanide is oxidised by it. In these cases, the amounts of cyanide and hydrogen peroxide used are uneconomical. It is possible to reduce the amounts used by applying the process of the invention.

Furthermore, it has been shown that the amounts of cyanide and !i hydrogen peroxide used even for leaching sulphide ores, for example gold ores containing iron pyrites and arsenopyrite, can be considerably reduced in accordance with the invention. This is clear from the examples using }I decomposition catalysts employed in accordance with the invention, compared ii to leaching in their absence. It is assumed in the explanation that the sulphide is oxidised in the case of a higher concentration of hydrogen i i peroxide in the leaching solution. Some of the sulphur compounds thus S formed can react extremely quickly with the cyanide and hence contribute to increased use of cyanide.

Ii The feature characterising one embodiment of the invetnion of adding a divalent to heptavalent manganese compound in the case of the stirred leaching process in an amount of 1-50 mg, preferably 1-10 mg, and in the case of the column leaching process in an amount of 0.01-1.0 mg, preferably 0.05-0.5 mg, calculated in each case as manganese and relative to 1 kg of S. leaching BJG/397P -8solution cannot be derived from prior known United States patent 732,605 as being obvious: whereas in accordance with the invention only catalytic amounts of manganese compounds are used, according to the prior known process the metal oxide which can be reduced by hydrogen peroxide had to be present in at least stoichiometric amount to hydrogen peroxide. Nhereas according to the prior known process, only metal oxides which can be reduced by H 202 have to be carefully added to the ore in an amount of at least 500 ppm before leaching, the manganese compounds of the invention are added to the leaching solution or to the ore pulp before or during the addition of hydrogen peroxide. It is thus possible to use, not only powder manganese oxides from the group comprising MnO, Mn 2 03, Mn 3 0 4 and '1 MnO 2 wherein MnO 2 is preferred, but also further divalent to heptavalent manganese compounds, such as for exazmple sulphates, hydroxides, carbonates or manganates. Apart from the addition of manganese compounds in the form of a powder, they can also be added in the form of a suspension or preferably as aqueous solution. Aqueous manganese II sulphate solution is preferably used, in particular in a concentration of 1 to 10 g Mn 2 to set the manganese concentration in the leaching solution. The use of a Mn (II) salt solution is particulary advantageous for the column leaching process, because careful addition of Mn is easily i possible in the required very low concentration using this solution.

*Barren solution means each leaching solution which is sprayed on the ore column. The so-called barren solution is largely free of noble metal; however, before spraying, the pH value and the sodium cyanide concentration are set. In addition, the amounts BJGI397P i ,ld according to the invention are added to the decomposition catalyst and

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2 0 Compared to the other compounds mentioned in United States patent 732 605, the use of manganese compounds according to the invention is also j characterised in that no cyano-complexes consuming cyanide are formed under leaching conditions, and also no adsorption, which limits leaching rate, of heavy metal ions added in larger amount can take place.

Instead of manganese compounds, organic or inorganic polymers which decompose hydrogen peroxide can also be used in accordance with the invention for the stirred leaching process, in an amount of 0.1 to 20 g, i preferably 1 to 10 g, per kg of leaching solution; the polymer, which is usually insoluble in the leaching solution, is added to the ore pulp before or during H22 addition.

Examples of inorganic polymers which may be used are silicate polymers, such as zeolites, examples of organic polymers which may be used S are cation exchangers. To strengthen or develop the required property, namely to effect decomposition of H 2 0 2 into water and oxygen, the S polymeric materials may contain chemically or physically bonded heavy metals or heavy metal compounds, preferably manganese or copper compounds.

The form of development using decomposition catalysts bonded to polymers is interesting if their decomposing action is to be excluded in cyanide detoxification.

11 According to a preferred embodiment of the process, a powdered BJG/397P i I ,i.

or granulated carbon is used in an amount of 1 to 50 g, preferably 1 to 50 g, preferably 1 to 20 g, per kg of leaching solution. Active carbons, in particular in granulated form and of a quality which can be used in the knrwn carbon-in-leach (CIL) or carbon-in-pulp (CIP) process, is particularly preferred. In general, an added amount of 1 to 10 g of active carbon per kg of leaching solution is sufficient. The use of active carbon in accordance with the invention is particularly advantageous if iydrogen peroxide is used as oxidising agent in the CIL process, and the cyano-complexes of gold and silver formed are adsorptively bonded to the same active carbon.

It has been found that leaching can be carried out particularly economically in the case of the stirred leaching process if the gold and silver cyano-complexes formed during leaching are removed continuously or in batches from the leaching solution during leaching. The concentration of cyano-complexes is thus kept lo', during leaching. This measure can shorten leaching time, in particular in those cases where hitherto leaching slowed down sharply with advancing leaching and where a high gold yield could only be reached after a long time or not at all. It is essential in this process that after 4 to 8 hours of leaching, virtually the entire amount of noble metal which can be leached with cyanide can be recovered.

Amount of noble metal which can be leached with cyanide means that amount i which could be extracted under the most favourable leaching conditions to date, including the use of hydrogen peroxide in the presence of dissolved decomposition catalysts, in general only after 24 to 48 hours of leaching from the ore or ore concentrate, and could be increased BJGi!97P -11no more after extending leaching.

The separation of the noble metal cyano-complexes can be carried out in various ways: Even a continuous or batchwise sluicing of a part of the leaching solution during leaching and replacing the sluiced amount with fresh, that is leaching solution free of noble metal or low in noble metal, leads to acceleration of leaching.

One alternative to separating the Au and Ag cyano-complexes by sluicing the mother liquor is to adsorptively bind the complexes to anion exchangers, polymer bonded chelating agents or powdered or granulated carbon. The adsorption on carbon is preferred. This embodiment is particularly advantageous because conventional carbons decompose hydrogen peroxide and also have a high adsorptive power for Au and Ag cyano-complexes. Leaching using hydrogen peroxide is thus carried out in the same way as the so-called carbon-in-leach technology (CIL). This process can be carried out in a manner known per se in one or, as is most often the case, by leaching in several leaching tanks connected in succession, wherein the latter case, the ore pulp and the carbon are guided in counter-current. The use of this embodiment of the invention is not only advantageous for ores having a high head grade of noble metals, but also ores having low gold content can be leached often more effectively and more economically then by means of prior known processes, using hydrogen peroxide as oxidising agent and carbon as H 2 0 2 decomposition catalyst and adsorption agent for the Au and Ag cyano-complexes.

S-12- It is known to carry out the carbon-in-leach process using pure oxygen instead of using air. This generally leads to acceleration of leaching and partly also to an increase in yield. This so-called CILO process (Company brochure from Messrs. Kamyr-Inc., Mineral Processing Glens Falls, USA) is carried out in the ore pulp at an oxygen concentration in the range from 20 to 35 ppm. After a leaching time of 5 hours, a gold

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yield is reached using the CILO process which leads to a further increase in yield of approximately 3 to 5 points when leaching is continued over 24 hours. Hence, after 5 hours leaching is not yet complete; particularly in the case of ores having a high gold content, it would not be desirable to dispense with this 3 to 5 and operate leaching considerably longer than 5 hours.

In the process of the invention using carbon and hydrogen peroxide, surprisingly, after 4 to 8 hours, the maximum amount of gold which can be leached using cyanide from ores could be extracted. When leaching is operated for a further 4 to 8 hours, there is virtually no further increase in yield. It could not be predicted that in spite of the low 02 content in the leaching solution 2 to 20 mg/l using H 2 0 2 and 20 to 35 ppm using 0 2 in accordance with the invention a higher gold yield could be achieved after 4 to 8 hours of leaching, than was possible using the CILO process. The embodiment of the invention shown above is suitable for leaching ore pulp which is very viscous and therefore is oxygen repellant.

Separating the carbon charged with cyano-complexes from the ore pulp takes place in the conventional manner, for example by means of sieves. I 4 1 BJG/397P -13- The solids concentration in the ore pulp of stirred leaching processes depends on the properties of the ore. Solids concentrations in the range of approximately 20-60 wt.% are conventional on the whole.

In accordance with the invention, aqueous hydrogen peroxide is used in concentration up to 70 Solutions having a content up to 30 wt.% are preferably used. Whereas in the prior known process of German Patentschrift 36 37 082, hydrogen peroxide mostly had to be added in the form of a very severely diluted aqueous solution, in particular 1 to 2 wt.% strength, and hence a high dilution was necessary, it is now possible to also use higher concentrated or average diluted hydrogen peroxide directly.

A further advantage of the invention consists in the fact that the most favourable 02 and H202 concentration in the leaching solution for leaching is set more rapidly in accordance with the invention, than was possible using hydrogen peroxide without the decomposition catalysts of the invention; this shortens the leaching time.

The leaching takes place at a pH value in the range from 8 to 13, 0 0 preferably 9 to 12. To set the pH, alkali hydroxides or alkali oxides and 0 0 alkaline earth hydroxides or alkaline earth oxides are preferably used as 0 o S. for the conventional leaching processes. Alkali cyanides or alkaline earth S 0000 cyanides are used as cyanide. The cyanide concentration is conventionally 0.01 to 0.1 wt.% in 0 0 o0 a 0 o Oe14 o <oo i the leaching solution, however, lower and higher concentrations are also possible.

The addition of hydrogen peroxide to the ore pulp or the leaching solution takes place continuously or periodically to the extent that an oxygen concentration of 2 to 20 mg 02/1, preferably 7 to 13 mg 02/1, is set in the leaching solution and is maintained during leaching. The oxygen concentration is measured in the process, for example by means of an oxygen electrode, and the deviation of the 02 value from the theoretical value is used to control the metering device for hydrogen peroxide.

The examples show how the measures of the invention reduce the use of chemicals compared to the process of German Patentschrift 36 37 082, and at the same time that the maximum gold yield can be achieved in a shorter time.

Example 1 Dump material containing gold from a previous incomplete leaching process was leached continuously in three stirred containers arranged in a cascade having a volume of 1 m 3 in each case. In the first container, the ore suspension having a solids content of approximately 55 wt.% was set at pH 10.5 by adding lime water. The cyanide ("Black Cyanide") and hydrogen peroxide were added to the second container. The addition of hydrogen peroxide, used as a 1.33 wt.7 strength solution, was carried out to the extent that the oxygen concentration remained constant during the 9 hour residence time in the second container. A cyanide concentration 4 1 BJG/397P corresponding to 0.04 wt.% of NaCN was set in each of the leaching solutions. In the second container, the manganese compounds of the invention were also added a aqueous MnSO 4 solution or as powdered MnO 2 The third container, the residence time was also 9 hours in this container, served to complete leaching.

The results taken from leaching three samples of different composition, but from the same mine, can be seen in the table. The results given are gold content of the sample used (head grade), the yield in and the amounts used for sodium cyanide and hydrogen peroxide, relative to 1 t of dump material used. Leaching took place at an 02 concentration of 8 or 12 mt/l, in each case with and without addition of a manganese II or manganese IV compound according to the invention (the Mn concentration details in mg/kg relate to the leaching solution). In all cases, the use of NaCN and H202 could be reduced by means of the process of the invention.

i BJG/397P -16- Sample No.

1leachi ng 0 concentration Mn II or Mn1 IV 2 Cyanide 11 20 2conSUmpti on (mg/i)concentration (mg/kg) Au content (head grade) yield() (g Au/t) cons umption (kg NaCN/t) (kg H 202 (70 18 -0.663 65.3 0.178 0.348 8 7 ppm as tMn(II) 0.798 67.0 0.161 0.191 2 12 0.O,04 35.6 0.163 0.639 12 2.3 ppm as Mn(II) 0.441 50.1 0.155 0.425 0.333 0.349 0. 159 0. 153 0 .513 0.440 7 ppml as Mn (IV) a 7~pq.. SExamples 2 to 4 Pretreated iron pyrite concentrates were leached using Black Cyanide and hydrogen peroxide while maintaining an oxygen concentration of 12 mg/l according to German Patentschrift 36 37 082, wherein Example 2 there was no addition of Mn compound in accordance with the invention, in Example 3 Smanganese (II) sulphate was added and in Example 4 manganese dioxide was added. A cyanide concentration corresponding to 1.5 wt.% NaCN was set in i each of the leaching solutions. The solids concentration was approximately wt.% in each case, the manganese (II) or (IV) concentration in Examples 3 and 4 was 10 mg/kg in each case, relative to the ore pulp. The gold yield and amounts used of sodium cyanide and hydrogen peroxide, added as wt.% strength aqueous solution, can be seen in the table as a function of leaching time. Surprisingly, hydrogen peroxide consumption in the presence of added manganese dioxide was lower, in particular in the first 9 hours of leaching, than in the case when manganese (II) sulphate was added. On the other hand, manganese (II) sulphate used less sodium cyanide than manganese S dioxide.

iT" The iron pyrite concentrate had been pretreated with the aid of Thiobacillus ferrooxidans biologically preoxidised concentrate.

BJG/397P -18i i

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Example 2 without Mn addition Example 3 with Mn(II) Example 4 with Mn(IV) Gold content g Au/t 146.0 154.8 141.1 Gold yield after hours leaching 1 64.9 69.3 62.8 4 84.7 86.4 80.4 9 89.9 90.6 87.5 24 94.0 93.2 93.0

H

2 02consumption (kg H 2 0 2 (70 wt.%)/t iron pyrite concentrate after hours leaching 1 4.42 2.91 1.78 4 5.03 4.26 2.70 9 8.52 5.51 4.72 24 11.80 8.92 8.45 o 00 S o o 0 0 0 00 0 0 oo a 0 0 0 00 0 0 0 G 0 NaCN consumption (kg NaCN (100%)/t iron pyrite concentrate after hours leaching 1 4 9 24 4.95 7.63 8.13 9.66 3.58 4.89 5.75 7.82 4.16 5.82 7.65 9.48 BJG/397P -19-

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Example An essentially oxidic gold ore was subjected to a column leaching process. the leaching solution (barren solution) fed to the column contained 0.03 wt.% sodium cyanide and had a pH value of 10.5.

The 02 concentration in the barren solution could only be increased by approximately 2 ppm by adding hydrogen peroxide to the barren solution in an amount corresponding to 0.05 but in the absence of a manganese compound according to the invention. However, the solution emerging from the column (pregnant solution) scarcely showed changed 02 values (approximately 5 ppm).

The 02 concentration of the leaching solution running in could be increased from 6 mg/l to 20 mg/l, and that in the solution running off could be increased from 5 to 8-12 mg/I, in the presence of 0.1 mg of Mn(ID/kg of leaching solution, added to the barren solution as manganese sulphate, under conditions which are the same otherwise. Hydrogen peroxide is thus decomposed as required. If 2.5 mg of Mn(II)/kg of leaching solution are added to the leaching solution, hydrogen peroxide is 0 decomposed virtually completely in the uppermost layer of the column, 0 o wherein gas is removed and in the greater part of the column there remains a oo a deficit of oxygen or hydrogen pero'xide.

Examples 6 and 7 Iron pyrite concentrates pretreated by means of biooxidation (in 00 0.o .o 0 0 r oo n 0 1 0 0 oo 0 0 CC BJG/397P accordance with Examples 2 to 4) were leached using sodium cyanide and hydrogen peroxide while maintaining an oxygen concentration of 12 mg/1 in accordance with German Patentschrift 36 37 082, wherein no decomposition catalyst was added in sample 6, wherein no decomposition catalyst was added in sample 6, whereas active carbon charged with palladium was added in sample 7. A cyanide concentration of 0.33 wt.% of NaCN was set in each leaching solution. The solids concentration was 25%. 7 g of active carbon/kg of ore suspension were added in Example 7, wherein the active carbon was charged with palladium up to 5 The amounts used of sodium cyanide and hydrogen peroxide, added as 1 wt.% strength aqueous solution, can be seen from the table as a function of the leaching time.

Surprisingly, the consumption of both hydrogen peroxide and sodium cyanide was significantly lower in the presence of the decomposition catalyst.

onG 7 000 00 0 f0 0oo 0 0 00 BJG/397P -21- Example 6 without decomposition catalyst Example 7 with Pd-charged active carbon H 2 0 2 consumpti on (Kg H 2 0 2 (70 of ore after hours leaching 2 4 6 0.65 1 .01 1 .76 2.70 0.48 0.76 1 .37 1 .78 NaCN consumption (Kg NaCN (100%)/t of ore after hours leaching 2 4 6 2.40 2.85 4.17 4.75 2.34 2.91 3.75 3.84 C'0 00 00 00 00 0 0 b0 0 00 0 010 00 0 8.JG/397P c- -l r-c narrr~--~ Example 8 A gravitational concentrate from the Osprey Mine, the head grade of which was 871.2 g Au/t of ore, was leached. The Ore pulp contained 16.7 wt.% solids; the pH was set at 11 and the starting concentration of NaCN was set at 10 g/l. Leaching took place in a stirred container with addition of a 5 wt.% strength H 202 solution and while maintaining an 02 level of 18 ppm. The gold yields are given in the table as a function of the leaching time, wherein test there was no exchange of leaching solution, in test the leaching solution recovered after sedimentation of the ore, was exchanged completely after 1, 2, 4 and 8 hours.

Gold extraction after hours 1 2 4 8 24 without exchange 46.5 50.3 56.1 65.7 99.2 °o'o with exchange 52.4 74.2 95.1 99.1 99.4 o o %Example 8, in which no decomposition catalyst was added, shows the o o effectiveness of exchanging leaching solution.

Example 9 The ore from Example 8 was leached under the same conditions, but without exchange of leaching solution and in the presence of 50 g of 0 granulated active carbon which acted as decomposition catalyst and as adsorber for the gold complex. The gold yields as a function of time, the o H 2 0 2 consumption *0C 2 2* BJG/397P -23-

?B

r~I and cyanide consumption can be seen in the table, wherein leaching took place in at an 02 concentration of 18 ppm, and in at one of 12 ppm: Gold extraction 2 4 18 ppm 02 97.4 98.7 99.2 99.5 99.6 12 ppm 02 92.7 97.1 99.6 99.6 99.6 For comparison Example 8 46.5 50.3 56.1 65.7 99.2 Cyanide consumption (2 g/t of ore) 18 ppm 02 8.5 11.1 13.2 25.6 42.4 12 ppm 02 6.5 7.3 9.9 14.7 30.7

H

2 0 2 consumption (kg 70 of ore) o 0 4 4 0004 0404 18 ppm 02 12 ppm 02 11.9 4.9 18.0 8.6 45.0 14.1 95.8 37.0 (00A0 41 04 04 4 41 44 410 4 4) 44 BJG/397P -24-

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Example A South African gold ore was leached once conventionally by the carbon-in-leach process and once in accordance with the invention (b) using the same carbon as in Solids content 50 pH 11 set using CaO; NaCN concentration of 0.05 head grade 2.1 g Au/t of ore; addition of active carbon 20 g/1 of pulp.

For the 02 value increases slowly in the pulp from 1-2 ppm to 8 ppm. For an 02 concentration of 12 ppm was set by adding 1 strength H 2 0 2 solution. The gold yield as a function of time can be seen from the table.

Gold yield after hours 1 2 4 8 24 CIL 56 80 88 97 99

H

2 0 2 /CIL 81 90 99 99 99 S Example 11 Comparison of leaching using H2 2 without active carbon and -0 using active carbon South African waste material, head grade 1.33 g/t of ore. NaCN concentration 0.05 solids concentration 50 pH 11. In and an 02 level of 12 ppm was set and maintained using 1 wt.% strength aqueous H 2 0 2 solution. In test 11 b, 20 g of BG 2 B.P carbon/l of pulp were used.

Gold yield after hours 1 2 4 8 24 only H202 70 72 81 88 89

H

2 0 2 carbon 80 83 92 93 93 2 2 After about 4 hours, approximately 4 points more gold were extracted in accordance with the invention at 92-93 gold yield, than after 24 hours using H 2 0 2 but without carbon under conditions which were the same otherwise. Shortening the leaching time leads to a reduction in the chemical consumption.

BG/397P -26- L WI MM M MM Q

Claims (7)

1. Process for leaching gold and silver from ores, ore concentrates or noble metal wastes, comprising: leaching said gold and/or silver at a pH of 8 to 13 from said ore with an aqueous solution containing cyanide in accordance with a stirred leaching process or a column leaching process to obtain an ore pulp or a barren solution respectively; adding an aqueous hydrogen peroxide solution to the ore pulp or to the barren solution and maintaining the oxygen concentration of the ore pulp or barren solution at 2 to 20 mg/l; and adding before or during addition of the hydrogen peroxide o' ~solution to the ore pulp, a decomposition catalyst selected from: 1 to 50mg Mn per kg of leaching solution of divalent to C C heptavalent manganese compounds. C (ii) 0.1 to 20g per kg of leaching solution of organic or inorganic polymers, 0 0 (iii) 0.1 to 50g per kg of leaching solution of powdered or Sgranulated carbon, or adding before or during addition of the hydrogen peroxide solution to the barren solution, a decomposition catalyst comprising 0.01 o l.Omg Mn per kg of the barren solution of divalent to heptavalent manganese compounds.

2. Process according to claim 1, wherein the ores or ore concentrates are oxide and sulphide ores or ore concentrates.

3. Process according to claim 1, wherein the noble metal wastes are dump materials from previous incomplete leaching processes.

4. Process according to any one of claims 1 to 3, wherein the decomposition catalyst is an aqueous solution of manganese II salts or a powdered MnO, MnO 2 Mn 2 0 3 and Mn 3 0 4 Process according to claim 4 wherein the manganese II salt is manganese sulphate.

6. Process according to claim 4 wherein the decomposition catalyst is MnO 2

7. Process for leaching gold and silver from ores, ore concentrates or noble metal wastes, substantially as hereinbefore described with reference to any one of the Examples but excluding any comparative example.

8. Gold and silver whenever produced by the process of any one of claims 1 to 7. "KX: 63y A, 28 DATED this EIGHTEENTH day of FEBRUARY 1991 Degussa Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON 0 0 0 C 00 ~0 0 0 KXN: 1263y