CA2050529A1 - Process for the removal of oxygen from cyanide leaching solutions containing noble metals before cementation of the noble metals with zinc - Google Patents

Abstract

Abstract In the Merrill-Crowe process for the removal of noble metals from cyanide leaching solutions by cementation with zinc, the dissolved oxygen has to be removed. According to the invention, oxygen scavengers are used to reduce the oxygen instead of the degassing technique normally applied.
Formamidine sulfinic acid and alkali metal sulfites are preferably used as the oxygen scavengers, alkali metal sulfites requiring the presence of an effective quantity of metal ions, preferably 25 to 100 mg Cu ions/l. The process is easy to carry out and improves the economy of the Merrill-Crowe process.

Description

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A process for the removal of oxygen from cyanide leaching solution~ containing noble metals be~ore cementation of the noble met~ls with zinc Description This invention relates to a process for the removal of oxygen from cyanide leaching solutions containing noble metals before cementation of the noble metals with zinc.
:~ In the recovery o~ noble metals, particularly gold, by leachislg oî: ores, ore concentrates, mechanically, chemical-ly and/or biologically pretreated ores and tailings from previous leachings with an alkaline cyanide solution in the presence of air or hydrogen peroxide as oxidizing agent, 10 the noble metals enter the cyanide leaching solution in the form of cyano complexes (see, for example, DE-PSS 36 37 082 and 38 30 703).
A method which has long been used to remove the noble metals from this leaching solution is cementation using 15 zinc powder. This process is generally Xnown as the Merrill-Crowe process although individual steps have been modified and improved in different ways, see G.G. Stanley "The Extractive Metallurgy of Gold in South Africa", Vol.
1 (1987), pages 331 to 343 (publ. by the South Africa 20 Institute of Mining & Metallurgy, Johannesburg) and R.I.
Adanson "Gold Metallurgy in South Africa", pages 120 - 135.
The Merrill-Crowe process essentially comprises the follow-ing steps: (a) clarification of the leaching solution freed from the leached material; (b) degassing of the leaching 25 solution to remove the dissolved oxygen; (c) addition of zinc powder and lead nitrate and monitoring of the alkali and cyanide concentration; (d) precipitation of the noble metals and filtration. The present invention is concerned with step (b) of the Merrill-Crowe process, degassing being 30 replaced by reductive elimination of the dissolved oxygen.
In the known process, the dissolved oxygen is removed .; . ~

2 ~ 2 9 in degassing tanks under reduced pressure. The degree of degassing critically determines the economy of the Merrill-Crowe process. Inadequate degassing leads to passivation of the zinc surface and to an increase in the consumption of zinc through dissolution in accordance with the follow-ing equation:

2 Zn + 8 CN + 2 + 2 H20 ~ 2 Zn (CN) 42 + 4 OH

0 Finally, the noble metal already precipitated can even be redissolved in the presence of oxygen, so that the yield of noble metal is reduced. Technically, degassing in degassing tanks is extremely complicated. In addition, a residue of dissolved oxygen of generally aboui l mg oxygen per liter remains in the leaching solution, so that the problems referred to are not completely avoided. The re-entrainment of oxygen cannot always be safely avoided during treatment of the leaching solution with zinc and removal of the precipitated products, so that the problems mentioned above may again be encountered.
The problem addressed by the present invention was to improve step (b) of the Merrill-Crowe process to the extent that the problems mentioned are alleviated so that the economy of recovery of the noble metals from the leaching solution containing them is increased.
This problem has been solved by a process for the removal of oxygen from cyanide leaching solutions contain-ing noble metals before cementation of the noble metals with zinc, characteri~ed in that the dissolved oxygen is reduced using an oxygen scavenger at pH 9 - 14 by adding one to five times the quantity of oxygen scavenger stoich-iometrically required for quantitatively reducing the oxygen and, if necessary, an effective quantity of a heavy metal catalyst to the leaching solution and continuing the treatment until the oxygen content has fallen to below l -:

2 ~ 2 9 Although the use of oxygen scavengers, which act by reducing the oxygen, has long been known in other industrial areas, for example in water/steam circuits, and although chemically different oxygen scavengers have been introduced, the use of such substances for removing oxygen from cyanide-containing leaching solutions was hitherto unknown. Since the Merrill-Crowe process has been in use for decades and the problems of step (b) are well-known, it is surprising that oxygen scavengers have never been used instead of the technically complicated degassing technique for the removal of oxygen. A
procedure such as this had evidently never been considered because an inadequate effect and/or unwanted secondary reactions had been expected in the complex system.

Oxygen scavengers which are active at pH values in the range from 9 to 1~ and which are not deactivated by cyanide ions may be used in accordance with the invention. The expert will determine the suitability of a selected oxygen scavenger or a combination of various oxygen scavengers and a combination of oxygen scavengers and heavy metal catalysts in a simple preliminary test. Preliminary tests such as these are useful because known oxygen scavengers from the group consisting of sulfites, hydrazine, hydroxylamines, ketoximes, ascorbic acid, and phenolic oxygen scavengers, such as for example hydroquinone, aminophenols and pyrogallol, show pronounced differences in activity. Some of the substances mentioned can only be used as oxygen scavengers in the process in question in the presence of catalytically active metal ions from the group consisting of manganese, iron, cobalt, nickel and copper. In selecting the oxygen scaven~ers, preference will be given to those which are highly efficient in removing oxygen at low :.

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temperatures, more particularly in the range from 5 to 25~C. A review of oxygen scavengers and their properties can be found, for example, in the following literature:
W.J.F. van der Wal, VGB Kraftwerkstechnik (1969), 296 -299; D.C. Scranton, Materials Performance, Sept. 1979, 46 -~8; EP-A O 161 822.
It can be seen from the Examples that sulfite develops hardly any effect at room temperature in the absence of known heavy metal catalysts. However, an excellent effect in the process in question is exhibited by sulfites soluble in the requisite concentration in the presence of a cata-lytically active ~lantity of metal ions from the ~roup consisting of Mn, Fe, Co, Ni and Cu. Preferred soluble sulfites are ammonium and alkali metal sulfites or bisul-1~ fites, more particularly Na2SO3. The oxygen scavenger is used in 1 to 5 times the quantity stoichiometrically necessary for quantitatively reducing the oxygen, i.e. 2 to 10 mmol Na2SO3 per mmol 2- The reaction rate increases and, hence, the treatment time for the complete removal of oxygen decreases with increasing ratio of oxygen scavenger to o~ygen. The oxy~en content may be determined, for example, with an oxygen electrode. An excess of sulfite and the resulting sulfate do not affect the Merrill-Crowe process.
The use of sulfite requires the presence of an effec-tive quantity of one or more heavy metal catalysts. Cu and Co ions are particularly effective. The necessary quantity is generally far above the quantity required for water/
steam circuits. A catalyst content of more than 20 ppm metal ions and preferably from 25 to 100 ppm is recom-mended; contents of Cu ions of at least 25 ppm are prefer-red, contents in the range from 25 to 100 ppm being par-ticularly preferred and contents of 50 to 100 ppm being most particularly preferred. The catalytically active metal ions may enter the leaching solution completely or partly during the cyanide leaching process and/or may be added to the leaching solution at the beginning of the treatment in the form of soluble metal salts, particularly in divalent form. Knowing the concentration of metal ions in the leaching solution, the concentration of catalyst required for optimal activity, which may readily be deter-mined in a preliminary test, may readily be adjusted by addition of the catalytically active metal salts before the removal of oxygen. The catalyst may optionally be added in '0 several portions during the treatment for the removal of oxygen should the removal OI oxygen come to a stop despite an adequate ouantity o~ sulfite.
Good oxygen scavengers for the field of application in question can be found among phenolic substances, for ex-1~ ample hydroquinone and pyrogallol. These oxygen scavengers do not require the presence of catalytically active metal ions, although on the other hand their application is restricted by toxicological considerations.
Surprisingly, formamidine sulfinic acid (FAS) has also proved to very effective without requiring the addition of catalytically active metal ions. The Merrill-Crowe process is not adversely affected by excess formamidine sulfinic acid or reaction products thereo.E. The quantities used are in the above-mentioned range; the FAS has to be used in a ~uantity of at least one mmol per mmol 2 and is preferably used in a quantity of up to 3 mmol and, more preferably, in a quantity of 1.5 to ~ mmol per mmol 2~ The FAS is added either in the form of powder or, with greater advantage, in the form o~ an aqueous alkaline solution.
The oxygen-removing treatment essentially comprises adding the oxygen scavenger and, where necessary, the heavy metal catalyst to the leaching solution, followed by uniform dispersion. Only after an adequate residence time following addition and dispersion, i.e. when the 2 content has fallen to below 1 ppm and preferably to zero, is the : ~ :

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2 ~ g leaching solution delivered to the next stage of the Merrill-Crowe process. The leaching solution is preferably stirred during the residence time - generally 1 to 15 minutes - which is determined by the selected concentra-tions of oxygen scavenger and, where necessary, catalyst, by the temperature and by the pH value. The preferred pH
range is 10 - 12 and, more particularly, 11 - 12.
The process according to the invention may also be combined with the conventional degassing technique by '0 initially removing part of the dissolved oxygen by degas-sing and then applying the treatment according to the lnvention for quantitative removal of the oxygen. The quantity of oxygen scavenger is preferably gauged in such a way that an excess thereof remains in the leaching solution even after the removal of oxygen, so that any oxygen subsequently entrained can still be removed.
The process according to the invention enables dis-solved oxygen to be quantitatively removed from the leach-ing solution. The consumption o:E zinc is thus reduced and the yield of noble metal increased. The time and effort hitherto involved in the removal of oxygen can be reduced.
Excess oxygen scavenger acts as a reserve for the removal of oxygen subsequently entrained, so that the cementation stage is not affected by any 02-i.nduced disturbances or by any reduction in economy.

Ex~mples 1 a to 1 c Quantities of 500 mg sodium sulfite and the quantities of Cu2' ions shown in Table l in the form of CuS04 5H20 were added at room temperature to 1 liter of an aqueous solution containing 200 mg sodium cyanide and having a pH
value adjusted with calcium oxide to 10.5. The reduction in the content of dissolved oxygen (ppm) was followed by continuous oxygen measurement while stirring using an oxygen electrode (see Table 1).

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Table 1 Example Cu added Time (mins.) (ppm) o 0.5 l 3 5 -la 0 8.6 8.6 8.2 8.1 8.2 lb lO 8.6 7.5 5.8 5.1 5.1 lc 50 8.3 3.3 0.2 0 Example 2 The procedure was as in Example l~, except that quantities of 10 mg Cu2~ ions were added at the beginning and after 4, 10 and 12 minutes. The reduction in the oxygen content takes place in steps and is shown in Fig.
1/2.

Example 3 Following the procedure o~E Example la, the same cyanide solution (pH 10.5) was treated with formamidine sulEinic acid (FAS) as oxygen scavenger instead of Na2S03.
The reduction in the oxygen content as a function of the quantity of FAS added is shown in Table 2.

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Table 2 FAS added Time (mins.) (g/l) 0 1 3 ~ 10 15 0.1 8.8 ~3.7 7.7 6.5 3.1 0 0.2 8.6 7.5 3.5 0.5 0 0.3 8.6 6.8 0.5 0.1 0 0.5 8.8 5.9 0.3 0 1.0 8.o ~.C' 0.1 0 Examples 4 - 7 Following the procedure of Example la, the same cyanide solution (pH 10.5) was treated with quantities of 500 mg of the oxygen scavengers listed in Ta~le 3 instead of NazSO3. The reduction in the oxygen content (ppm) is shown in Table 3.

Table 3 Example Oxygen scavenger Time (mins.) 0 0.5 1 3 5 -4 Hydrazine 8.1 3.2 8.0 7.8 7.8 Ascorbic acid 8.4 a . 3 8.1 7.5 7.7 6 Hydroquinone 9.o 2.1 0.9 0.2 0 7 Pyrogallol 9.0 0.3 0 E~amples 8 - 11 and Comparison ~xample A gold-containing ore from the Beatrix Mine (SA) was leached in accordance with DE-PS 36 37 082. The leaching solution contained 4.36 ppm Au, 258 ppm NaCN and approx. 13 " " ~

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ppm Cu, 3 ppm Zn and 5 ppm Fe; this solution was used in Examples 8 and 9 and in the Comparison Example; another batch with a different Au content of 5.06 ppm and an NaCN
content of 412 ppm was used in Examples 10 and 11.
Quantities of 1 liter of the solution were adjusted to pH 11.0 with CaO. The oxygen scavenger and, in the case of Na2S03, CuS04 5H20 as catalyst were then added at room temperature. The oxygen was removed while stirring over a period of 10 minutes. In the Comparison Example, the oxygen was displaced by introductlon of nitrogen (10 minutes). Ste~ (c) o~ the Merrill-Crowe process was then carried out by adding 10 mg Pb(NO~) 2 and 500 mg Zn powder and stirring for 60 minutes. The zinc/gold deposit was filtered off and the residual Au content in the filtrate was determi~ed by AAS. The results are shown in Table 4 below.

Table 4 Merrill-Crowe process in dependence on the oxygen removal method Oxygen Residual gold ~ Gold scavenger/ content in precipitated quantity filtrate added (mg~l) Comparison Example N2 0.0510 98.9 Example 8 0.1 g FAS/l 0.014 99.7 Example 9 0.2 g FAS/l 0.005 99.9 Example 10 0.1 g FAS.l 0.020 99.6 Example 11 0.126 g Na2SO3/l and SO mg Cu2+/l 0.020 99.6 -2 ~

Figure 2/2 shows the reduction in oxygen where the leaching solution used for Examples 10 and 11 is treated with 1 mmol FAS/l, with 1 mmol Na2SO3/l in the absence of a catalyst and with 1 mmol Na2SO3/l in the presence of 50 ppm Cu2+ ions.

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Claims (17)

1. A process for the removal of oxygen from cyanide leaching solutions containing noble metals before cementation of the noble metals with zinc, wherein the dissolved oxygen is reduced using an oxygen scavenger at pH 9 - 14 by adding one to five times the quantity of oxygen scavenger stoichiometrically required for quantitatively reducing the oxygen, and containing the treatment until the oxygen content has fallen to below 1 ppm.

2. A process as claimed in claim 1, wherein an effective quantity of a heavy metal catalyst is added to the leaching solution.

3. A process as claimed in claim 2, wherein a sulfite soluble in the leaching solution is used as the oxygen scavenger and the removal of oxygen is carried out in the presence of a catalytically active quantity of a metal ion selected from the group consisting of Mn, Fe, Co, Ni and Cu.

4. A process as claimed in claim 3, wherein the removal of oxygen is carried out in the presence of at least 20 ppm of a metal ion selected from the group consisting of Mn, Fe, Co, Ni and Cu.

5. A process as claimed in claim 3, wherein the removal of oxygen is carried out in the presence of from 25 to 100 ppm of a metal ion selected from the group consisting of Mn, Fe, Co, Ni and Cu.

6. A process as claimed in claim 4, wherein the removal of oxygen is carried out in the presence of at least 25 ppm Cu ions.

7. A process as claimed in claim 4, wherein the removal of oxygen is carried out in the presence of from 25 to 100 ppm Cu ions.

8. A process as claimed in claim 4, wherein the removal of oxygen is carried out in the presence of from 50 to 100 ppm Cu ions.

9. A process as claimed in claim 6, 7, or 8 wherein the Cu ion concentration is adjusted by the addition of a soluble Cu(II) salt.

10. A process as claimed in claim 1, wherein formamidine sulfinic acid is used as the oxygen scavenger.

11. A process as claimed in claim 1, wherein hydroquinone is used as the oxygen scavenger.

12. A process as claimed in any one of claims 1 to 8, 10 or 11 wherein the removal of oxygen is carried out at pH 10 to 12.

13. A process as claimed in any one of claims 1 to 8, 10 or 11 wherein the removal of oxygen is carried out at pH 11 to 12.

14. A process as claimed in any one of claims 1 to 8, 10 or 11 wherein part of the dissolved oxygen is initially removed by degassing or displacement by inert gases and the remaining dissolved oxygen is removed using an oxygen scavenger.

15. A process as claimed in any of claims 1 to 8, 10 or 11 wherein the treatment is continued to an oxygen content of zero, as measured with an oxygen electrode.

16. A process as claimed in any one of claims 1 to 8, 10 or 11, effected at from 5 to 25°C.

17. A process as claimed in claim 10, wherein the concentration of formamide sulfinic acid is up to 3 mmol per mmol of oxygen.

13. A process as claimed in claim 10, wherein the concentration of formamide sulfinic acid is in the range of fro 1.5 to 2 mmol per mmol of oxygen.