Leaching of minerals

Abstract

A method for treating a mineral composition containing iron, arsenic or other Group VA compounds comprises milling the mineral composition to a particle size of P.sub.80 of less than 25 m and leaching the mineral composition in the presence of lime and/or limestone and a soluble alkali complexing agent and in the presence of an oxygen containing gas at a pH in the range of from 3.5 to 6.

Claims

1. A method for treating a mineral composition containing iron, arsenic or other Group VA compounds comprising milling the mineral composition to a particle size of P.sub.80 of less than 25 m and leaching said mineral composition in the presence of lime and/or limestone and a soluble alkali complexing agent in the presence of an oxygen containing gas at a pH in the range of from 3.5 to 6.

2. A method as claimed in claim 1 wherein a second complexing agent forms a soluble complex with iron, arsenic or other Group VA compounds during the leaching.

3. The method of claim 2 wherein the second complexing agent is selected from the group consisting of soluble alkalis, soluble carbonates, soluble hydroxides, and carbon dioxide gas.

4. A method as claimed in claim 1 wherein the soluble alkali complexing agent forms a soluble complex with iron, arsenic or other Group VA compounds during the leaching.

5. The method of claim 1 wherein limestone is present during the leaching and lime is not added during the leaching.

6. The method of claim 1 wherein the soluble alkali complexing agent comprises a soluble carbonate or soluble hydroxide selected from the group consisting of sodium hydroxide, potassium hydroxide, magnesium hydroxide, ammonium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, and ammonium carbonate.

7. The method of claim 1 wherein the mineral composition comprises a refractory material containing precious metals.

8. A method as claimed in claim 7 wherein the refractory material comprises: (a) refractory sulfides in which precious metals are encapsulated; (b) ores which contain carbonaceous material and telluride materials; or (c) ores which contain carbonaceous material and selenide materials.

9. The method of claim 7 wherein the mineral composition also contains one or more of stibnite, tetrahedrite, argentopyrite, calaverite, altaite, gold bearing selenides, tennantite and pentlandite, or the mineral composition comprises a composition including carbonaceous matter, where the carbonaceous matter would otherwise interfere with the precious metals recovery process.

10. The method of claim 9 wherein the refractory material comprises flotation concentrates.

11. The method of claim 1 wherein the composition is finely ground to a particle size range of 80% passing 2-25 m, or about 80% passing 2-15 m.

12. The method of claim 1 wherein the method is carried out at ambient pressure.

13. The method of claim 1 wherein the leaching is conducted at a temperature of between about 50 C. up to the boiling point of the mixture.

14. A method as claimed in claim 13 wherein the leaching conducted at a temperature of between about 50 C. up to about 98 C.

15. The method of claim 13 wherein the leaching is carried out in the presence of an oxygen containing gas selected from oxygen, air or oxygen enriched air.

16. The method of claim 1 wherein after the composition has been leached, the mixture is further treated to recover precious metals.

17. The method of claim 1 wherein the soluble alkali complexing agent comprises a soluble alkali compound that forms a complex with arsenic or other group VA elements or compounds, the complex comprising a short-lived complex that subsequently migrates away from particles of the mineral composition and then precipitates.

Description

EXAMPLES

Example 1

Armenian Refractory Concentrate

(1) A sample of refractory sulphide concentrate of the following composition was used for the testwork:

(2) TABLE-US-00001 Arsenic % 4.5 Antimony % 0.5 Iron - % 22 Sulphide - % 22 Au - g/t 55 Ag - g/t 57 Te - g/t 155

(3) Micrographic analysis was carried out on the concentrate sample to identify the major gold deportment, and this is summarised below:

(4) TABLE-US-00002 FREE/CYANIDABLE GOLD 48.2 TELLURIDE LOCKED GOLD 5.25 CARBONATE LOCKED GOLD 3.36 ARSENICAL MINERAL (ARSENOPYRITE) 42.21 PYRITIC SULPHIDE MINERAL 0.6 SILICATE (GANGUE) ENCAPSULATED 0.3

(5) The majority of the gold within the sample was housed in arsenic sulphide phases

(6) The sample was then milled in a horizontally stirred bead mill to an 80% passing size of 11.5 microns.

(7) A series of tests were then carried out on the sample under the following set of conditions:

(8) TABLE-US-00003 Level of Sulphide Control Temperature Oxidation NaOH addition CaCO.sub.3 addition Test No pH ( C.) Durationhrs % (kg/tonne) (kg/tonne) 1 5.5 95 48 87.6 0 181 2 5.5 95 48 95.8 10 176 3 5.5 95 48 73.4 15 128 4 5.5 95 48 77.8 20 132

(9) The level of sodium hydroxide addition was varied for all tests to improve selective oxidation of the arsenic sulphide phases.

(10) The sodium hydroxide and limestone were added progressively to all tests to control the pH to the required setpoint. On completion of the tests, the oxidised slurry was filtered.

(11) The filter cake from each oxidation test was re-slurried in tap water to level of 40% solids and then leached for 24 hours in a 500 ppm NaCN solution, with the pH held at 10 using hydrated lime. Activated carbon was added at the start of the test. On completion of the test, the cyanide leach slurry was filtered, and the final filter cake, solution and carbon phases analysed for gold and silver to determine recovery.

(12) A summary of the results of the testwork is presented in Table 2, below:

(13) TABLE-US-00004 TABLE 2 Testwork Results - Armenian Refractory Concentrate Test ID Au Recovery % 1 88.9 2 93.1 3 95.0 4 95.5

(14) The addition of the sodium alkali to tests 2-4 resulted in superior gold recovery from the oxidised residue, due to improved oxidation of the arsenic rich gold phases. These improved gold recoveries were also achieved at lower overall levels of sulphide oxidation.

Example 2

Central American Refractory Concentrate

(15) A sample of refractory sulphide concentrate of the following composition was used for the testwork:

(16) TABLE-US-00005 Arsenic % 11.02 Iron - % 38.4 Sulphide - % 36.8 Au - g/t 32 Ag - g/t 61

(17) The sample consisted of predominantly arsenopyrite and pyrite. The sample was milled in a horizontally stirred bead mill to an 80% passing size of 9.7 microns.

(18) Two tests were then carried out on the sample at varying control pH levels. The pH levels of 5.2 and 5.5 were tested. Arsenic solubility, while very low across this entire pH range, is marginally higher at a pH of 5.2 relative to 5.5, and so the improved complexing of arsenic and migration of the arsenic complex away from the leaching surface at the lower pH was expected to translate to an improved overall oxidation rate. The conditions for the two tests are outlined below:

(19) TABLE-US-00006 Level of Sulphide CaCO.sub.3 Oxidation NaOH addition addition Test No Control pH Temperature ( C.) Durationhrs % (kg/tonne) (kg/tonne) 1 5.5 95 48 41 61 346 2 5.2 95 50 62 90 790

(20) The sodium hydroxide and limestone were added progressively to all tests to control the pH to the required setpoint. The reduction of the pH to 5.2 from 5.5-6 resulted in a 50% increase in the rate of oxidation of the sulphide minerals.

(21) On completion of the tests, the oxidised slurry was filtered.

(22) The filter cake from each oxidation test was re-slurried in tap water to level of 40% solids and then leached for 24 hours in a 500 ppm NaCN solution, with the pH held at 10 using hydrated lime. Activated carbon was added at the start of the test. On completion of the test, the cyanide leach slurry was filtered, and the final filter cake, solution and carbon phases analysed for gold and silver to determine recovery.

(23) A summary of the results of the testwork is presented in Table 2, below:

(24) TABLE-US-00007 TABLE 2 Testwork Results - Central American Refractory Concentrate Test ID Au Recovery % Ag Recovery % 1 90.8 92.9 2 90.9 97.7

(25) The arsenic phase that will form and precipitate on the leaching mineral surface will be scorodite under the leaching conditions employed. Data on the solubility of scorodite in the pH range tested has been reported [P. M. Dove and J. D. Rimstidt. Am. Miner. 70, 838-844 (1985)]. At the two pH ranges tested, the arsenic solubilities are expected to be:

(26) pH 5.2=0.8 mmol/L

(27) pH 5.5=0.1 mmol/L

(28) The control of the pH in Test 2 to a level where the arsenic solubility while still very low, was optimized, resulted in a 50% increase in the oxidation rate within the oxidative leach test.

Example 3

Mexican Refractory Concentrate

(29) A sample of refractory sulphide concentrate of the following composition was used for the testwork:

(30) TABLE-US-00008 Arsenic % 14.4 Iron - % 37.1 Sulphide - % 33.2 Au - g/t 18.4 Ag - g/t 18.2

(31) The majority of the gold within the sample was housed in arsenopyrite.

(32) The sample was then milled in a horizontally stirred bead mill to an 80% passing size of 10 microns.

(33) A series of tests were then carried out on the sample under the following set of conditions:

(34) TABLE-US-00009 Level of Sulphide Oxidation NaOH addition CaCO.sub.3 addition Test No Control pH Temperature ( C.) Durationhrs % (kg/tonne) (kg/tonne) 1 (8) 5.5 95 51 39 0 107 2 (1) 5.5 95 49 65 49 179 3 (4) 5.5 95 48 52 96 106

(35) The level of sodium hydroxide addition was again varied for the three tests to improve selective oxidation of the arsenic sulphide phases.

(36) The sodium hydroxide and limestone were added progressively to all tests to control the pH to the required setpoint. On completion of the tests, the oxidised slurry was filtered.

(37) The filter cake from each oxidation test was re-slurried in tap water to level of 40% solids and then leached for 24 hours in a 500 ppm NaCN solution, with the pH held at 10 using hydrated lime. Activated carbon was added at the start of the test. On completion of the test, the cyanide leach slurry was filtered, and the final filter cake, solution and carbon phases analysed for gold and silver to determine recovery.

(38) A summary of the results of the testwork is presented in Table 2, below:

(39) TABLE-US-00010 TABLE 2 Testwork Results - Mexican Refractory Concentrate Test ID Au Recovery % 1 48.6 2 83.6 3 84

(40) The addition of the sodium alkali to tests 2 and 3 again resulted in superior gold recovery from the oxidised residue, due to improved oxidation of the arsenic rich gold phases.

Example 4

New Zealand Refractory Concentrate

(41) A sample of refractory sulphide concentrate of the following composition was used for the testwork:

(42) TABLE-US-00011 Arsenic % 11.3 Iron - % 27.0 Sulphide - % 24.2 Au - g/t 56.9 Ag - g/t 2.0

(43) The sample consisted of predominantly arsenopyrite and pyrite. The sample was milled in a horizontally stirred bead mill to an 80% passing size of 6.0 microns.

(44) Ten tests were then carried out on the sample at varying control pH levels. The pH levels of 4.4, 4.8 and 5.5 were tested. Arsenic solubility, while very low across this entire pH range, is marginally higher at a pH of 4.8 relative to 5.5, and so the improved complexing of arsenic and migration of the arsenic complex away from the leaching surface at the lower pH was expected to translate to an improved overall oxidation rate. The conditions for the tests are outlined below:

(45) TABLE-US-00012 Level of Sulphide Na.sub.2CO.sub.3 CaCO.sub.3 Specific Temperature Oxidation addition addition Rate Test No Control pH ( C.) Durationhrs % (kg/tonne) (kg/tonne) constant 2 5.5 90 10.5 19 0 150 0.08 8 4.8 90 12 22 38 165 0.24 9 4.4 90 12 21 38 167 0.17-0.24 10 4.8 90 48 63 38 428 0.20

(46) The sodium carbonate and limestone were added progressively to all tests to control the pH to the required setpoint. The reduction of the pH to 4.8 from 5.5 resulted in a 50% increase in the rate of oxidation of the sulphide minerals.

(47) On completion of the tests, the oxidised slurry was filtered.

(48) The level of sodium carbonate addition was proportional for all tests to improve selective oxidation of the arsenic sulphide phases.

(49) The arsenic phase that will form and precipitate on the leaching mineral surface will be scorodite under the leaching conditions employed. Data on the solubility of scorodite in the pH range tested has been reported [P. M. Dove and J. D. Rimstidt. Am. Miner. 70, 838-844 (1985)]. At the two pH ranges tested, the arsenic solubilities are expected to be:

(50) pH 4.4=0.8 mmol/L

(51) pH 4.4=0.8 mmol/L

(52) pH 5.5=0.1 mmol/L

(53) The control of the pH in Test 10 to a level where the arsenic solubility while still very low, was optimized, resulted in a 50% increase in the oxidation rate within the oxidative leach test.

(54) In the present specification and claims (if any), the word comprising and its derivatives including comprises and comprise include each of the stated integers but does not exclude the inclusion of one or more further integers.

(55) Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

(56) In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.