FR2476137A1 - Gold and silver recovery from sulphidic material - by low temp oxidative roasting and cyanide or thiourea leaching - Google Patents

Abstract

The roasting step is carried out at below 650 (pref. 500- 625, esp. 575-625) deg.C. Roasting may be carried out in two stages, the exothermic oxidising stage being effected at below 650 deg. C, or may comprise a single exothermic stage. Pref. the roasted residue is pre-leached in a slightly acid medium, pref. in the presence of a reducing agent such as SO2 and magnetic pyrites (e.g. pyrrohotine), to eliminate soluble iron oxides and soluble sulphates. A preferred leaching operation comprises (i) pre-leaching with a sulphuric acid soln. in the presence of SO2 followed by (ii) leaching with an acidic thiourea soln. Good yields of Au and Ag are obtd.

Description

Treatment of sulphurous substances in order to recover the metals v contained.

 The subject of the present invention is a process for treating sulphurous materials containing gold and silver, with a view to recovering these metals.

 Some ores and some metallurgical residues are known which contain gold and silver in a gangue of various sulphides, such as pyrites in particular.

 Precious metals are found in various forms more or less easily accessible for leaching. Thus gold may be partially trapped in the native state and the silver is often present mainly as Ag2S, some of which may exist in solid solution in PbS.

 It is already known to extract precious metals from such starting materials, but the recovery yields obtained are rarely entirely satisfactory.

 For some ore deposits or some metallurgical residues, direct leaching with cyanide solution or thiourea acid solution may already yield exploitable yields.

 In most cases, however, it is necessary to subject the starting sulfurous materials to a roasting operation for removing the labile sulfur, or even to oxidize the sulfides to oxides and SO.sub.2.

 The sulphide roasting is commonly done in two distinct stages, namely a reducing-mode roasting step (endothermic) and an oxidizing roasting step (exothermic), or a single step of highly exothermic oxidizing roasting.

 So far, the roasting of sulphurous materials such as pyrites has always been conducted in industrial practice at high temperatures. The roasting temperatures depend on the type of equipment used: floor oven, rotary kiln or fluidized bed oven. These temperatures are between 700 and I0000C.

The main reason for roasting at these temperatures is that, in general, higher roasting rates and lower residual sulfur contents are obtained at higher temperatures,
The behavior of the various elements present in the sulphides depends on the roasting temperature. It is known, in fact, that at a lower temperature sulphates can be more easily formed.

The general decomposition equation for these sulphates is as follows

 This equation depends on the temperature.

Around 7000C, for example, sulphate formation is observed for zinc and copper, whereas at higher temperatures (850 to 9000C) these sulphates are decomposed into oxides.

 With regard to iron, iron oxide is even formed at lower temperatures (500 ° C.). The characteristics of the iron oxide formed, however, are variable and depend on the temperatures and the duration of the roasting.

 If leached a grilled material at temperatures below 6500C, in an acidic medium, there is a partial solution of the iron present in these materials.

 Contrary to this generally accepted trend, the applicants have now unexpectedly discovered that the recoverability of precious metals from the sulphurous materials by roasting and leaching can be considerably improved if the roasting operation is carried out. a lower temperature.

 The present invention therefore relates to a process for the treatment of sulphurous, gold-bearing and silver-bearing materials, by roasting these materials and by lixiviating the roasting residues by means of cyanidation solutions or thiourea acid solutions, in which the reaction is carried out. roasting operation at a temperature below 6500C in order to improve the recovery of gold and silver.

 In the process according to the invention, this roasting operation can take place either in 2 steps, in which case the endothermic reducing operation can be carried out at the usual temperatures while the exothermic oxidizing step is conducted at a temperature below 6500.degree. in a single exothermic step the temperature being maintained below 6500C throughout the roasting operation.

 The preferred conditions of the process according to the invention correspond to roasting temperatures of between 500 and 6500.degree. C., more particularly between 550 and 6250.degree.

 According to one embodiment of the invention, it is possible, after roasting, to subject the material obtained to a prior leaching in a slightly acidic medium in order to substantially eliminate the soluble iron oxides and sulphates. soluble.

 Then the residue of this operation is subjected to a leaching operation by means of an acid solution of thiourea.

 In this embodiment of the invention it is preferable to conduct the pre-leached leaching operation, intended to remove soluble iron, in the presence of a reducing agent. This reducing agent may especially and preferably be selected from SO 2 and magnetic pyrites, such as pyrrhotite.

A most preferred embodiment of the invention consists in carrying out the following sequence of operations: roasting in one or two stages of the material
sulphurous, gold and silver at a temperature
between 500 and 6500C; - lixiviation of the roasting residue with a
sulfuric acid solution in the presence of SO2; leaching of the residue thus obtained with the aid of a
acid solution of thiourea.

 Other features and details of the invention will become apparent from the following description of some embodiments thereof, given for purely illustrative purposes.

Example 1

A material containing Au 35 g / t, Fe 35.85, S 29.5 is roasted in a reducing atmosphere for 2 hours at a temperature of 7000C.

It is then divided into several fractions which are roasted for 4 hours in an oxidizing atmosphere at different temperatures (500 ° C., 55 ° C., 6000 ° C., 650 ° C. and 700 ° C.).

The various roasted products are then leached in a thiourea solution.

The operating conditions are as follows
dilution: 40 g of roasted product
solution
composition of the solution
water: 1 liter
thiourea: 20 g
pH: 0.8 adjusted with sulfuric acid
temperature: 600C
duration: 4 hours.

 The leach residue is filtered and washed with half a liter of thiourea solution.

Analysis of leaching yields of different products.

Temperature
<tb> 500 C <SEP> 550 C <SEP> 600 C <SEP> 650 C <SEP> 700 C
<tb> of <SEP> roasting
<tb> Fe <SEP> At <SEP> Fe <SEP> At <SEP> Fe <SEP> At <SEP> Fe <SEP> At <SEP> At <SEP> At
<tb>% <SEP> g / t <SEP>% <SEP> g / t <SEP>% <SEP> g / t <SEP>% <SEP> g / t <SEP>% <SEP> g / t
<tb> roasted <SEP> 48.75 <SEP> 49 <SEP> 51.70 <SEP> 51 <SEP> 52.35 <SEP> 52 <SEP> 52.80 <SEP> 52 <SEP> 53.1 <SEP> 53
<tb> residue <SEP> of
<tb> leaching <SEP> 11.7 <SEP> 1.1 <SEP> 41 <SEP> 2 <SEP> 51.35 <SEP> 2.5 <SEP> 57.9 <SEP> 9.9 <SEP > 57.9 <SEP> 12
<tb> weight <SEP> residue
<tb> leaching <SEP> 0.32 <SEP> 0.535 <SEP> 0.685 <SEP> 0.875 <SEP> 0.9
<tb> on <SEP> weight
<tb> grilled
<tb> yield <SEP> of
<tb> setting <SEP> in <SEP> 92.32 <SEP> 99.28 <SEP> 57.57 <SEP> 97.90 <SEP> 32.81 <SEP> 96.71 <SEP> 4.05 <SEP> 83.34 <SEP> 1.86 <SEP> 79.62%
<tb> solution
<Tb>
It can be seen that the dissolution efficiencies of ltor- are practically constant and are close to 100% between 500 and 6000C and that abruptly at 6500C there is a decrease of the yield of nearly 15%.

 Example 2

The material, identical to that of Example 1, was roasted in a first test at 6000C for 4 hours and in a second test at 6500C for 4 hours also.

The following grilled products were obtained
6000C 6500C Temperature
Fe 49.3% 51.3%
At 48 g / t 50 g / t
S sulfate 1.2846 t, 25%
S sulphide 0.15% 0.15%
These two samples were leached in a thiourea solution.

The operating conditions are as follows
dilution 40 g of grilled product / liter of solution
composition of the solution: water: 1 liter
thiourea: 20 g
pH: 0.8 adjusted to
medium sulfuric acid
temperature: 600C
duration: 4 hours.

The leach residue was filtered and washed with half a liter of thiourea solution.

Final results
Toasting temperature 6000C 6500C weight of leach residue on grilled weight 0.723 0.917 residue analyzes
Fe 52.5% 56s1fo
At 2.8 g / t 11 g / t
Solution yields
Fe 23.01% 0%
At 95.78% 79.83%

Claims (6)

CLAIMS 1. Process for the treatment of sulphurous materials, gold and silver, by roasting these materials and by lixiviation of the roasting residues using cyanidation solutions or thiourea acid solutions, intended to improve the recovery of gold and silver, characterized in that the toasting operation is carried out at a temperature below 650 C. 2. Method according to claim 1, characterized in that the toasting operation takes place in two stages and that the exothermic oxidizing step is conducted at a temperature below 6500C.  3. Method according to claim 1, characterized in that the toasting operation takes place in a single exothermic step, the temperature being maintained below 6500C throughout the roasting operation. 4. Method according to any one of the preceding claims, characterized in that the toasting operation is performed at a temperature between 500 and 6250C. 5. Method according to any one of claims 1 to 3, characterized in that the toasting operation is performed at a temperature between 575 and 6250C. 6. Method according to one or other of the preceding claims, characterized in that after the roasting operation, the residue is subjected to a prior leaching to substantially eliminate the soluble iron and soluble sulphates formed during the roasting below 6500C, and then the residue of this operation is subjected to a leaching operation by means of an acid solution of thiourea.  Process according to Claim 6, characterized in that the leaching step prior to removing soluble iron and sulphates is carried out in the presence of a reducing agent. 8. Process according to claim 7, characterized in that the reducing agent used for the preliminary lixiviation operation is chosen from SO 2 and magnetic pyrites, such as pyrrhotite. 9. The method of claim 1 characterized by the following sequence of operations: - grilling in one or two stages of the sulfide material  reuse, gold and silver at a temperature  between 500 and 6250C; - lixiviation of the roasting residue with a  sulfuric acid solution in the presence of SO2; leaching of the residue thus obtained with the aid of a  acid solution of thiourea.