AU2009101298B4 - Process for recovering gold otherwise lost to the antimony bearing PLS from alkaline leaching of aurostibnite ores - Google Patents

Description

Confidential AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Process for recovering gold otherwise lost to the antimony bearing PLS from alkaline leaching of aurostibnite ores The following statement is a full description of this innovation including the best known method for performing it. Rohan Bose Page 1 16/12/2009 Confidential Description 1 Overview Antimony occurs in aurostibnite bearing ore, a mineral that also contains gold. Due to the recent increases in the gold metal price, the minerals industry perceives aurostibnite bearing ores primarily for the production of gold with antimony as a by-product. As gold is a precious metal, maximising its recovery is paramount during the processing of these aurostibnite bearing ores. Gold is recovered from aurostibnite through the process of cyanidation, as it is the most economical method currently available. However, the presence of antimony during cyanidation significantly reduces the recovery of gold. The presence of only 100 ppm antimony (also perceived as 0.01% of antimony in ore) reduces the gold recovery during cyanidation to below 4% (Bose 2009). This reduction in gold recovery during cyanidation facilitates the need for a process prior to cyanidation that can effectively remove antimony. One plausible process is alkaline leaching of aurostibnite ores using sodium sulphide (Na 2 S) and sodium hydroxide (NaOH) (Ubaldini, Vegli6 et al. 2000; Bose 2009). During this process antimony dissolves into the pregnant leach solution (PLS) from alkaline leaching as sodium ortho thioantimonite which can then be electrowon to form an antimony metal product on the cathode (Ubaldini, Vegli6 et al. 2000). This method gives antimony recovery between 80 and 100% (Bose 2009). Unfortunately, during the alkaline leaching process up to 10% of the gold, possibly that which is directly associated with the aurostibnite mineral itself, also dissolves into the alkaline leach PLS giving rise to a need for a process to be found that succeeds alkaline Rohan Bose Page 2 16/12/2009 Confidential leaching and recovers the gold dissolved in the PLS. If this dissolved gold is not recovered, it is lost to the antimony electrowon product. This document outlines a postulated theory that forms the basis for this innovation and a process that is validated after practical research conducted to recover the gold dissolved (i.e, lost) to the antimony bearing PLS during the alkaline leaching process. In so doing, this process increases the overall gold recovery from aurostibnite bearing ores during its processing for gold and antimony. 2 Basis of innovation 2.1 Background to the theory During alkaline leaching the aurostibnite bearing ore is mixed in an aqueous solution medium comprising of sodium sulphide (Na 2 S) and sodium hydroxide (NaOH). During this process, the antimony from the ore dissolves as antimonites. The sodium sulphide dissolves the antimony from the stibnite to form soluble sodium thioantimonite (Ubaldini, Vegli6 et al. 2000). This is essentially a two-step process. In the first step, sodium sulphide reacts with stibnite to form sodium meta-thioantimonite (NaSbS 2 ). In the second step, sodium meta-thioantimonite further reacts with sodium sulphide to form sodium ortho-thioantimonite (Na 3 SbS 3 ) (EQ 1 & 2) (Anderson 2000). Na2S (aq) + Sb 2

S

3 (s) - 2NaSbS 2 (aq) (EQ 1) NaSbS 2 (aq) + Na 2 S (aq) - Na 3 SbS 3 (aq) (EQ 2) The presence of NaOH in the alkaline leach solution prevents the hydrolysis of sodium sulphide, which produces toxic hydrogen sulphide (H 2 S) gas (EQ 3) (Ubaldini, Vegli6 et al. 2000). Rohan Bose Page 3 16/12/2009 Confidential Na2S(aq) + 2H 2 0(l) - H2S(g) + 2NaOH(aq) (EQ 3) During the process of alkaline leaching, to a lesser extent, the sodium hydroxide dissolves the stibnite to sodium oxo-thioantimonites and sodium meta-thioantimonite. (EQ 4) (Ubaldini, Vegli6 et al. 2000). The sodium meta-thioantimonite further reacts with the sodium sulphide to form sodium ortho-thioantimonites (EQ 2). Sb 2

S

3 (s) + 2NaOH(aq) <-+ NaSbOS(aq) + NaSbS2(aq) + H 2 0(l) (EQ 4) 2.2 Postulated they Given that the alkaline leach PLS contains sodium hydroxide, if a neutralising agent, (i.e., an acid such as HCI) is introduced to the PLS from alkaline leaching, the H+ ions from the acid will react with 01- ions from the NaOH to form water. Thus it is postulated that this reduction in 01- ion concentration in the PLS will lead to the reversal of (EQ 4) thus forming a Sb 2

S

3 precipitate. In doing so, it will facilitate a need for NaSbS 2 in solution, which will in turn reverse (EQ 2). This reversal of EQ4 and EQ2 simultaneously should remove antimony from the PLS as a Sb 2

S

3 precipitate, leaving its gold content as dissolved ions in solution. This otherwise lost gold, which was previously unrecoverable, can now be recovered from the antimony ion free gold bearing solution. 3 Materials and methods used to prove the theory 3.1 Reagents The following reagents were used: 32% hydrochloric acid (Analytical Reagent, Sigma Chemicals Pty. Ltd.), distilled water (laboratory grade, Kalgoorlie Metallurgical Laboratory). Rohan Bose Page 4 16/12/2009 Confidential 3.2 Samples Used The samples used were the PLS collected following the alkaline leaching bottle roll tests conducted. 3.3 Equipment The following equipment was used in the study: General laboratory glassware; 2 L vacuum flask, vacuum pump (Oerlikon Leybold Vacuum); Analytical filter paper (Hollingsworth and Vose Company Ltd.); Aschefrerie Rundfilter MN 640dd 12.5cm filter paper (Macherey-Nagel Inc.); Magnetic stirrer and hot plate (Model: 4803-02, Cole Parmer Instrument Company Inc.); 25mm and 50mm stirring magnets. 3.4 Sample Preparation A 5 L batch of alkaline leach PLS was prepared by mixing samples collected from each of the alkaline leaching bottle roll tests conducted. 3.5 Analytical Method Analyses of the solution from all tests were conducted by an inductively coupled plasma optical emission spectrometer (ICP-OES). All PLS were sent to the Kalgoorlie Metallurgical Laboratory where they were analysed for their gold and antimony contents. The gold content of the solutions were analysed using 5, 10 and 15 ppm standard solutions. The standards used for analysing the antimony content in the solution were 5, 10,15 and 100 ppm. Rohan Bose Page 5 16/12/2009 Confidential 3.6 Testing Method to validate the postulated theory Place an 1100 mL sample of the alkaline leach PLS (prepared as mentioned in Section 3.4) into a 2 L beaker and take a 20 mL sample of the solution using a 20 mL pipette. Place the sample of the PLS in a sealed container. Analyse the sample for its gold and antimony contents using an ICP-OES. Fill a I L volumetric flask to the mark with alkaline leach PLS from the 2 L beaker. Place a 24 or 50 mm stirring magnet in a 1 L conical flask and fill it with the PLS from the 1 L volumetric flask. Place the 1 L conical flask on a magnetic stirrer and set the stirring rate to a setting of 3 on the dial. Fill a 100 mL graduating burette to the 0 mL mark. Drop wise add the desired amount of 32% HCI from the burette to the 1 L conical flask containing the alkaline leach PLS. Let the solution react for the desired amount of time and take a 20 mL sample using a pipette. Filter the sample using a MN 640dd filter paper placed in a funnel, whose output end is placed in a sealable container. This will collect a solids free solution sample which can be analysed for its gold and antimony content using an ICP-OES. Repeat the process from the acid addition point to the analysis of sample for any extra HCI added from here on. Once the acid addition section of the test is completed it is noticed that a significant amount of precipitate is formed in the solution. The precipitate can be separated from the solution by doing the following: A I L vacuum filter funnel with a latex seal on the output spout is placed on a 2 L vacuum flask attached to a vacuum pump. Start the vacuum pump and arrange MN 640dd filter paper on the input of the vacuum funnel so that all holes to the input are covered. Wet all the MN 640dd filter paper with distilled water to check that a sufficient vacuum is created. Place an analytical filter paper over the wetted MN 640dd; wet the analytical filter paper in order for it to set along the shape of the vacuum funnel. Empty the distilled water collected in the vacuum flask to avoid dilution of the sample. Slowly place the Rohan Bose Page 6 16/12/2009 Confidential contents of the 1 L conical flask into the vacuum funnel. Once all the solids have been separated from the liquid take a 20 mL sample of the liquid and analyse it for its gold and antimony contents using an ICP-OES. 4 Summary of the Description The antimony in the aurostibnite significantly reduces the gold recovery during cyanidation. An antimony content of 0.01% can effectively reduce the gold recovery to below 4% (Avraamides, K. Drok et al. 2000; Bose 2009). This facilitates the need for a process to precede cyanidation that removes the antimony. A plausible method is alkaline leaching using Na 2 S and NaOH. This process can recover/remove 70 to 100% of the antimony from the aurostibnite bearing ore, depending on operational/experimental parameters, namely reagent addition and leaching time (Ubaldini, Vegli6 et al. 2000; Bose 2009). However, during the alkaline leaching process 4 to 9.25% of the gold from the aurostibnite bearing solids ore feed also dissolves in the PLS (Bose 2009). This section contains (a) Results of a series of tests using the method (as explained in Section 3.6) to substantiate the theory that a pH neutraliser (i.e.an acid) added to the antimony-gold bearing alkaline leach PLS will precipitate the antimony as Sb 2

S

3 . This precipitate can be removed via a solid/liquid separation to isolate the Sb 2

S

3 solids from a gold ion bearing filtrate. The section also contains (b) Recommendations for handling and further treating the two byproducts of the proposed process. 4.1 (a) Results of a series of tests The acid addition to a point around the neutral pH precipitates practically all the antimony as an orange precipitate. This precipitate has the industrial name antimony orange with a chemical formula Sb 2

S

3 and is a saleable product. This coincides with the chemical formula of the compound theorised to be precipitated. However, given that the precipitate has the same chemical formula as the mineral Rohan Bose Page 7 16/12/2009 Confidential stibnite, its orange colour can be attributed to the change in its chemical bond structure. The antimony orange precipitated can be separated from the filtrate theorized to contain the gold originally dissolved in the alkaline leach PLS. Primary investigations reveal that the gold does remain in solution after the removal of the antimony orange. However, to maximize the gold recovery as dissolved gold ion in the PLS it is paramount to facilitate an appropriate reaction time and a pH below 7. This was validated by conducting two experiments. In the first, a 30 minute reaction time and 20 mL acid addition (pH = 7) results in 90% of the gold remaining dissolved in the PLS/filtrate. In comparison, in the second, a 30 minute reaction time and 25 mL acid addition (pH = 1.35) ensures that 99% of the gold remains dissolved in the PLS/filtrate. In addition, the second experiment reveals that surprisingly the gold first precipitates out of solution along with the antimony and then re-dissolves back into solution. 4.2 (b) Recommendations for handling and treatment of the two byproducts The proposed process of adding an acid to the alkaline leach PLS produces two byproducts, namely an antimony orange precipitate and a gold bearing acidic solution. Following are recommendations for handling and further treatment of the two process byproducts: 4.2.1 Antimony orange precipitate Following the proposed process of acid addition to the alkaline leach PLS, the antimony orange precipitate produced can be separated from the gold bearing filtrate using a Larox C or M series pressure filter. This will produce an antimony orange filter cake that can be collected and sold as a pigment. Rohan Bose Page 8 16/12/2009 Confidential However if it is decided that antimony metal is to be produced via electrowinning, possibly to recover the sulphide ions which can be recycled back into the alkaline leach process, the antimony orange cake can be introduced to a NaOH solution which will dissolve it at room temperature (Hurst and Stack 1917). This can be done by two methods: one is where the filter cake is collected mixed with a NaOH solution in a continually stirred tank reactor (CSTR). The other method of dissolving the antimony orange filter cake utilises a feature specific to the Larox M or C Series pressure filters thus justifying their use. These pressure filters have a wash cycle between subsequent filtration cycles. The wash cycle inlet can be attached to a NaOH bearing solution stream and the outlet stream can be sent to be electrowon for antimony recovery. 4.2.2 Antimony free gold bearing solution Following the solid/liquid separation process, the gold bearing acidic filtrate can be collected. The gold from this acidic solution can be recovered using the following methods: 1. By adding NaOH or any other pH altering agent to increase the pH of the solution to between 11 and 12. The solution can then be merged with the cyanidation electrowinning or adsorption (i.e. CIP) streams depending on the effect of cyanidation on the recovery of gold ions dissolved in the filtrate. 2. Gold dissolved in the acidic filtrate can be recovered using MINIX resin marketed for its selectivity to gold. The rate of gold adsorption onto resin is greatly enhanced with the presence of abundant chloride ions in the antimony free gold bearing solution resulting from the proposed process. Rohan Bose Page 9 16/12/2009 Confidential 3. The other method of recovering the gold is to electrowin the gold bearing acidic solution produced by the proposed process directly or after the addition of NaOH to ensure an alkaline pH. Rohan Bose Page 10 16/12/2009

Claims (5)

1. When an acid is added to the aqueous PLS from alkaline leaching containing antimony and gold ions, it removes the antimony from the PLS as an orange coloured Sb 2 S 3 precipitate while leaving the gold ions in solution. This otherwise lost gold which is present in antimony free solution can be recovered thus increasing the overall recovery of gold from aurostibnite ores.

2. If the process as claimed in Claim 1 is employed, it removes antimony from an alkaline leach PLS as an orange antimony precipitate. It is claimed that in order to remove 99% of the antimony as the antimony orange it is paramount that the amount of acid added is enough to adjust the pH of the PLS to 7 or below.

3. When employing the process described in Claim 1, the amount of acid needed to precipitate/remove antimony from the alkaline leach PLS is directly proportional to the hydroxide ions in the solution matrix that were introduced during the preceding alkaline leaching process. It is therefore claimed that to minimize acid addition, the amount of sodium hydroxide added during the alkaline leaching process is optimized to a point enough to prevent the effervescence of hydrogen sulphide.

4. It is further claimed that the amount of gold that remains dissolved in solution is directly influenced by the amount of acid added. During the process of acid addition, the gold first precipitates out of solution and then beyond this point, as acid addition is increased the gold re-dissolves. Rohan Bose Page 11 16/12/2009 Confidential

5. Given that the process outlined in Claims 1 to 4 hold up with successful removal/recovery of otherwise lost gold, it is further claimed, that the tail stream is recyclable back to the beginning of the proposed process contained in Claim I thus reducing acid consumption. ROHAN BOSE 16 DECEMBER 2009 (Name of Applicant) (Date) Rohan Bose Page 12 16/12/2009