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Technology Overview
Various processes exist for treating cyanide and to a lesser extent thiocyanate; and may be generally categorised as a recovery process, or a destruction process. The latter processes typically involve the breaking of the carbon nitrogen bonds, thereby destroying the cyanide species producing less toxic species.

Biological degradation of cyanide uses various species of bacteria, fungi and algae known to enzymatically oxidise cyanide. In the case of certain bioleach effluent streams, the reaction of cyanide with reactive sulphide and partially oxidised sulphur surfaces inadvertently produces more thiocyanate and lower residual cyanide, making most of the aforementioned processes unsuitable, and those that are suitable, economically unfavourable. Moreover, the degree to which thiocyanate is produced is significant, ranging anywhere from 20 mg/L to 5 g/L. The reaction pathways as well as the overall reaction that is believed to occur are summarised as:

SCN- + H2O + → HCNO + HS-	                                             undefined

HCNO + 2H2O → NH4 + HCO3-	                               undefined

HS- + 2O2 → SO42- + H+	                   undefined

SCN- + 3H2O + 2O2→ HCO3- + NH4 + SO42- + H+ 	      undefined

ASTER, an acronym for Activated Sludge Tailings Effluent Remediation, is a biological thiocyanate and cyanide destruction process, rendering a non-toxic solution for return and re-use in BIOX plants.

The ASTER process was developed in the late 1990s to deliver improved and integrated water balances in commercial BIOX applications by supporting the ever-tightening environmental legislation on water usage and availability in both arid and tropical regions.

The microbial culture for the ASTER process was isolated at the Pan African Resources Barberton Mine - Fairview Plant and is constituted as a number of composites of organisms drawn from the tailings dam during the 1990s. Figure 1 shows photographs of just two of the usually dominant species, viz. Bosea thiooxidans and Ralstonia eutrophia, while Figures 2 and 3 show the migration of the test scales from developmental through to large scale trialling.

Like the BIOX microbial consortium, the identified ASTER species are non-pathogenic.

Previously, a common challenge for BIOX plants was the inability to recycle cyanide containing water due to the low tolerance of the organisms to thiocyanate and cyanide species. In most gold processing installations, the final plant effluent streams contain thiocyanate and cyanide, and the tailings dam return water cannot be re-utilised upstream in BIOX plants without treatment.

The ASTER process involves the continuous feeding of tailings solutions containing cyanide and thiocyanate to aerated reactors containing the microorganisms to which nutrients are added to sustain and promote microbial growth. Principal constituents of the nutrient suite are phosphorous and potassium salts and molasses.

The microorganisms also need nutrients to sustain optimal bacterial growth added in the form of phosphorous and potassium salts and molasses.

ASTER Plants
The BIOMIN ASTER technology has currently to date been implemented on two commercial operations in South Africa and Kazakhstan. The first installation was completed in 2010 for Pan African Resources at their Consort Metallurgical complex and the second in 2013 for Nordgold at their Suzdal operation.

A third installation is targeted for commissioning in 2015 at the FCF Minerals Philippines BIOX® installation.