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Focus on improving gold processing

26th February 2016

By: Lynne Jackson

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South African State-owned research and development organisation Mintek is trying to advance gold processing amid the weak market to reduce the cost involved in extracting the yellow metal and aid struggling mining companies.

The gold industry has been using the same processing methods for decades, with very little development taking place. The new methods being introduced by Mintek are “more efficient, saving mining companies time and money”.

Mining Weekly reported in February that the gold price was expected to average $1 103/oz for the full year – $57/oz below the actual average price in 2015 – with those surveyed predicting prices of between $960/oz and $1 225/oz.

This forecast is according to London Bullion Market Association’s 2016 Forecast Survey, in which 36 analysts participated, and in sharp contrast to the gold price of $1 921.17/oz in 2011.

However, the ‘Gold Survey 2015 Q4 Update and Outlook’ report released by Thomson Reuters GFMS last month predicts that the gold price is set for a gradual recovery this year, trading above $1 200/oz towards year-end and averaging $1 164/oz.

With this expected uptick, the new methods developed by Mintek are being positively received.

Mintek principal scientist in the gold processing group Sonestie Janse van Rensburg tells Mining Weekly that to solve the major challenges faced by the gold industry in South Africa, a paradigm shift may be needed with regard to the methods of mining. The current gold extraction technology used in South Africa is effective but not always as efficient as it can be.

“The processing of gold ore, especially in South Africa, is becoming more expensive, the profit margins are shrinking and the pressure is on operations to work as effectively as possible.

Advanced Leach Facility

One of Mintek’s processing developments involves improving the efficiency of gold plant para- meters, such as the operation of gold recovery circuits, using information from Mintek’s advanced leach facility (ALF).

The facility was commissioned in 2009 and has been servicing commercial projects for the past five years. It aims to provide an understanding of the leach chemistry of a specific sample to assist in advancing selected plant operating conditions.

The ALF laboratory can perform five automated leach tests simultaneously, with online analysis of cyanide (including cyanide speciation) and dissolved oxygen, as well as the simultaneous measurement of pH levels, reduction-oxidation, temperature and conductivity.

ALF can be used to determine the best timing of mining operations, reagent consumption, minimum environmental impact and gold recovery in relation to other parameters, such as oxygen consumption.

Smart System

Mintek has developed a gold process advisor (AuSimPro) system over the past three years, which has been prompted by the expansion of existing mines, the reclamation of dumps and the processing of complex and refractory low-grade orebodies.

Many South African gold projects are experiencing time and budget constraints, which necessitate focused development and testwork programmes to provide the relevant information to compare available processing options and economic trade-offs.

The system is a growing database in which Mintek’s knowledge on historical and current projects are captured, including information on gold grades, site location, mineralogical data on the ore and the occurrence of gold, gold recovery rates and the treatment steps applied.

Mintek’s clients are using AuSimPro free of charge to test and improve the system.

“The knowledge contained in AuSimPro results in better decision- making and cost and time savings by increasing the efficiency and focus of development work,” explains Janse van Rensburg.

POX Facilities

Pressure oxidation (POX) involves the oxidation of sulphides by oxygen at elevated temperature and pressures. During recent years, there has been an increasing interest in this technology especially with regard to the processing of refractory gold ores, which requires oxidation prior to gold recovery.

Mintek has, thus, expanded its POX leaching facilities, which now include three batch autoclave vessels and a continuous autoclave pilot plant.

A standard testwork programme when investigating the feasibility of POX leaching includes extensive batch POX optimisation testwork followed by an expensive continuous POX campaign. However, owing to the current economic climate, not all clients can afford the entire testwork programme, thus, Mintek has developed a model that can predict plant performance should POX be included in the process flowsheet.

The POX model was simulated using kinetic data from batch tests conducted on a flotation concentrate.

“Significant information regarding a suitable operating window for the treatment of material was obtained from the model. “During the economic study, it was found that none of the reagent costs seem to be excessive and the gold to sulphide ratio in the concentrate was still high enough to justify POX as an economic pretreatment method,” concludes Mintek engineer Jakolien Strauss.

Edited by Tracy Hancock
Creamer Media Contributing Editor

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