New Arseno refractory process of interest to gold miners

When City Resources starts processing ore at its Cinola gold mine late in 1989, it will be using a new refractory treatment process, one that has not previously been used commercially.

The Arseno process, developed by Arseno Processing, is currently being tested at the laboratories of Bacon, Donaldson & Associates and promises to be an economical pretreatment for those ores which in the past have presented problems to gold miners. A brief description of the new process, provided by Dr M. Beattie, P.Eng., follows.

Arseno Processing has developed process technology for the extraction of gold from refractory ores and concentrates. These ores, which do not respond to conventional extraction processes, are rapidly becoming the primary targets for mining companies around the world.

It has been estimated that by the mid 1990s, annual gold production from the Pacific Basin Epithermal Arc in southeast Asia could be as high as 550 million tonnes.

Within this area, the Porgera deposit under development by Placer, Renison and nim, and the Lihir Island deposit under development by Kennecott have been shown to be refractory and require pretreatment before the gold can be extracted. Additional deposits which are now starting to be developed in this area are also expected to require such pretreatment.

Numerous refractory deposits are also known in Australia, North America, China and other parts of the world. Samples from many deposits throughout the world have been tested using the Arseno process conditions and consistently successful results have demonstrated its widespread application. The samples which have been tested include ores in which the gold is locked in pyrite, marcasite or arsenopyrite.

The process is insensitive to the sulphur content of the feed. Feed materials containing from 1% to 50% sulphur have been tested successfully with the process.

Sulphide minerals are dissolved so that the contained gold becomes accessible for subsequent gold recovery. This gold recovery is achieved by cyanidation of the process residue. For many of the materials which have been tested, gold extractions in excess of 95% have been achieved.

The advantages of the Arseno process over the traditional roasting include, in addition to increased gold recovery, the elimination of any need to handle sulphur dioxide gases or arsenic trioxide products. The sulphur in the feed ore or concentrate is oxidized to sulphate and is subsequently precipitated as calcium sulphate. Any arsenic present in the feed is oxidized to arsenate and is precipitated with the iron as ferric arsenate. Process description

Process flowsheets for treating ore or concentrate are shown in Figures 1 and 2. The operation of the Arseno leach is the same in each case.

The leach is a nitrate catalyzed oxygen pressure leach. The catalytic nature of the process results in very high reaction rates so that sulphide decomposition is complete in less than 15 minutes. The conditions required to maintain the process reactions are moderate with standard conditions being a temperature of 100 degrees C and oxygen pressure of 700 kPa. When treating concentrate, the reactor must be cooled to remove the heat generated by leaching reactions.

The slurry discharges from the reactor into a gas separation vessel to allow for the recovery of the nitric oxide gas generated in the process. This gas is recycled to the reactor as the source of the catalyst.

It has been determined that more than 99% of the nitrate reagents are recycled in the process. The total concentration of nitrate species available in solution varies from 0.2 N to 3 N depending on the nature of the feed.

The slurry leaving the gas separation vessel consists of the solid residue which includes the gold and a solution which contains the dissolved iron, sulphur and arsenic. When ore is being treated this slurry is neutralized through the addition of limestone and lime. Approximately 80% of the neutralization can be accomplished with limestone while the balance is carried out with the lime. The neutralized slurry is treated for gold recovery by conventional methods as the carbon-in-pulp process.

When concentrates are being treated by means of the process the slurry leaving the gas separation vessel is subjected to a solid-liquid separation. The solid residue resulting from the leaching of concentrates frequently represents only 10% to 20% of the feed weight. The gold recovery section of the plant is therefore very small.

Gold leaching rates are also generally high so that retention times of less than 12 hours may be appropriate. The solution derived from the solid-liquid separation is treated for impurity control by means of limestone and lime additions. The precipitate which is formed contains calcium sulphate, hematite and ferric arsenate. The stability of these precipitates has been tested and results indicate them to be suitable for disposal in a tailings pond.

An added advantage to the Arseno process over alternate technologies is that high silver recoveries can be achieved without the need for additional process steps. Since jarosite precipitates are not formed during the leach, any silver which is dissolved can easily be recovered from solution. Testing the process

Initial development of the Arseno process was carried out with batch laboratory-scale autoclaves utilizing 100-400 grams of feed for each test. Once the basic process parameters were defined, additional test work was done to demonstrate that the process could operate in a continuous mode.

This work was performed by means of two laboratory autoclaves operating in series. The throughput of this apparatus was nominally 2.5 kg per hour. Results were very encouraging and indicated that continued development of the process was warranted.

Patents were granted in Canada and the United States covering the basic process chemistry. Additional coverage relating to the over-all process and reactor design is pending.

During 1986 a test campaign was undertaken at a scale of 40 kg per hour. The objective was to obtain additional engineering data for scale-up of the process. Data was also collected regarding the removal of heat from the leaching reactors and the recovery of nitric oxides used to catalyze the oxidation reactions. These studies were carried out utilizing instrumented heat exchangers. Results demonstrated that heat dissipation within the process will not pose any problems and can in fact be controlled for optimum performance of the over- all process.

Similarly, nitric oxide recoveries greater than 99% were achieved. The results achieved in continuous test work carried out to date indicate that scale-up of the process will be achieved readily.

During September and October, 1987, a continuous pilot plant was operated. The capacity of this pilot plant was one tonne per day when treating low sulphide ore. The test campaigns which were conducted demonstrated the process to be viable on a continuous basis and indicated that scale-up to a commercial size would not pose any problems. Samples from two deposits were tested in the plant and both gave acceptable results in terms of sulphide oxidation and subsequent gold extraction.

Batch laboratory testing of several other ores and concentrates is currently under way. This test work is performed at the laboratories of Bacon, Donaldson & Associates, under contract to Arseno processing.