Improving Gold Flotation Recovery at Porgera by Reducing Non-Sulphide Gangue Entrainment

摘要

The Porgera mine and treatment plant are locatedin Enga Province of Papua New Guinea. Gold in the Porgeradeposit is found free and in association with the primarysulphide mineral pyrite. The refractory nature of the gold in thepyrite requires the decomposition of the mineral matrix byoxidation to yield the gold for recovery via cyanidation. Thesulphide concentrate, which is fed to pressure oxidation andsubsequent leaching. is the product of mining, milling, flotationand regrinding.The target of the Porgera flotation circuit is to produce a 14 percent sulphur concentrate grade at maximum gold recovery. Thissulphur concentrate grade is required to provide satisfactorythroughput and oxidation of the refractory ore in the autoclaves.Overall plant gold recovery is therefore controlled by the goldrecovery/sulphur grade relationship in the flotation circuit.Flotation selectivity on certain ores has shown to be poor withthe current circuit design/operational strategy. Reduced goldrecovery with increasing concentrate sulphur grade has been noted.Laboratory testing of the flotation feed has indicated that there isa high percentage of slime ( <5 #mu#m) in the feed, constitutingup to 35 per cent of the total mass. The presence of the slimeresults in excessive non-selective entrainment of non-sulphidegangue, which subsequently dilutes the concentrate and reducesthe gold recovery in order to maintain the desired 14 per cent sulphur grade.Entrainment is unavoidable because of the size of the particlespresent. By manipulating the variables that affect the recoveryof water (or reduce the solids concentration per unit volume), itis possible to improve the concentrate grade and recovery viapartial non sulphide gangue rejection (predominantly silicates).Laboratory testwork examined several operational and chemicalmethods for reducing the level of entrainment. These included:1. Low-density flotation (18 per cent solids, where hinderedsettling does not occur and entrainment minimised).2. Split flotation at 38 microns.3. Bulk roughing followed by dilution cleaning.4. Addition of a gangue depressant -Carboxyl Methyl Cellulose (CMC).Gold recoveries at the target 14 per cent sulphur grade haveshown in the laboratory to increase significantly by the abovemethods depending on the ore type being treated. Plant trials atlow flotation density have verified the laboratory results,however water restrictions and limited pumping capacity haveprevented this option from full-scale implementation. CMCplant trials were on-going at the time of writing. Operational andcircuit modifications consisting of a bulk rougher/scavengerstage followed by a cleaner/scavenger circuit expansion toreduce the excessive trade-off in gold recovery for sulphur gradeare to be installed in November 1997. A projected conservativegold recovery improvement of 1.6 per cent is estimated for allore types to be processed over life of mine.