MINERAL PROCESSING RESIN ON THE RISE

“They offer a reduction in extraction time from days to hours,” Dr Riveros told The Northern Miner Magazine “Pulps can be advanced on a cycle of approximately two hours rather than a whole day The time saved amounts to about a 10-fold improvement The ion exchange processes enable higher loading capacities and cheaper elution, and regeneration of the resin ”

The most promising system replaces activated carbon with a new resin produced by Dow Chemical At the pilot plant stage, the resin was fed into a 5-stepped tank system The next stage will employ a single column, and CANMET is looking for an industrial partner for an on-site demonstration test Dr Riveros feels the process at its current stage of development is particularly attractive for small- and medium-sized operators

The ion exchange systems overcome current inefficiencies of gold losses in the fines (which can amount to 2% or 3%), slow kinetics, and carbon’s susceptibility to fouling, right down to contamination by traces of petroleum from mill machinery Resins do not suffer from the same low tolerance as activated carbon to clays and fine materials The cost of the resins is higher than activated carbon, but the time advantage and the relatively low cost of resin regeneration should more than offset that negative The system is under patent registration

“We have had calls from the mining industry asking for help with activated carbon problems in their gold extraction processes,” Dr Riveros said “Some mills experience carbon contamination by materials that occur in the ores The basic method developed in the current research will be adjustable within limits to counter an operator’s specific problems with activated carbon ”

In addition to the bench model and mini-pilot plant at CANMET, ion exchange processes have been tested extensively under contract by Witteck Development of Mississauga and the Ontario government’s ORTECH (formerly the Ontario Research Foundation) The field tests will be conducted as an Energy, Mines and Resources- sponsored project of ORTECH and IWT Himsley International IWT Himsley will supply its patented countercurrent continuous ion exchange column The mobile unit will handle 65 to 120 l of solution per minute Dr Riveros is confident that an industrial partner will be found soon and that the technique will provide a badly needed solution for the gold industry The chemistry of gold recovery with ion exchange resins has been studied extensively South African research has identified three specific cases in which rip appears to be more effective than cip The extraction of gold from pulp obtained by cyanidation of roasted pyrite concentrates was found to produce loading that was severely retarded by the adsorption of a skin of fine particulate calcine on the carbon surface In addition, the temperature of these pulps (generally some 10 degrees to 20 degrees c above ambient), reduced the capacity of the activated carbon for gold significantly It is believed that highly oxidized materials, clays and shales will behave similarly to calcine and that rip could perform more efficiently than cip in the treatment of these materials

In the event of pulp contaminated with flotation reagents reporting to cyanidation and cip without prior roasting, the carbon will become poisoned Such contamination has a serious effect on the capacity of carbon for gold, but it has little effect on the capacity of ion exchange resins

According to their chemical properties, ion exchange resins are classified as strong-base or weak-base Strong-base resins can extract gold at almost any ph Other metal cyanide complexes that may be present in the cyanide solution are also extracted Unless the concentration of base metals is exceptionally high, the gold loading on a strong-base resin is of the same order as the gold loading on activated carbon This lack of selectivity can be a positive aspect, because the treatment of the effluent solutions is facilitated and, simultaneously, a considerable amount of cyanide can be recycled

Strong-base resins are difficult to elute, and this fact has prevented the development of a commercial process in the past In recent years, however, a series of new or improved elution methods have been developed that show a great deal of promise The oldest elution method, developed in the U S S R , uses acidic solutions of thiourea Under strongly acidic conditions, the gold-cyanide complex breaks down and a gold-thiourea complex forms which, being positively charged, is not retained by the strong- base resin

In the Soviet process, zinc, nickel and free cyanide are eluted first by dilute mineral acid Elution of copper follows from an ammoniacal solution of ammonium nitrate and, finally, gold and silver are eluted with acidified thiourea and electrowon continuously

South African researchers have focused their efforts on the use of zinc cyanide and thiocyanate as eluants for gold Their results are also promising In Canada, joint research of CANMET and ORTECH has led to an improved elution procedure that uses thyocyanate dissolved in di-methyl formamide (DMF) This method appears to be superior to others

Weak-base resins are those having primary, secondary or tertiary amine groups Characteristically, these amine groups are active for ion exchange only in neutral and acidic media In alkaline solutions, the amine groups become inactive Because of this, weak-base resins do not extract gold significantly unless the cyanide solution is neutralized This gives rise to some hazards because of the evolution of hydrocyanic acid However, the great advantage of weak-base resins is that they can be eluted readily with an alkaline solution CANMET is in the process of patenting a procedure to neutralize cyanide solutions safely and efficiently Alternatively, research is under way to synthesize weak-base resins that are active at a higher pH in order to avoid the need to lower the pH of the cyanide solution Dow Chemical has recently introduced one of these resins

South African researchers investigated the long-term performance of the weak-base resin Duolite A7 DU through 45 cycles of adsorption, elution and regeneration at ambient temperature It was concluded that the performance of the weak-base resin will remain satisfactory for a period of at least two years in a cyclic operation involving elution with sodium hydroxide and regeneration with sulphuric acid There is no reason why the service life of the resin should not be considerably longer than two years Pilot Testing

The only problem found was that other metals (silver, zinc, lead, and nickel) tend to be poorly eluted by pure sodium hydroxide This is probably the result of the decomposition of some of the cyanide complexes at low ph The presence of free cyanide in the eluant remedies the problem, but the operational costs would increase significantly Alternatively, these metals can be eluted during the regeneration of the resin with sulphuric acid, although the acid consumption increases Despite the added cost of removing thes
e metals from the resin, it is believed that a process using a weak-base resin is a promising alternative for the recovery of gold

Three pilot plant tests were run by the Council for Mineral Technology (MINTEK) in South Africa to compare the performances of ion exchange resins and activated carbon The conclusion of this work was that both strong- and weak-base resins extract gold cyanide efficiently from pulps under the same operating conditions as those generally adopted in CIP

Further South African testing with ion exchange resins was carried out in a NIMIX (National Institute of Metallurgy Continuous Ion Exchange) column This equipment had been used successfully in the treatment of unclarified solutions in the uranium industry The study concluded that either a weak- or strong-base resin can be used successfully in a NIMIX column to recover gold Both types of resin can be satisfactorily eluted and regenerated, permitting repeated utilization of the resins in cyclic operation

Canadian research that has built on the overseas experience includes CANMET projects carried out by Witteck Development and ortech Witteck undertook to demonstrate the feasibility of a continuous circuit using Dow Chemical’s promising new resin (XU-40114 00) A 10-day pilot plant campaign was run on a resin-in-pulp process No operational problems were encountered during the pilot plant run The pulp flowed readily through the 48-mesh interstage screens, while the resin was retained The interstage screens were not plugged or blocked by the resin beads Although the performance of the experimental resin was irregular (depending on the batch used), Dow Chemical claims it can produce a resin of consistent quality Ion Exhaange Column

ORTECH compared the performances of both activated carbon and ion exchange resins in a CANMET Deep Fluidized Ion Exchange Column originally designed to recover uranium After extensive screening testwork, ORTECH chose the strong- base resin Amberlite IRA-400 (Rohm & Haas) and the activated carbon CALGON GRC 22

The single-stage, fluidized bed, ion exchange column used by ORTECH was built according to the description given by CANMET The loading column was operated with a metered upflow solution, sufficient to fluidize the bed and maintain a desired height The novel CANMET/ORTECH procedure for stripping gold from the strong-base resin was applied in the program It involves stripping with 4 88 M KSCN (potassium thiocyanate) in a 50% water/50% di-methyl formamide solution The claimed advantage of the method is that the stripped gold combines with KCN in the solution and crystallizes out of the solution as KAUCN, from which gold can be obtained

While the gold loadings on both activated carbon and resins were similar, the latter presented considerably fewer transportation problems than carbon In fact, carbon did not fluidize well and it tended to block the valves By contrast, the spherical beads of Amerlite IRA-400 fluidized well and no operational problems were encountered

To conclude, the Canadian research consolidates and advances ion exchange technology for the extraction of gold by improving techniques such as the elution of strong-base resins and showing that cyanide solutions can be neutralized safely Use of the new Dow Chemical resin and its likely improvement holds great promise for the metals industry The work has resulted in a series of potential processes that will be further demonstrated in the pilot plant, using the IWT Himsley column The on-site plant will test all the alternatives and adjust the processes to the specific needs of the processor Norman Avery is a freelance writer based in Ottawa Dr Patricio Riveros joined CANMET about two years ago as a post-doctorate fellow He received his PhD in metallurgical engineering from Queen’s University, in Kingston, Ont , and was a post- doctoral fellow at Alcan, in that city His paper is entitled “Advances in Ion Exchange and Solvent Extraction for Gold Recovery” (Mineral Sciences Laboratories Division Report MSL 88-108 (OP&J)

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