AN EFFICIENT APPROACH TO COBALT, COPPER AND NICKEL RECOVERY FROM RAFFINATES, EVAPORATION PONDS, AND OTHER LOW GRADE STREAMS P.A.(Tony) Treasure Managing Director Electrometals Technologies Limited TABLE OF CONTENTS 1.0 BACKGROUND 2 2.0 SUPERLIG PERFORMANCE DATA 4 3.0 EMEW PERFORMANCE DATA 9 4.0 THE PROCESS FLOW SHEET 12 5.0 TYPICAL PROJECT MODEL 15 6.0 SUMMARY OF BENEFITS 16
2 1.0 BACKGROUND The following paper presents an efficient process flow sheet for the recovery of low grade cobalt values, along with attendant copper and nickel, from mine (and other) related primary leach solutions and waste streams. The circuit was developed for recovery of cobalt from raffinate from large scale copper SX-EW operations, but can be adapted to a number of other process environments - including recovery of these metals from tankhouse bleed streams; and the treatment of waste solutions generated by natural leaching of old dumps. The flow sheet utilises two well-proven technologies which together allow for an easier and lower cost approach than alternative methodologies. Each of these technologies - a powerful Molecular Recognition Technology (MRT) product developed by IBC Advanced Technologies, Inc. (U.S.A.); and a high performance electrowinning cell (EMEW ) developed by Electrometals Technologies Limited (Australia) - has its own unique potential benefits in hydrometallurgy in general. For the recovery of low grade metals, their combination provides a resolution to two of the main problems previously faced: 1. The SuperLig product allows selective recovery of both cobalt and copper (either separately or together) from relatively dirty and complex solutions. The SuperLig material is designed to reject both ferric and ferrous iron, thus allowing the recovery of the target metals without any prior chemical treatment. One of the major obstacles faced in treating such solutions with present technology is the high cost and process complexity involved in preparation of the solution prior to extraction of the target metals. For example, more conventional routes generally require prior chemical adjustment to remove ferric iron (or convert it to ferrous state) - a costly process which can be extremely difficult to control in a production environment; and which may lead to significant acid losses in a closed leach circuit. 2. The EMEW cell offers two basic advantages - copper can be recovered, as LME Grade A cathode, from the low grade eluate produced by the MRT system; and copper and cobalt can be selectively electrowon to the exclusion of each other. The recovery of cobalt and copper from low grade streams has always faced significant challenge in separating the two metals. As well as being necessary for the production of a high grade cobalt product, most extractants (both IX and SX) have significantly different affinities for the two metals - to the point where they effectively compete with each other. EMEW allows their effective separation in an easy, non-chemical manner.
3 Final configuration of individual steps in the flow sheet will be determined by site factors - including solution chemistry and purity required in final products. A model flow sheet for combined Cu and Co recovery with approximately 500 ppm Cu and 300 ppm Co in the feed incorporates the following steps. 1. Joint extraction of cobalt and copper from the main raffinate bleed stream using SuperLig 156. 2. Elution of copper and cobalt together from the SuperLig column with a heated, high acid concentration solution. 3. Continuous electrowinning of copper from the eluate at relatively low grade (2 g/l) to produce a high grade cathode using the EMEW cell. 4. Progressive increase of cobalt grade in the eluate through recycling through the column (to 40 g/l) 5. Bleed of a small volume of high cobalt, low copper solution from the elution circuit to a single line of EMEW cells in series to strip residual copper down to below 10 ppm. 6. ph adjustment of the de-coppered solution to 3-3.5, using lime, magnesia or caustic. 7. Electrowinning of cobalt (and minor nickel) on a batch or continuous basis through an EMEW circuit to low concentration (10 ppm) prior to disposal to waste or return to the column feed. This step will require minor continuous ph adjustment with caustic during electrowinning. A final step can be added, using either SX (eg Cyanex 272) or IX to achieve separation of cobalt and nickel from the low volume end solution. The flow sheet has been developed for an operating copper SX-EW project in Australia. It has been independently reviewed by a major engineering group; and judged to be robust from both process and financial viewpoints. Specific details of the facility may change from project to project depending on the copper/cobalt grades, recovery objectives, and site characteristics. This paper provides a summary of one flow sheet, as well as typical performance characteristics of the two technologies; and financial projections for a model project. All process and operating cost characteristics of the flow sheet have been examined in great detail and it is possible, through inputting local cost and project factors, to readily generate a preliminary process and financial model for a particular client. Confirmation of process performance at specific sites can now be rapidly followed by design and installation of a complete plant package, for efficient extraction and recovery of these metals.
4 2.0 SUPERLIG PERFORMANCE DATA The following provides a summary of pertinent data derived through laboratory testing of solution provided by an operating SX-EW operation in Australia, as a typical indication of the IBC SuperLig product s high performance and selectivity. It is noted that the work was conducted on a solution which is higher in copper than will generally be contained in a solvent extraction raffinate - these data therefore being presented as a worst case, with a higher than normal proportion of the lower value metal in solution. The main characteristics of the solution tested were as follows: Co : 270 ppm Cu : 2200 ppm Ni : 68 ppm Fe : 13700 ppm ph : 0.8 The solution was treated through a laboratory lead column trail column circuit; the following charts illustrating the results of progressive assaying of the effluent from each column. It is noted that the data are calculated back to metal loading and flow per kg of SuperLig in the column, in order to facilitate initial analysis by a client of an individual project s needs. The following tables and graphs summarise the result of this programme. Raffinate assays lead column Total column feed (litres/kg of resin) Raffinate assays (ppm) Co Cu Ni 6 0 2 0 12 9 19 12 18 37 100 23 24 68 213 28 30 138 742 48 36 262 796 58 42 301 2190 73
5 Raffinate assays trail column Total column feed (litres/kg of resin) Raffinate assays (ppm) Co Cu Ni 6 0.2 0.1 bdl 12 bdl 0.1 bdl 18 bdl 0.1 2.0 24 0.4 0.1 9.9 30 1.7 0.1 16.2 36 12.4 0.2 17.7 Cumulative recovery cobalt and copper Cumulative gm Cu/Co loaded First column 80 8 gm Cu per kg Superlig 60 40 20 6 4 2 gm Co per kg Superlig Cu Co 0 0 0 10 20 30 40 50 litres per kg Superlig Note : This graph illustrates extremely well the key features of the SuperLig resin in this application - that loading of cobalt and copper occurs at the same rate, proportional to their original concentration in solution, and it is only when the column becomes fully loaded that copper starts to displace the cobalt.
6 Percent recovery cobalt lead column Percent recovery Co First column 120 100 80 %Co 60 40 Periodic Cumulative 20 0 0 10 20 30 40 50 litres per kg Superlig Percent recovery cobalt trail column 120 100 Percent recovery Co Second column 80 %Co 60 40 20 0 0 10 20 30 40 litres per kg Superlig Elution of the first column was carried out with a hot (90 o C) 4 molar sulphuric acid solution; and the metals were shown to completely transfer to the eluate. Less than 2% of the total iron fed to the column reported to elution - this quantity being insufficient to cause difficulties in subsequent direct electrowinning of the target metals. Approximately 50% of the nickel in solution was recovered to the eluate. Although actual operating data will be needed from sites considering implementation of this flow sheet, there are a number of clear observations that can be made from the example provided:
7 1. Assays of effluent from the lead column show depletion of the target metals almost to zero during the first stages of the loading cycle - with break through only occurring when total loading of metal on the SuperLig reaches apparent saturation of the resin. The trail column extracted all of the remaining copper and cobalt in the feed solution. 2. Recovery in the lead column plateaus when total metal loading (Cu+Co) reaches around 65 gm per kg of SuperLig. When loading begins to plateau it is clear that copper starts to push cobalt off the column - indicating that loading should cease at or before this point. 3. Plots for cumulative recovery of copper and cobalt parallel each other very closely - with each metal being recovered at a similar rate in proportion to its original grade in the solution. 4. Secondary treatment of the solution through a trail column resulted in recovery of virtually all of the cobalt in the solution sample. The IBC technology is well proven and can be confidently scaled up from the laboratory testwork provided as an example above. SuperLig systems for the recovery of Copper and other base and precious metals have been scaled up at various locations worldwide. In a production plant, the configuration and size of the MRT circuit required will vary greatly from project to project, as a function of: Copper and cobalt concentration in solution The respective proportions of the two metals Total required cobalt output (tonnes per annum of available cobalt) Nature of the primary process at the site Projected longevity of the primary site process Availability of start-up capital The choice of single column or lead-trail will be largely dependant on the latter. Both approaches can lead to close to 100% recovery of cobalt in a single pass - but, if using a single column approach, total flow to the column before elution will be restricted. From a practical point of view, it should be considered that between 75% and 85% of the cobalt will be recovered in one pass through a single column, 95+% through a lead-trail circuit. Most of the applications currently being considered for this flow sheet entail treatment of a bleed stream from a primary copper leach-sx-ew operation. Due to the closed nature of this style of circuit any residual cobalt in solution after the MRT step will not be lost to future recovery. Thus, the use of a single column system becomes feasible. The choice between single column and lead-trail can only be made after detailed investigation of the project. In general: a) A single column method will reduce initial capital in the project, but will reduce percentage recovery of the target metal in each pass. It is likely that this approach would be recommended for most closed circuit operations.
8 b) A lead-trail system will ensure virtual 100% recovery of the target metal in a single pass through the facility, but will incur higher capital cost in resin inventory and plant. At a given metal recovery level, likely operating life of the resin will be significantly increased. This approach would generally be recommended in cases where the feed solution is sent to waste (rather than being returned to a leach circuit) after treatment.
9 3.0 EMEW PERFORMANCE DATA In this flow sheet, the EMEW cell will perform the following functions: 1. Recovery of high grade cathode from a high acid, low copper concentration solution 2. Stripping of residual copper prior to cobalt recovery 3. Direct electrowinning of cobalt from the de-coppered solution The capacity of the technology has been thoroughly tested in each of these applications in full sized commercial cells. Key economic factors in this flow sheet relate to the number of cells required to achieve desired production rates and the expected power cost in the electrowinning of each product. The following data exemplify expected performance in each of the above steps: Cathode from low grade eluate As designed for this application, the flow sheet requires continuous electrowinning of copper from the elution circuit, at an expected tenor of 2 g/l in solution. Although current density can be increased, due to the high conductivity of the target solution, a current density setting of 300 a/m2 has been chosen for initial plant design. The following graph provides an example of the recovery profile achieved in closed cycle electrowinning of a high acid electrolyte in the range of copper concentration in question. This work was performed at a current density of 300 a/m2 - achieving a dense cathode plate at current efficiency of 90% down to a copper level of 0.4 g/l. 6 4 Cu (g/l) 2 0 0 2 4 6 8 amp hours The straight line nature of this profile confirms the expected high current efficiency (85%+) that will be achieved in treating this relatively clean electrolyte. It also demonstrates that the process window under which this step can be achieved is extremely wide and that careful control of electrolyte copper grade will not be required. It is noted that this is a key feature of the flow sheet presented here - process operating windows have been kept broad. The facility will be extremely flexible to
10 changing chemistry of the feed solution; and tight process controls from step to step are not required. The resultant reduction in operating (and therefore financial) risk is significant. Copper polishing As designed, the circuit provides for continuous copper cathode production from the eluate cycle. One of the purposes in this step is to allow build up of cobalt concentration, through recycling of the elution inventory, whilst maintaining copper constant. In the standard flow sheet cobalt concentration will be built up to around 40 g/l in the elution circuit, prior to bleeding a small stream for downstream recovery. This bleed will contain an equivalent concentration of copper to the elution inventory, which must be removed to facilitate electrowinning of the cobalt as a high grade product. The cobalt bleed stream will be de-coppered in a single pass through a line of EMEW cells, from around 2 g/l down to less than 10 ppm. Again, this is a setting in which the EMEW cell has been well tested and developed. As the solution will remain highly conductive, these cells will be operated at a current density of 200 a/m2. On the basis of known performance of the cell in this grade range, a cumulative current efficiency in the order of 40-50% is expected. The following chart provides an example of expected performance of the technology over this grade range: 4 Low grade copper polish 3 Cu (ppm) Thousands 2 1 0 0 5 10 15 20 Time Final copper concentration in this test run was 8 ppm and cumulative current efficiency from the initial 3200 ppm down to this level was 48%. Dependant on local requirements, it is proposed that this step be configured to produce powder rather than plate and that the product be automatically harvested by flushing of the cells. This is a technique that has now been established in full scale cells for treatment of acid mine drainage solutions at Grasberg.
11 Cobalt electrowinning There are a number of potential configurations in this flow sheet for the final electrowinning step, but essentially: 1. The step can be performed at a constant cobalt tenor by recycling the bleed stream through electrowinning and back to the elution circuit, with a chosen delta cobalt in the EW, or 2. The step can be performed on a batch basis with all cobalt being recovered from solution before sending the solution to waste. In addition operating parameters can be selected to produce either plate or powder as a product. The final choice will affect the number of cells required to achieve the given production level and their performance characteristics (current efficiency). For the purposes of this case illustration it is assumed that the circuit will produce powder at very high current density. The following table illustrates current efficiency achieved in recent tests on a cobalt solution grading around 20 g/l at various current densities: Current Current density efficiency amps/m2 % 490 90+ 1600 88 3200 78
12 4.0 THE PROCESS FLOW SHEET The schematic diagram provided overleaf outlines the typical MRT-EMEW process flow sheet for recovery of cobalt and copper from a low grade SX-EW raffinate stream. This flowsheet is flexible and can be changed to suit specific site conditions with: Little consequent increase in capital or operating costs Little or no additional process complexity No impact on its versatility and ease of operation Individual process streams in the circuit are detailed as follows: Process Flows 1 Raffinate feed 2 Bleed from pre-elution wash 3 Bleed from post elution wash 4 Total feed to SuperLig column 5 Barren raffinate 6 Pre-elution wash 7 Elution 8 Eluate to copper cathode EW tank 9 Overflow to elution holding tank 10 Post elution wash 11 Copper cathode EW recycle 12 Bleed to copper polish 13 To ph adjustment (if required) 14 To cobalt EW 15 To waste or main circuit Reagents 16 H2SO4 17 SO2 18 MgCO3 19 NaOH 20 Water pre-elution wash tank 21 Water post elution wash tank 22 Water to eluate tank A detailed metallurgical balance has been generated for this flow sheet. The process is capable of catering for wide variations in conditions at any particular site. A complete copy of this model can be provided on request from potential clients, once sufficient information has been made available to complete specific data input.
13 3 20 21 CO-CU FROM SX RAFF TYPICAL FLOW SHEET PRE ELUTION WASH TANK POST ELUTION WASH TANK 6 P2 10 2 6 10 5 19 15 SUPERLIG RESIN COLUMN 6 10 8 CO-NI EW TANK 14 P5 CO-NI EW CO/NI METAL 1 4 7 9 12 COPPER POLISH 13 ph ADJUST 18 ELUATE TANK COPPER EW TANK 16 17 P1 22 P3 11 P4 COPPER POWDER COPPER EMEW CATHODE COPPER
14 5.0 TYPICAL PROJECT MODEL Operating parameters The table at the end of this paper (Table 1) provides broad modelled operating parameters for an SX raffinate application, with 250 ppm cobalt and 470 ppm copper content. This model would not necessarily treat the whole of the raffinate stream from the main plant. The volume of the bleed will be governed by selected total cobalt production for the facility - which would be expected to be in excess of annual input to the overall circuit (either from the ore being leached or from an electrolyte bleed stream), plus a factor which would lead to gradual depletion of the metal in the total site solution inventory. In such a project, it is envisaged that the stream after treatment would return to the leach circuit. Financial Outcome Again, the financial outcome from implementing this circuit will depend on a number of factors, which will vary considerably from site to site. For the purpose of providing potentially interested parties with a first pass look at its potential value, however, the operating parameters presented here have been used to generate a preliminary financial for the case study under investigation. The following tables provide a summary of the key financial inputs to the model, together with a simple one year cash result (based on an initial cobalt value equivalent to US$10) for the two production rates envisaged in the typical operating parameters. The cost analysis assumes an Australian based operation and local values for costs and consumables. It is noted that there may be significant variation in some individual costs (for example for sulphuric acid) in other countries. OPERATING REVENUE ANALYSIS - single column Assumptions Realisable value cobalt US$/lb 10 Realisable value copper A$/tonne 2,600 Exchange rate 0.60 Labour and supervision per annum 200000 Power c/kwh 10 Sulphuric acid $/tonne 180 Caustic $/tonne 450 Limestone $/tonne 88 SO2 $/tonne 750 Maintenance per annum 80000 Government Royalties %of gross 2 Transport $tonne product 100 EW power Cu kwh/kg 2 EW power Co kwh/kg 5 EW power cost (Cu) $/tonne 200 EW power cost (Co) $/tonne 500 A$
15 BROAD ESTIMATE 350 DAY YEAR Operating days/year 350 Cobalt production tonnes/annum 94.5 Copper production tonnes/annum 177.66 SALES A$/annum Cobalt 3,471,300 Copper cathode 461,916 ---------------- Total 3,933,216 OPERATING COSTS 686,839 PROJECTED SURPLUS 3,246,377 Preliminary Capex 3,870,000 6.0 SUMMARY OF BENEFITS The benefits of this process to the operator can be clearly summarised as follows. 1. The SuperLig rejects iron and other contaminant metals to produce a pure solution for electrowinning. This eliminates any pre-treatment of the feed stream and greatly reduces the costs of the process. 2. The EMEW system produces LME Grade A cathode from the eluate produced by the SuperLig columns, and can selectively electrowin the Co and Cu from each other, thus eliminating any further processing steps and reducing costs. 3. Due to the high selectivity and efficiency of the two technologies, the capital and operating costs are significantly reduced. 4. Financial payback on a typical case is less than 18 months, providing an attractive rate of return. 5. The overall process is flexible and can treat a wide range (varying Cu/Co tenor, ph, and flowrates) of streams ranging from raffinates, tankhouse bleed streams, and waste solutions generated by natural leaching of old dumps.
16 TABLE 1: DESIGN AND OPERATING PARAMETERS UNITS 94 tpa Co Solution feed to circuit Raffinate bleed flow m3/hr 60 Cu ppm 470 Co ppm 250 Raffinate bleed flow m3/day 1440 Cu kg/day 676.8 Co kg/day 360 MRT SuperLig resin Loading capacity gm Cu+Co/kg resin 50 Method of operation Column single Metal recovery Percent loading before % 100 elution Percent recovery cobalt % 75 Percent recovery copper % 75 SuperLig requirement Flow rate l/hr 91,953 Quantity in columns Kg 5,108 Annual production metals Operating days/year 350 Cobalt production tonnes/annum 94.5 Copper production tonnes/annum 177.66 PLANT SIZING SuperLig Resin in inventory Kg 5,108 Number SuperLig columns 2 Volume each column Litres 12,163 No EMEW cells - Cu cathode 99 No EMEW cells Cu polish 10 No EMEW cell Co EW 93 Total EMEW cells 202