Eastern Finland Office C/MT/2011/13 Outokumpu. Beneficiation Tests. Taivaljärvi Ore

Transcription

1 Eastern Finland Office C/MT/2011/13 Outokumpu Beneficiation Tests on Taivaljärvi Ore

2 C/MT/2011/13 GEOLOGICAL SURVEY OF FINLAND DOCUMENTATION PAGE Date / Rec. no. 2 nd May 2011 C/MT/2011/13 Authors Satu Peltoniemi Markku Klemetti Type of report Research Report Commissioned by Sotkamo Silver AB Title of report Beneficiation Tests on Taivaljärvi Ore Abstract A flotation test work was conducted in the Mineral Processing laboratory of the Geological Survey of Finland in the spring 2011 on the three Taivaljärvi silver ore samples sent by Sotkamo Silver AB. The composites received were expected to represent the head grades in the production. The flotation tests comprised flotation of the main minerals, galena and sphalerite. The target was first to achieve the required concentrate qualities by open circuit batch flotation tests and then carry out a locked cycle test to study the effect of closed circuit into the flotation results. In the open bench-scale flotation tests on Taivaljärvi samples two cleaning stages was requested both on lead and zinc cleaning to yield close to 50 % Pb and Zn grades into the concentrates. In the locked cycle test the lead recovery into the cleaned lead concentrate was 86,7 % and the lead grade 55 %. The silver grade in the lead concentrate was 1,7 % and the corresponding recovery 69,5 %. In addition 7 % of the silver was recovered into zinc concentrate and the grade was high enough to raise the commercial value of the zinc concentrate. The gold grade in the lead concentrate was 70 g/t and the recovery 90 %. Zinc recovery into the cleaned zinc concentrate was 90,8 % the zinc grade being 51 %. Taivaljärvi ore is in general simple to be concentrated ; with a simple process and reasonable reagent consumptions, fair concentrate grades can be yielded with satisfactory recoveries. Based on the bench-scale and locked cycle tests two alternative pilot circuits for Taivaljärvi ore at GTK are presented in this report. After experience of the present test work, the basic engineering study reported by GTK in 2007, C/MT/2007/28, is still considered valid. The up-date will be an issue after the pilot plant, especially in the case gravity option will indicate benefits in terms of silver recovery and concentrate grades. Keywords silver, galena, sphalerite, flotation, locked cycle Geographical area Finland, Sotkamo, Taivaljärvi Report serial Research report Archive code C/MT/2011/13 Total pages Appendices Language English Price Confidentiality Confidential Unit and section ISY 408 Project code Signature/name Signature/name Kauko Ingerttilä, Laboratory Manager Satu Peltoniemi, Research Scientist

3 C/MT/2011/13 Contents Documentation page 1 INTRODUCTION 1 2 HEAD SAMPLES 1 3 TEST PROGRAM 1 4 RESULTS Batch Flotation Tests Locked Cycle Test Pilot Scale-Up 7 5 CONCLUSIONS 9

4 C/MT/2011/ INTRODUCTION A flotation test work was conducted in the Mineral Processing laboratory of the Geological Survey of Finland in the spring 2011 on the samples sent by Sotkamo Silver AB. The test work comprised crushing and grinding the samples and flotation of the main minerals, galena and sphalerite. The used recipe was based on the flowsheet reported in the late 1980 and had been updated based on the tests made at GTK in HEAD SAMPLES Sotkamo Silver AB delivered in February 2011 three ore samples to GTK Mineral Processing Laboratory, 60 kg each. The samples were labeled based on the silver grade: Ag50, Ag100 and Ag150. Each sample was crushed in a laboratory crusher to 100 % -4 mm, homogenized and divided into sub samples for the flotation tests and head assays. The crushed samples were stored in a freezer to avoid oxidation. For the locked cycle flotation a blend of the three composites were prepared and also divided to sub samples. Each composite was analysed for silver and gold by Fire Assay, for lead, zinc and iron by nitric acid digestion and atomic adsorption spectrometer, for sulphur by Eltra and for about 40 elements by XRF. The chosen head assays are presented in Table 1. The XRF results are shown in Appendix 1. Table 1. Head Assays Ag50 Ag100 Ag150 LC Blend Ag (FA) 49,2 98,8 164,1 104,0 Au (FA) 0,18 0,28 0,50 0,32 Pb (HNO 3 +AAS) 0,142 0,180 0,375 0,232 Zn (HNO 3 +AAS) 0,339 0,402 1,074 0,605 Fe (HNO 3 +AAS) 2,29 2,76 2,29 2,45 S (ELTRA) 1,89 1,80 2,50 2,06 The composites are expected to represent the head grades in the production and are of lower grade than the Taivaljärvi ore samples used earlier in the flotation tests at GTK. 3 TEST PROGRAM Prior to flotation five kilogram flotation feed batches were milled in a laboratory rod mill (DxL 258x300 mm) to achieve the requested 75 % - 75!m passing. Slurry density of 50 % solids was used in grinding. Rougher flotation stages were carried out in a 15 litre cell of GTK flotation machine equipped with a froth scraper. The cell sizes of 4 and 2,5 were used in cleaner flotation stages. The used reagents in lead flotation were zinc sulphate (ZnSO 4 ) and Aerophine 3418A in rougher flotation and sodium cyanide (NaCN) and Aerophine 3418A in cleaner stages. Reagents used in zinc flotation were copper sulphate (CuSO 4 ) and sodium isobutyl xanthate (SIBX). The zinc flotation was carried out at ph 11,5. Methyl isobutyl carbine (MIBC) was used as frother roughly 50 g/t. Lime was used to increase ph in the lead cleaner flotation and zinc flotation.

5 C/MT/2011/13 2 The target was first by batch flotation tests to produce commercially valuable lead and zinc concentrates. The lead or zinc contents in the concentrates should then be at least 50 %. The final flowsheet of the batch flotation tests are shown in Figure 1. The tests were carried out on each of the three composites. LEAD FLOTATION ZINC FLOTATION Tailing PbCT1 ZnCT1 PbCT2 ZnCT2 PbCC2 ZnCC2 Figure 1. Flowsheet of Best Batch Flotation Test After the required result had been achieved by open circuit batch flotation test, a locked cycle test was carried out to study the effect of closed circuit into the flotation results. The feed for the locked cycle test was a blend of the three composites. The head grade for the silver was then 104 g/t. Locked cycle test was also made to produce flotation tailings for environmental studies. The flowsheet of the locked cycle test is shown in Figure 2. LEAD FLOTATION ZINC FLOTATION Tailing PbCC2 ZnCC2 Figure 2. Flow Sheet of Locked Cycle Test

6 C/MT/2011/ RESULTS 4.1 Batch Flotation Tests The first batch flotation tests were carried out with the same recipe used in the flotation tests Only one cleaner flotation stage was then needed for both rougher concentrates. In the second tests the activator and collector dosages were cut into half due to the lower head grades compared with the former sample. In the third tests the rougher concentrates were cleaned twice to raise the lead and zinc grades in the concentrates. Lead, silver and gold grades vs. recoveries in the lead concentrate are shown in Figures Pb Recovery, % Ag50-1 Ag50-2 Ag50-3 Ag100-1 Ag100-2 Ag100-3 Ag150-1 Ag150-2 Ag ,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0 Pb Grade, % Figure 3. Lead Grade vs. Recovery in Lead Concentrate The best lead recoveries were achieved with the richest composite, Ag150. There were not big differences between the ore types Ag50 and Ag100 as there were not in the head grades for lead either. When the lower reagent dosages were used, lead recoveries into the rougher concentrates were 84 % for the Ag50, 80 % for Ag100 and 89 % for Ag150 as an average of two tests. The lead grades in the rougher concentrates of the two poorest ore types stayed below 10 % and was raised roughly to 40 % by one cleaner flotation. A lead grade of 52 % was achieved in a single cleaning stage when the richest composite Ag150 was processed. The lead grades after two

7 C/MT/2011/13 4 cleaning stages were notably above the required 50 % limit being %. The details of the concentrates produced in the third tests are shown in Table Ag Recovery, % Ag50-1 Ag50-2 Ag50-3 Ag100-1 Ag100-2 Ag100-3 Ag150-1 Ag150-2 Ag Ag Grade, g/t Figure 4. Silver Grade vs. Recovery in Lead Concentrate When the lower reagent dosages were used, silver recovery into lead rougher concentrate was roughly 70 % for the two poorest ore types and 75 % for the Ag150. After two cleaner flotation stages, the silver grades in the lead concentrates were 1,1-2,3 % and the recovery had sank roughly to 40 %.

8 C/MT/2011/ Au Recovery, % Ag50-1 Ag50-2 Ag50-3 Ag100-1 Ag100-2 Ag100-3 Ag150-1 Ag150-2 Ag Au Grade, g/t Figure 5. Gold Grade vs. Recovery in Lead Concentrate The shapes of the gold grade vs. recovery curves are very similar compared with the silver curves of each ore type. The highest gold grade was achieved with Ag100. The gold recoveries into lead rougher concentrate were roughly between 70 and 80 % with each sample Zn Recovery, % Ag50-1 Ag50-2 Ag50-3 Ag100-1 Ag100-2 Ag100-3 Ag150-1 Ag150-2 Ag ,0 10,0 20,0 30,0 40,0 50,0 60,0 Zn Grade, % Figure 6. Zinc Grade vs. Recovery in Zinc Concentrate

9 C/MT/2011/13 6 The best zinc recoveries were yielded with the samples Ag50 and Ag150. The zinc recoveries were at 10 %-units lower level with the corresponding zinc grades for the sample Ag100. Two cleaning flotation stages were required to raise the zinc grade of the concentrate to the desired level. Still, the poorest ore type stayed slightly under the 50 % zinc grade target. Because the desired concentrate qualities were achieved using the third recipe, it was chosen for the locked cycle test. The details of the best batch flotation test concentrates are shown in Table 2. Table 2. Batch Flotation Test Lead and Zinc Concentrate Grades and Recoveries Ag50 Ag100 Ag150 Grades and Recoveries Pb (HNO 3 +AAS) Zn (HNO 3 +AAS) Fe (HNO 3 +AAS) Ag (FA) Au (FA) S (Eltra) % Rec% % Rec% % Rec% g/t Rec% g/t Rec% % Rec% PbCC2 58,7 65,7 3,91 1,6 6,98 0, ,63 41,3 48,0 17,9 1,5 ZnCC2 0,315 1,4 48,7 78,3 11,5 2, ,94 1,04 4,8 33,1 11,0 PbCC2 62,9 47,3 5,44 1,9 4,38 0, ,20 81,6 48,2 16,9 1,4 ZnCC2 0,457 1,2 54,2 65,7 7,65 1, ,01 1,15 2,3 33,3 9,6 PbCC2 67,2 60,7 3,58 1,3 3,60 0, ,15 52,2 45,7 15,3 2,5 ZnCC2 0,270 0,8 56,3 70,3 7,20 3, ,11 0,68 2,0 33,1 18,4 Taivaljärvi ore is in general simple to be concentrated ; with a simple process and reasonable reagent consumptions, fair concentrate grades can be yielded with satisfactory recoveries. 4.2 Locked Cycle Test The locked cycle test was carried out as sequential batch flotations. The cleaner tailings were returned to previous stages on the next cycle as in a continuous flotation circuit (Fig 2). The second lead and zinc concentrates as well as the lead and zinc tailings from each cycle were assayed. The test was stopped after five cycles. The last tailing sample was not dried but sent as slurry for further studies. The balances from each cycle are shown in Appendix 3. The results after the final cycle are presented in Table 3. Table 3. Locked Cycle Test Results Grades and Recoveries weight Pb (HNO 3 +AAS) Zn (HNO 3 +AAS) Fe (HNO 3 +AAS) Ag (FA) Au (FA) S (Eltra) % % Rec% % Rec% % Rec% g/t Rec% g/t Rec% % Rec% Feed 100,0 0,23 100,0 0, ,0 2,77 100,0 92,2 100,0 0,29 100,0 2,00 100,0 Lead Con 0,37 54,6 86,7 5,43 3,1 4,67 0, ,5 69,6 89,6 15,9 2,9 Lead Tailing 99,6 0,03 13,3 0,625 96,9 2,77 99,4 28,2 30,5 0,03 10,4 1,95 97,1 Zinc Con 1,14 0,62 3,0 51,0 90,8 11,9 4, ,7 2,96 11,8 32,8 18,8 Zinc Tailing 98,5 0,02 10,3 0,040 6,1 2,66 94,5 22,6 24,1 0,03 10,3 1,59 78,3 After reaching the balance, the Pb grade of the lead concentrate stayed around 55 % with the corresponding 87 % recovery. The silver grade in the lead concentrate was approximately 1,7 % and the corresponding recovery 70 %. In addition 7 % of the silver was recovered into the zinc concentrate. The silver grade in zinc concentrate was 544 g/t, which is clearly above the limit zinc refineries normally pay for in addition to zinc. Zinc grade in the zinc concentrates was 51 % and the corresponding recovery 90 %. The XRF results of the concentrates and tailings are shown in the Appendix 4. The reagent consumptions in the locked cycle test were equal to the open circuit: ZnSO 4 150, AP 6,25, CuSO 4 150, SIBX 4,5, NaCN 0,5, MIBC 50 and lime 1400 g/t.

10 C/MT/2011/ Pilot Scale-Up Based on the bench-scale and locked cycle tests the pilot circuit for Taivaljärvi ore at GTK is shown in Figure 7. The flowsheet is shown also in Appendix 5. Figure 7. Primary Taivaljärvi Pilot Flowsheet The ore feed rate into the rod mill-ball mill circuit is 1 tph. In the pilot circuit the rod mill circuit is open. The rod mill and ball mill discharges are reported onto a vibrating screen, the over-size is returned to the ball mill and the under-size is pumped to cyclone classification. The cyclone overflow is reported to flotation and the underflow back to the ball mill. Table 4 shows the estimated cell volumes and residence times in flotation. Table 4. Taivaljärvi Pilot Flotation Cell Volumes and Residence Times

11 C/MT/2011/13 8 In lead flotation the Pb roughing has a cell volume of 2 x 500 litres. The two cleaning stages have 1 x 50 litre volumes each. The cleaning circuit is a traditional closed cleaning with reporting the cleaning tails to the beginning of the previous stage. In zinc flotation the roughing cell volume is totally 2 m 3. The first 500 litres is in an open circuit with reporting the rougher concentrate straight to the second cleaning stage. The rest of the roughing is in closed circuit with the two zinc cleaning stages. The reagents in lead and zinc flotations are according to the locked cycle tests. Both concentrates will be thickened and filtered with ceramic or pressure filters. In addition to the flowsheet in Figure 7, an alternative flowsheet comprising gravity separation of silver and gold in the grinding circuit shown in Figure 8 should be included into the schedule of the coming pilot plant in autumn Gravity concentration could be an option especially for a high grade silver concentrate. On the other hand gravity increases the investment and operating costs and makes the circuit more complex. As well removing silver from the produced lead concentrate lowers the smelter return for the concentrate. Thus the possible benefit of producing a high grade concentrate for grade and recovery of silver and gold should be proved in a realistic scale. Figure 8. Alternative Taivaljärvi Pilot Flowsheet

12 C/MT/2011/ CONCLUSIONS In open bench-scale flotation tests on Taivaljärvi samples two cleaning stages was requested both on lead and zinc cleaning to yield close to 50 % Pb and Zn grades into the concentrates. The locked-cycle head had a silver grade approximately 100 g/t, but the lead, zinc and sulfur grades were higher than in the single Ag100 sample, and so, the respective results are not totally comparable. Remembering this fact, in the closed circuit like the locked-cycle test, the concentrates showed slightly lower grades for Pb and Zn, but simultaneously the recoveries were higher as expected and both the lead and the zinc concentrate grades remained in a reasonable or good level. The Pb recovery into the lead concentrate was 87 %, silver recovery 70 % and the Zn recovery into the zinc concentrate 91 % and silver recovery 6,7 %. It is worth to mention that the zinc smelters pay also for silver. In this case this means that the total silver recovery according to the locked cycle test was 69,5 % + 6,7 % = 76,2 %. The final smelter return is then of course according to each appropriate contract. Figures 9 and 10 show comparisons of the present tests with the tests done late 1980 s. Figure 8. Lead Grade-Recovery Comparison of Present and Old Samples

13 C/MT/2011/13 10 Figure 9. Silver Grade-Recovery Comparison of Present and Old Samples When comparing the bench-scale tests done on Taivaljärvi samples late 80 s where especially the silver recovery showed rather high figures from 77,8 to 83,9 % with different samples, it must be noted that the silver head grades were then g/t, so, in practice double compared to the reported locked-cycle tests. Also Pb and Zn grades were higher in the old tests. The results of the performed tests are to be confirmed in the pilot plant on the ore in autumn Only then the possible benefit of for example gravity separation on the silver (and gold) recovery from the ore can be proved. Thus, at this stage the basic engineering study reported by GTK in 2007, C/MT/2007/28, is still considered valid. The up-date will be an issue after the pilot plant, especially in the case gravity option will indicate benefits in terms of silver recovery and concentrate grades. The proposed flowsheets for 250 and 450 ktpa in 2007 are shown in Figures 10 and 11.

14 C/MT/2011/13 11 Figure 10. Proposed Full Scale Taivaljärvi Tpa Flowsheet 2007

15 C/MT/2011/13 12 Figure 11. Proposed Full Scale Taivaljärvi Tpa Flowsheet 2007