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1

Thiosulfate-copper-ammonia leaching of pure gold and pressure oxidized concentrate

Porvali A., Rintala L., Aromaa J., Kaartinen T., Forsen O., Lundstrom M.

Physicochemical Problems of Mineral Processing

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2017

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Vol. 53, iss. 2

1079--1091

EN

In this research cyanide-free leaching of pure gold and pressure oxidized refractory gold concentrate by thiosulfate-copper-ammonia solutions were examined. A quartz crystal microbalance (QCM) was used to study gold leaching as a factorial series where the best gold leaching rate (2.987 mg/(cm2∙h)) was achieved with a solution consisting of 0.2 M (NH4)2S2O3, 1.2 M NH3, 0.01 M CuSO4 and 0.4 M Na2SO4. Temperature had the greatest effect on the gold leaching rate. An increase in thiosulfate concentration (0.1–0.2 M) increased gold dissolution. The combined effect of temperature and ammonia concentration had a statistically significant effect on the gold leaching rate at 0.1 M M2S2O3. Combination of applied potential and NH3:S2O3 ratio had a statistically significant effect on the gold leaching rate at 0.2 M M2S2O3. An increase in applied potential decreased the gold dissolution rate at low ammonia concentrations but increased it at high concentrations. A pressure oxidized gold concentrate was leached for 6 hours in the batch reactor leaching experiments. The effect of rotative velocity (1.26–1.56 m/s) and slurry density (10–30 wt%) was investigated at the following leaching parameters: 0.2 M Na2S2O3, 0.6 M NH3, 0.01 M CuSO4, 0.4 M Na2SO4. Lower slurry density (10 wt%) resulted in a higher Au leaching efficiency. An increase in the rotation rate did not have an effect on the final Au leaching recovery. The best Au leaching efficiency (89%) was achieved with 590 rpm mixing, 1.56 m/s rotative velocity and 10 wt% slurry density.

2

Oxidative pressurized acid leaching of waste printed circuit boards

Lundstrom M., Seisko S., Jascisak J., Orac D., Aromaa J., Havlik T., Forsen O.

Physicochemical Problems of Mineral Processing

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2017

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Vol. 53, iss. 2

781--792

EN

The amount of waste printed circuit board (WPCB) increases continuously. There is an economic and environmental need to recover valuable metals from WPCBs. In this study, the applicability of oxidative pressurized acid leaching of WPCBs rich in copper and tin was investigated. The effect of leaching parameters such as temperature, partial oxygen pressure and chloride concentration on metal dissolution from WPCBs was studied in sulfuric acid media. It was shown that non-metallic elements present in WPCBs initiated gas formation, namely CO2 and CO, during oxidized pressure acid leach. Decomposition of plastic components already started at 90 °C with 1.5 MPa oxygen overpressure in the presence of 1 g/dm3 chlorides. Gas formation was shown to have a negative impact on the process since copper extraction was reduced to 27% compared to 80-90% extraction achieved from anode slimes used as a reference material. It was suggested that gas formation related to plastic components could have a severe impact on metal yields. The highest dissolution of tin equal to 36% was achieved at room temperature. It was found that higher temperature promoted tin hydrolysis.

3

Development of characterization methods for adherent anode slimes in copper electrorefining

Kiviluoma M., Aaltonen M., Aromaa J., Lundstrom M., Forsen O.

Physicochemical Problems of Mineral Processing

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2016

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Vol. 52, iss. 1

295--302

EN

Adherent anode slimes can cause anode passivation in copper electrorefining and lower the efficiency of copper electrorefining. Declining concentrate grades cause larger impurity levels in anodes, thus creating larger quantities of slimes in the refining process. In order to investigate the characterization methods for adherent anode slimes in copper electrorefining, experiments were conducted for the Boliden Harjavalta Pori refinery material. Methods such as particle size determination, chemical (ICP) analysis, settling rate determination, XRD, SEM-SE, SEM-BSE and SEM-EDS were applied. In addition, adherent anode slime samples were compared to optical micrograph and SEM-BSE images of respective anode copper samples. It was shown that SEM-EDS and SEM-BSE provided precise information about phases formed during electrorefining. The settling rate and particle size had a correlation only with a copper content of anode slime. The main phases in the anode slime were copper and lead sulphates as well as copper-silver selenides. NiO was shown to be the major Ni-bearing phase in the adherent slime. Nickel, tellurium and lead had the strongest, whereas arsenic, selenium and antimony had the weakest tendency to report to the anode slime.

4

Dissolution of gold with cyanide replacingreagents

Aromaa J., Rintala L., Kahari M., Forsen O.

Physicochemical Problems of Mineral Processing

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2015

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Vol. 51, iss. 1

269--279

EN

Cyanide is used in the leaching phase of hydrometallurgical recovery of gold. The toxicity and environmental risks have created a need for safer alternatives. There is a vast amount of information about gold extraction and a selection has been collected and formalized to be used in a decision support tool. The tool Auric Advisor uses case-based reasoning (CBR) to provide process alternatives for the user queries. The aim of this paper is to study leaching of gold with cyanide replacing alternatives such as ammonium thiosulfate, chloride-hypochlorite, thiourea and thiocyanate to provide new cases for the tool. The behaviour of gold was examined with electrochemical tests and weight loss measurements a using quartz crystal microbalance (QCM) were used to determine the dissolution rate. Based on literature, the dissolution rate determined for cyanide solution was 2.5 mgcm-2h-1. Based on polarisation experiments and potential measurements, the ammonium thiosulfate system with cupric ion as oxidant was estimated to dissolve gold slower than cyanide. In the QCM tests the chloride-hypochlorite alternative showed highest dissolution rate of 8.6 mgcm-2h-1 at the redox potential of 900 mV vs. SHE. The thiocyanate system with ferric ion dissolved gold at a rate of 3.4 mgcm-2h-1 at the redox potential of 620 mV vs. SHE. Thiourea system with ferric ion showed the dissolution rate of 1.65 mg cm-2 h-1 at the redox potential of 450 mV vs. SHE. The results of the rapid tests were comparable with literature and were included in the case base of the Auric Advisor decision-support tool.

5

Copper deposition on stainless steel sheets in copper nitrate solution

Kekki A., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2015

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Vol. 51, iss. 1

247--256

EN

The aim of this study was to examine factors that influence copper nitrate based electrorefining of copper and to search the best process parameters for high-purity copper deposition on AISI 316L steel blanks. Considering impurities, the most important goal was to minimize sulfur content in a copper cathode. The effect of Cu2+ concentration, current density, temperature and pH were studied. The best parameters for the best copper purity were sorted out. The most important factors for quality copper deposition are sufficiently low Cu2+ concentration, low current density, right zone and careful control of pH. Active nitrate ion reduction reactions catalyzed by copper ions are suggested to affect detrimentally both copper deposition current efficiency and purity. Furthermore, nitrate ion reactions seem to elevate an electrolyte pH so that the deposition appears to be dark brow copper oxide. The 6N purity for copper was not reached with this cell construction and it felled behind about 7 ppm (99.9993% Cu). Both, sulfur and silver concentration were slightly above 1 - 2 ppm. To minimize the impurities, electrolyte circulation and filtration are needed. Also, either a separate silver cementation cell or cementation membrane is needed.

6

Applicability of published experimental works as a knowledge source in recommendation of gold ore processing workflows

Rintala L., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2015

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Vol. 51, iss. 2

707--717

EN

The experimental work is the most time consuming and expensive part of the process design. A case-based reasoning (CBR) methodology can be used to assist in the process design. Auric Adviser is a CBR system under development for recommendation of gold ore processing workflows. In Auric Adviser the knowledge in gold ore processing is represented in two models. The first model holds the knowledge needed to recommend process chains already used in industry. The second knowledge model is intended to recommend the most efficient unit processes based on research results. The objective of this study was to analyse the information richness of journal articles and other publications concerning single process steps of gold extraction. The aim was to study the applicability of these publications as a source for the second Auric Adviser knowledge model. In this study, 25 publications concerning leaching of gold were analysed and information was extracted in a case base. The case base was taken as either a process or experimental description with clearly defined differences to other descriptions. In total, 218 cases that described results of gold leaching were extracted from the sources. The analysis of descriptions showed that the knowledge necessary for design the second Auric Adviser model can be elicited from journal articles and other publications concerning single process steps. The trends in the case description were that the gold content and process outcome were usually well described. Nevertheless, the information richness varied in descriptions of raw materials, experimental arrangements but the results were often missing details. The incompleteness of information causes challenges in the process comparison although it does not prevent the CBR system to work.

7

Alkaline leaching of zinc from stainless steel electric arc furnace dusts

Stefanova A., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2015

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Vol. 51, iss. 1

293--302

EN

Stainless steel production generates dust and sludge that are considered as a harmful waste. These dusts contain also valuable metals but extraction and recovery of valuables is difficult due their complex composition. Zinc is the most troublesome element in the dust and it hinders direct recycling of dust back into furnaces. In this paper two different stainless steel electric arc furnace dusts (EAF1 and EAF2) from Outokumpu Stainless (Tornio, Finland), were leached using NaOH solutions. The purpose was to selectively leach out zinc from the dusts and to find factors that affected most dissolution of zinc. From all leaching factors temperature, agitation speed and NaOH concentration were found to be statistically strongly significant, whereas a liquid-to-solid ratio and bubbling gas were only somehow significant. Two experiments from the test series gave clearly higher zinc extraction, that is around 60% for EAF1 and 30% for EAF2. For those experiments, a strong 8 M NaOH solution with the high temperature and agitation speed was used but bubbling gas and liquid-to-solid ratio changed. Zinc was leached selectively and practically no iron, chromium and nickel dissolved.

8

Pressure oxidation of pyrite-arsenopyrite refractory gold concentrate

Rusanen L., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2013

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Vol. 49, iss. 1

101--109

EN

Refractory gold ores have poor gold recoveries with direct cyanide leaching. Typically the refractoriness is due to encapsulation of the gold particles inside the host mineral. To liberate gold for leaching the host mineral must be broken by mechanical or chemical means. The aim of this study was to study the effect of temperature, oxygen partial pressure and slurry density on pressure oxidation of pyrite-arsenopyrite gold concentrate. Batch oxidation tests in an autoclave were done using a factorial design. Different responses were measured and analysed to study effect of the three factors and oxidation kinet-ics. Generally, high slurry density required high temperature and oxygen partial pressure to reach com-plete oxidation. Oxidation kinetics at 225°C temperature, with 1050 kPa oxygen partial pressure and 15% slurry density was found to be fastest resulting in complete conversion of sulfides in 30 minutes. At 195°C, 700 kPa oxygen partial pressure and 10% slurry density, the oxidation kinetics for complete sul-fide conversion was about 60 minutes. Slurry densities above 10% had an adverse effect on the oxidation rate, when the temperature was below 225°C and oxygen partial pressure below 1050 kPa.

9

Determination of leaching parameters for the recovery of platinum group metals from secondary materials

Lillkung K., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2013

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Vol. 49, iss. 2

463--472

EN

The leaching parameters for the recovery of platinum group metals were investigated with potentiostatic measurements using wire electrodes. The dissolution rates of Pt, Pd and Rh were measured in concentrated NaCl solution. The parameters inspected were the temperature, redox potential and chloride content. Measurements were done at temperatures 25–94 °C using four different NaCl concentrations, 62.5–250 g/l. Measurements were done at potentials of 950–1250 mV vs. SCE for Pt and Rh and 380–470 mV vs. SCE for Pd. Although higher temperature, potential and chloride content generally increased the dissolution rate, the effect was found to be nonlinear and slightly different for different metals. Based on the results, strongly oxidizing conditions are needed for the leaching process in order to achieve reasonable dissolution rates.

10

Comparison of chemical and biological leaching of sulfide tailings

Aromaa J., Makinen J., Vepsalainen H., Kaartinen T., Wahlstrom M., Forsen O.

Physicochemical Problems of Mineral Processing

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2013

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Vol. 49, iss. 2

607--620

EN

Sulphidic tailings from Finnish Hitura nickel mine and Pyhäsalmi multi-metal mine were leached using sulphuric acid and bioleached. The aim was to recover minor amounts of valuable Cu, Ni, Zn and Mn. Both tailings consisted mainly of iron and magnesium-containing minerals and acid neutralizing minerals. The solution after chemical leaching tests contained mostly iron and magnesium, in Hitura up to 11 g/dm3 Fe and 38 g/dm3 Mg while in Pyhäsalmi 8–9 g/dm3 Fe and 4 g/dm3 Mg. Amount of these metals was 20–100-fold larger than amount of valuable metals, which were typically 100-300 mg/dm3. Problems in chemical leaching were high consumption of acid and poor selectivity. Bioleaching using iron and sulphur oxidizing bacteria was more selective towards the valuable metals. Both in leaching and bioleaching the high concentration of iron and magnesium in solution will make metals recovery challenging.

11

Alkaline leaching of zinc from argon oxygen decarbonization dust from stainless steel production

Stefanova A., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2013

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Vol. 49, iss. 1

27--46

EN

Stainless steel production generates solid wastes such as dust and sludge that are considered as harmful in most industrial countries. Today dusts are recycled in separate treatment plants as these dusts contain valuable metals such as alloying elements. A direct recycling of dust back to steel production is hindered due to the presence of elements, especially zinc, that cause operational difficulties in the stain-less steel making process. In this paper two different stainless steel converter argon oxygen decarboniza-tion dusts (AOD1 and AOD2), from Outokumpu Stainless (Tornio, Finland), were leached using NaOH solutions. The purpose was to selectively leach zinc out from the dusts and to find factors that affected most dissolution of zinc. The dust samples were leached under atmospheric pressure and the factors tested were temperature, sodium hydroxide concentration, liquid to solid ratio (L/S), stirring rate and oxygen or nitrogen gas bubbling. All the studied factors had statistically significant effect on the dissolution of zinc. The maximum zinc extraction was achieved at 95°C, with 8M NaOH solution, stirring rate of 400 rpm and L/S ratio of 30 and was around 80% for AOD1 dust and around 50% for AOD2 dust. Difference in maximum zinc extraction arose from the mineralogical differences of the dusts. Zinc was leached selec-tively. Among alloying elements only molybdenum was leached and practically no iron, chromium and nickel were dissolved.

12

Leaching characteristics of eaf and aod stainless steel production dusts

Kekki A., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2012

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Vol. 48, iss. 2

599-606

EN

In this paper sulfuric acid leaching of four different stainless steel production flue dusts is studied. The main objective is to study and compare the valuable metal Zn, Cr, Ni, Fe, Mo dissolution present in the EAF 1&2 (electric arc furnace) and AOD 1&2 (argon oxygen decarburization converter) dusts. The effect of sulfuric acid concentration, temperature and liquid-to-solid ratio are tested for maximum and selective Zn leaching into solution for recycling purposes. Leaching tests were done in 0.1 M, 0.5 M and 1.5 M sulfuric acid, at temperatures of 30?C, 60?C and 90?C with liquid-to-solid ratios L:S = 10 and 20 under 1 bar pressure for 120 minutes. Maximum Zn dissolution yield was achieved with 1.5M, 90?C, L:S = 10 where the Zn dissolution yield varied from 65% to almost 100%, depending on the dust type (AOD, EAF) and production line (1 or 2). At the same time Cr was leached 7 - 17%, Ni 37 - 48%, Fe 48 - 89% and Mo 82 - 100%. The best zinc selectivity vs. Cr, Ni, Fe, Mo was achieved with 0.5 M, 30?C and L:S 10. The Zn dissolution varied between 33 - 72%, Cr 2 - 4%, Ni 6 - 9%, Fe 2 - 9% and Mo 1 - 55%. Higher temperature and acid concentration resulted in faster dissolution of metals. AOD dust in both lines 1 and 2 has better valuable metal recycling possibilities than EAF dust due to better maximum Zn dissolution and better selective dissolution of Zn vs. Cr, Ni, Fe, Mo.

13

Laboratory tool for electrochemical study of sulphide minerals

Galfi I., Aromaa J., Forsen O.

Physicochemical Problems of Mineral Processing

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2007

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Vol. 41

301-312

EN

Sulphide mineral dissolution in aqueous media occurs by different mechanisms. The rate of the overall reaction is controlled by the rate of the slowest reaction step. Most important factor in the anodic dissolution of mineral sulphides is the formation of a reaction product layer, which affects the mass transfer rate, often the rate determining step of the overall reaction. In this work we have used electrochemical tests for evaluating factors affecting sulphide mineral dissolution. As an essential tool, an electrochemical measurement method for leaching behaviour of sulphide minerals has been developed. Development of a graphite paste electrode (GPE) for the electrochemical measurements has made it possible to investigate powdered sulphide mineral samples. This ensures that the measurement conditions are as close as possible to the real leaching conditions. The graphite paste electrode makes it possible to run electrochemical measurements reproducibly and reliably.

PL

Roztwarzanie minerału siarczkowego w roztworach wodnych zachodzi na drodze różnych mechanizmów. Szybkość całkowitej reakcji jest kontrolowana przez szybkość najwolniejszego etapu reakcji. Najważniejszym czynnikiem w anodowym rozkładzie minerałów siarczkowych jest tworzenie warstwy produktu reakcji, która wpływa na szybkość wymiany masy na granicy faza stała – roztwór, która jest często etapem kontrolującym proces roztwarzania minerału. W tej pracy użyliśmy elektrochemicznej metody oceny podatności surowców siarczkowych na proces ługowania. Jako podstawowe narzędzie tej metody zastosowaliśmy pomiary elektrochemiczne na elektrodach wykonanych z badanego surowca. Elektrody użyte w pomiarach są wykonane w postaci pasty z udziałem grafitu i drobnoziarnistego materiału siarczkowego. Taka konstrukcja elektrody zapewnia warunki bliskie tym, jakie występują w rzeczywistym procesie ługowania. Grafitowa elektroda w formie pasty zawierającej siarczki metali gwarantuje dużą powtarzalność i niezawodność.