奥托昆普哈加瓦尔塔镍加压浸出流程 (1).doc

NEW NICKEL PROCESS INCREASING PRODUCTION AT OUTOKUMPU HARJAVALTA METALS OY,FINLAND TABLE OF CONTENTS1.0 Introduction 22.0 Nickel Smelter 33.0 Nicke! Refinery 54.0 Conclusions 18 Presented atALTA 1997 Nickel/Cobalt Pressure Leaching & HydrometallurgyForum, May 19-20,1997,Perth Ausffalia1.0 INTRODUCTION Outokumpu Oy has produced nickel at Harjavalta Works in Finland since 1960. The production has been based on flash smelting technology and electrowinning of nickel. The capacity of the process has been increased gradually to 18000 tpa by several small expansions and process improvements. The production relied on domestic nickel concentrates until thaAt the turn of the 1990s a new nickel strategy was developed and Outokumpu started nickel exploration in Austraiia and Canada. The work secured long term delivery contracts of nickel concentrates from Mt. Keith and starting of mining operations at Forrestania and Silver Swan in Australia.At the same time Outokumpu studied different alteratives to expand the nickel and copper production. n 1993, a decision was made to invest in the expansion and modernisation of Harjavalta operations. Outokumpu decided to increase nickel production from 18000 tpa to 32000 tpa and blister copper production from 100000 tpa to 160000 tpa. The copper refinery in Pori was expanded from 72000 tpa to 125000 tpa. In addition, the expansion included a new oxygen plant, two hydrogen plants and a new sulphuric acid plant. The construction startad in 1993 and the Harjavalta project was completed in 1996. During the construction the nickel production was decided to increase further to 40000 tpa . The realised capital expenditure of the whole expansion project of Harjavalta was FIM 1600 million ($US370 million) .The expansion of the nickel production has meant several process changes in tha nickel smelter and refinery . The decision to invest in nickel was preceded by research and development work at Outokumpu Harjavalta Matals Oy and at Outokumpu Rasaarch Oy . The investigation program led to the following improvements of the process: Development of direct Outokumpu Nickel Smelting (DON) Process.Leaching process for high iron nickel matteUtilization of Outokumpu-developed and designed reactors for atmospheric and pressure leaching.Utilization of Outokumpu-developed solvent extraction process in the nickel/cobalt separationUtilization of the Sherritt process for hydrogen reduction of nickal and cobalt.Sulphur outlet as ammonium sulphateThe process improvements were incorporated in the existing nickel flash smelter,matte refinery and electrowinning procaiss. The capacity of the nickel refinery is 18000 tpa nickel cathodes and 22000 tpa briquettes. 2.0 NICKEL SMELTER In tha 1980s Outokumpu started to develop a converterless direct smelting process to praduce a high grade nickel matte for further refining. The process was tested on a pilot scale at Outokumpu Research Oy in Pori and found to be viable . The benefits of this process were in a simplified flowsheet and high grade S02 gas flow to the acid plant. The process was named the direct Outokumpu Nickel Smelting Process or DON Process /1-3/. At tum of 1990s Outokumpu continued the development work on the DON Process based on a feed with high nickel and magnesia content and low iron content. As a result of this work the DON Prccess was further developed to produce two mattes: Low iron matte in the flash smalting fumace (FSF) to be leachad in the existing leaching circuit of Harjavalta. High iron matte in the electric furnace (EF) to be Ieached in a new EF matte leaching circuit.By producing two mattes it was possible to eliminate the converters which has meant a simple process with low investment, maintenance and operating costs. In addition, it is now possibla to smelt concentrates with a high magnesium content. The ferric residue from the EF matte leaching is added to the smelting charge to increase the iron to magnesium ratio and to reduce smelting temperature of the slag. Futher, the cobalt recovery of the DON Pracess is higher than in the conventional smelting process. In Harjavaltas case, it was essential that the process could use the existing flash furnace and electricfurnace as well as the existing leaching plant. Figure l shows the flowsheet of the DON Process at Harjavalta nickel smelter. The primary feed of the nickel smelter is a blend of various concentrates. Also nickel containing secondary materials are used as a feed. In addition to the flue dust from the waste heat boiler and the electrostatic precipitator, the iron precipitate from the EF matte leaching is circulated back to the flash furnace. This makes it possible to convert leach residue to disposable fayalite slag. Concentrates and sand are dried in an oil-fired rotary drum. After drying, the feed mixture and oxygen enriched air are fed through the concentrate burner into the reaction shart of the flash smelting fumace. During the oxidation prccess the iron content of the nickel matte is currently reduced to five per cent. The nickei matte is granulated directly into water dewatered and fed to the leaching plant. The slag from the flash fumace is tapped into the electric fumace for cleaning. In the purificatton process, coal is used as a reductant and dry nickel concentrate as a sulphidizing agent to produce a desired quality of matte and slag. The matte is granulated by a similar procass as used for the FSF matte and is fed to the new EF matte leaching plant. The slag from the EF fumace is also granulated and finaily stored in slag dumps. Hot gas from the flash furnace is cooled down to 350 in a waste heat boiler and before feeding to the acid plant, dust is removed in an electrostatic precipitator. Table l gives a typical composition of the feed mixture, flash furnace matte, and electric furnace matte and slags /4/. The smelting capacity of concentrates is 250000 tpa corresponding 40000 tpa of nickel at the current blend of concentrates. Table l Composition of feed mixture, flash smelting furnace matte, electric furnace matte and slags. 3.0 NICKEL REFINERYAn oyerall flowsheet of the nickel production process at Harjavalta is shown inFigure 2. The granulated mattes from the FSF and the EF are the starting materials for the nickel refinery, where, after gnnding, the mattes are leached in separate circuits. The product solution from the leaching plant is led to a solvent extraction (SX) process for cobalt separation. The SX plant has replaced the former cobalt removal process /5, 6/, where cobalt was precipitated as a cobaltic hydroxide. There are two final products; cathode nickel produced in an electrowinning step and nickel briquettes produced in a hydrogen reduction step. There are also facilities for hydrogen reduction of cobalt. The nickel refinery, or in fact the entire Harjavalta plant, is automated using the VALMET Damatic XD process control system. In the nickel refinery, Outokumpu Mintec on-stream analysers form an essential part of the automation, The Courier 30AP analysers measure liquid phase concentrations of copper, iron and arsenic using X-ray fluorescence whereas automatic OTI titrators are used for measuring sulphuric acid concentration. 3.1 LEACHING PLANTThe leaching plant at Harjavalta is especially designed for treatment of the two mattes from the DON Process. The chemistry of the Ieaching process is described prein section 3.1.1, whereas the actual circuit at Harjavalta and its parameters are sented in section 3.1.2. 3.1.1 Outokumpu nickel matte leaching processBased on experiences from the Hajavalta leaching plant and research work using small-scale equipment, Outokumpu has developed its method for Ieaching two different nickel mattes. A flowsheet of the Outokumpu nickel matte Ieaching process is shown in Figure 3. According to the Outokumpu nickel matte leaching concept, the FSF matte is leached in three steps connected in counter-current and the EF matte is Ieached in two steps in series. Main leaching agents are oxygen and sulphuric acid. The main product of the Ieaching plant, i.e. the solution going to cobalt and nickel separation, is achieved from the copper removal step. The copper is obtained as copper sulphide precipitate from the nickel Ieaching autoclave in the FSF pressure leaching step. Any precious metals in the FSF matte are collected into the residue from the total Ieaching autoclave /7/. The iron is precipitated as goethite or hematite in the EF Ieaching circuit. a) Atmospheric copper removalThe atmospheric copper removalin the FSF Ieaching circuit produces a Ni- and Co-sulphate solution with impurities at levels low enough for subsequent cobalt extraction and nickel reduction. The main minerals in the matte are heazlewoodite Ni3S2, copper sulphide Cu2S and a NiCu alloy. The other elements occur partly in separate phases but also mixed into the three main phases. The most reactive phase is the metallic alloy followed by Ni3S2 and then Cu2S.Important reactions are the precipitation of copper and the simultaneous leaching of nickel. Most important net reactions are shown in the following: CuSO4 + NiCu + 0.5O2 Cu2O+NiSO4 (1)2CuSO4 + 2Ni3S2 + 0.5O2 Cu2O + 2NiSO4 + 4NiS (2)3CuSO4 + 2Ni3S2 + O2 + 2H2OCu3SO4(OH)4 + 2NiSO4 + 4NiS (3)2CuSO4 + NiCu + O2 +2H2O Cu3SO4(OH)4 + NiSO4 (4)lron in the solutions fed to the copper removal step is virtually as Fe+, and it is oxidized and precipitated as FeOOH according to FeSO4 + Ni3S2 + 0.75O2 + 0.5H2O FeOOH + NiSO4 + 2NiS (5)but some Fe(OH)3 is also formed. Arsenic, antimony and bismuth are mainly precipitated together with iron in poorly soluble ferric compounds similar to ferric arsenate, FeAs04. Thus, the removal of these elements from the solution is enhanced by an increased iron precipitation. b) Nickel atmospheric leach In the atmospheric leach, the main objective is to leach remaining Ni3S2 as completely as possible leaving NiS as the secondary phase. Other objectives are to control Cu, Fe, As, Sb and Bi in the solutions at levels that can easily be handled in the atmospheric copper removal.The main reaction is leaching of Ni3S2, and for the succeeding nickel pressure leach, it is important that Ni3S2 is converted to NiS as completely as possible. Ni3S2 + H2SO4 + 0.5O2 NiSO4 + 2NiS + H2O (6)Also Cu2S reacts, but more slowly than Ni3S2. A part of the copper is leached, and the remaining part is converted into more oxidized sulphide species. The most common species found are Cul.8S and CuS. Cu2S + 0.2H2SO4+ 0.1O2 Cu1.8S + 0.2CuSO4 + 0.2H2O (7) Cu1.8S + 0.8H2SO4+ 0.4O2 CuS + 0.8CuSO4 + 0.8H2O (8)NiS is partly converted into CuS through a reaction path probably involving an oxidation and a reduction mechanism. As in the copper removal step, most of the incoming Fe, As, Sb and Bi is precipitated at the end of the atmospheric leach. Since the pH is lower in this step than in the copper removal, the solubility of Ni and Cu arsenates will be much higher and the As concantration in the solution is mostly determined by the more stable compounds formed with iron. lron is chiefly precipitated as FeOOH. c) Nickel pressure leachThe aim of the pressure leach is to bring unleached or precipitated nickel and iron into the solution and produce an outlet for copper in the form of copper sulphide. The step consists of a selective nickel leaching part /6, 8/ where no or little oxygen js used and a total leaching part with efficient oxidization. In the selective nickel leaching autoclave, NiS and eventually remaining Ni3S2 are converted into the copper sulphide, digenite, Cu1.8S. The reactions can be written as: 6NiS + 9CuSO4 + 4H2O - 5Cu1.8S + 6NiSO4 + 4H2SO4 (9) 8Ni3S2 + 27CuSO4 + 4H2O - 15Cu1.8S + 24NiSO4 + 4H2SO4 (10) NiS reacts very fast, whereas the leaching rate of Ni3S2 is much slower. Also CuS and Cu+ ions are reacting forming Cu1.8S and sulphuric acid, according to the reaction: 6CuSO4+3CuSO4 + 4H2O5Cu1.8S + 4H2SO4 (11)As the solution is acidic, Fe3+ in solution will be reduced by the sulphide Fe2(S04)3 + l.2bCul.8S 2Fe3O4 +1.25CuS + CuSO4 （12）When Fe3+ is removel from tne solution, a dissolution of FeOOH, FeAsO4 as well as Sb and Bi compounds will proceed as long as free acid and reducing suiphide are available.Thus ,dissolution of the iron compounds is enhaced by keeping a high conlcentration of sulphuric acid in the reactor. On the contrary, Se is precipitated from the solution I this step. In the total pressure leach, any remaining Ni3S2 is leached according to whereas NiS and CuS are leached according to MeS +2O2MeSO4, Me = Ni, Cu （13）Thus, the sulphide of the matte are leached almost completely, and the leach residue from the total leaching autoclave is mainly an outlet for precious metals. d) EF matte leachThe objective of the EF leaching circuit is to leach nickel, copper and cobalt of the matte and precipitate the iron as goethite-hematite. The main component of the EF matte is a metallic alloy NiFe. Also copper may be mixed into the ailoy, Pentlandite （NiFe)9S8) is the major sulphide component of the matte. The EF matte can react very vigorously with sulphunc acid and form hydrogen and hydrogen sulphide if the mixing and the feed of oxygen are not adequate. Especially at high temperatures, the risk of hydrogen formation is high. Therefore, the leaching circuit consists of an atmospheric leach and a pressure leach. The metailic alloys are leached in the atmospheric leach whereas the sulphides are leached in the pressure step. Thus, a high recovery of nickel, cobalt and copper is achieved with a short residence time and also the iron precipitation is optimized by this process arrangement. The EF matte Ieaching can roughly be described by the following overall reactions .Nickel, copper and iron in the metallic phase are leached according to Me + H2S04 +1/2O2 MeS04+ H2O, Me=Ni, Cu, Fe (14)The pentlandite reacts according to (NiFe)9S8 + H2S04 +16.502 9(Ni,Fe)S04 + H20 (15)Hydrogen can be produced according toNiFe+2H2S04 NiS04+FeS04+2H2 (16)Hydrogen sulphide may be formed according to (NiFe)9S8 +4H2S04 5NiS +4FeS04 +3H2S + H2 (17)The formation of these unwanted products is prevented by proiding for an intensive mixing and an excess of oxygen in the solution. Iron is mainly precipitated as goethite in the atmospheric leach: 2FeS04 + 3H20 + 0.502 2FeOOH + 2H2S04 (18)and as hematite in the pressure leach: 2FeSO4 +2H2O +0.5O2 Fe2O3 +2H2SO4 (19)If the pH of the atmospheric leach is lower than 2, iron can be precipitated as jarosite. So, with pH control the iron can be precipitated into a desired form. At the same time, a careful pH monitoring is needed to get a good recovery of nickel and copper. 3.1.2 Harjavalta leaching plant The actual circuit at Harjavalta differs somewhat from the flowsheet in Figure 3.Prior to the expansion project at Harjavalta, there was not any industrial scale experience of leaching high iron mattes. Thus, certain safety measures were taken to ensure the removal of the EF matte iron. A1so, there was naturally a need to utilize existing equipment which led to some special arrangements in the circuit. The main differences compared to the flowsheet in Figure 3 are:The autoclaves in the FSF pressure leach are connected in parallel with respect to the solid material, and the leach residues are combined. The solution from the FSF nickel pressure leach is led to the FSF atmospheric leach.A solid/liquid separation is made after the EF atmospheric leach, and the solution is led to an iron removal autoclave with subsequent liquid-solid separation. The solution from the iron removal autoclave is led to the copper removal step in the FSF circuit whereas the solution from the EF pressure leaching step is recycled to the EF atmospheric Ieaching step.The product solution from the leaching plant is led to the solvent extraction plant for cobalt separation. The copper suiphide precipitate from the FSF leaching circuft is led to the copper smelter whereas the iron residue from the EF leaching circuit is recycled to the fiash smelting furnace. a) Feed and product streamsb) The composition of the raw material for the leaching plant, the FSF and the EF mattes, is specifled in Table l. Sulphuric acid is fed in concentrated form and in the form of anolyte, which is a recycle stream from the nickel electrowinning circuit with. 100 g/l Ni and 50 g/l H2S04.The product solution, going to cobalt SX, is a nickel-cobait sulphate solution. The nickel concentration is 130 g/l. The main targets for the product solution are low concentrations of iron, copper, arsenic, and other impurities. A specification of its composition is given in Table 3.The solid products of the leaching plant are the copper precipitate from the FSF circuit and the iron precipitate from the EF circuit. The copper precipitate, whlch is mainly composed of copper sulphides and iron as well as a small amount unleached nickel, is led to the copper smelter. Any precious metals in the FSF matte are contained in the copper precipitate. The iron precipitate, i.e. goethite or hematite with small amounts of copper and nickel, is recirculated to the nickel flash