NIckel , copper aNd coBalt coaleSceNce IN copper clIff coNverter

The aim of this investigation is to assess the effect of various additives on coalescence of nickel, copper and cobalt from slags generated during nickel extraction. The analyzed fluxes were silica and lime while examined reductants were pig iron, ferrosilicon and copper-silicon compound. Slag was settled at the different holding temperatures for various times in conditions that simulated the industrial environment. The newly formed matte and slag were characterized by their chemical composition and morphology. Silica flux generated higher partition coefficients for nickel and copper than the addition of lime. Additives used as reducing agents had higher valuable metal recovery rates and corresponding partition coefficients than fluxes. Microstructural studies showed that slag formed after adding reductants consisted of primarily fayalite, with some minute traces of magnetite as the secondary phase. Addition of 5 wt% of pig iron, ferrosilicon and copper-silicon alloys favored the formation of a metallized matte which increased Cu, Ni and Co recoveries. Addition of copper-silicon alloys with low silicon content was efficient in copper recovery but coalescence of the other metals was low. Slag treated with the ferrosilicon facilitated the highest cobalt recovery while copper-silicon alloys with silicon content above 10 wt% resulted in high coalescence of nickel and copper, 87 % and 72 % respectively.


Introduction
Vale's the Copper Cliff smelter is one of the largest in the world and extracts nickel, copper, cobalt and precious metals from a concentrate that is approximately equal parts nickel and copper and a nickel-to-cobalt ratio of about 30.Sudbury basin deposits are gaining more interest since nickel and copper containing minerals are accompanied with minor amounts of silver, gold and platinum group metals [1][2].Vale's Copper Cliff operation extracts nickel from a combination of pentlandite ((Fe,Ni) 9 S 8 ) and variable amounts of chalcopyrite (CuFeS 2 ), pyrrhotite (Fe 7 S 8 ) and/or pyrite (FeS 2 ) and violarite (Ni 2 FeS 4 ) [3][4][5].Similarly to other pyrometallurgical extraction processes [6], it is technically difficult to produce high-grade nickel concentrate without loss of nickel due to the intergrowth of the various sulphide minerals and the presence of dissolved nickel in the pyrrhotite crystal lattice.Therefore, a bulk concentrate of about 20 wt% CuNiCo is treated in two stages; flash smelting and converting [7][8].
During flash smelting, a 20 wt% CuNiCo concentrate is fed in to an INCO flash furnace and is blown with oxygen burners located on opposite end walls of the vessel.Silica is added as a flux to ensure immiscibility of nickel sulphide rich matte and iron oxide based slag.Once the target matte level is reached the matte is tapped and sent to converters.A segment of Vale's Copper Cliff Smelter flowsheet is given in Figure 1.
The matte generated during flash smelting is a homogeneous mixture of nickel, copper, cobalt, iron and sulphur.Removal of the remaining iron and sulphur is accomplished by the use of Peirce-Smith converters.Charging of the converter occurs when matte is poured into the vessel through an opening in the top.The molten matte is blown with oxygenenriched air Sulphur to remove the remaining iron and sulphur in the following generalized reaction: 2FeS + 3O 2 + SiO 2 = 2FeO·SiO 2 + 2SO 2 (1) The iron oxide forms a slag phase that floats above the newly formed matte phase of low iron content (0.5 wt%-2.5 wt%).Due to high oxygen pressure and the lower temperature required to eliminate residual iron from the matte, a large amount of matte remains entrained in the slag phase.Since the output of a converter is nearly directly proportional to the amount of oxygen blown through the charge, oxygen enrichment is a natural choice for use in converter practices.The oxygen enrichment not only increases

J. Min. Metall. Sect. B-Metall. 52 (2) B (2016) 143 -150
the conversion rate but also allows for addition of cold charge, which allows for the recovery of nickel and cobalt from various solids formed in the smelter.This cold charge prevents temperature rise that would erode the refractory lining of the converter [10].A typical chemical composition and operational parameters of smelting process is summarized in Table 1.
This study follows an experimental process developed by Wolf and Utigard [12] for converter slag settling.The effect of various industrial and prospective reductants and/or flux additions on coalescence of valuable metals entrained in converter slag is investigated and compared with settling tests with no reductant/flux addition.The matte recovery rate for different additions is reported.The matte and slag produced during the tests are characterized with optical and scanning electron microscopy to determine the composition and phases present.Finally, X-ray diffraction is used to analyze selected slags to determine the compounds present in the slag.

Factors affecting metals loss during smelting and converting
For slags formed during the smelting or converting steps, the amount of metal loss is proportional to the matte grade, i.e. as the matte grade increases the amount of metal lost also increases.These metal losses are either dissolved as oxides or are present as physically entrapped matte prills.Dissolved metal losses are related to sulphide and oxygen activities and the matte grade.During flash smelting, a combination of a low matte grade and low oxygen potential causes the dominant metal losses to be sulphides [8].During converting, the matte grade is high with increased activities of nickel and copper sulphides and decreasing activity of FeS.The first cause of the matte losses is the increased sulphide activity that drives the system to form more metal oxides that further dissolve into the slag according to general chemical reaction: Another cause for the matte losses occur when the matte particles are carried up by the gaseous bubbles that form during this converting process [13].Presence of gas bubbles at the matte-slag interface is an important factor affecting matte's settling rate due to the limited residence time that causes matte droplets to not settle through the slag layer.
A third significant cause for the matte losses are low temperature zones in the converter bath.The solubility of matte decreases with temperature and this leads to the precipitation of matte particles from the slag [14].
Considering the large amount of slag produced during nickel smelting, a significant fraction of nickel, copper and cobalt are contained in slags.In pyrometallurgical 'slag cleaning' two concurring phenomenon contribute in to coalescence and subsequent valuable metals recovering from slag, namely physical settling of matte particles and chemical reduction of dissolved metals.The primary reducing agents used in this process are coke, coal, pig iron or other carbon-containing materials as well as ferrosilicon.The reductant addition will reduce metal oxides into either metallic or sulphide forms, while at the same time reduce solid magnetite to liquid FeO which decreases slag viscosity and improves settling rate [15].It is also common practice to add sulphide concentrate, pyrrhotite or pyrite to the process.This addition of sulphur will favor the transformation of nickel, copper and cobalt oxides into sulphides and at the same time will lower the melting temperature of the matte to less than 1300 ºC from the 1400 ºC of the nickel iron alloys [8][9]16].

experimental
Converter slag samples from Vale are periodically received in approximately one kilogram batches.400 g samples were taken from the respective batch.Each of the slag samples were poured into a cylindrical alumina crucible (17 cm high x 3.8 cm inner diameter) and placed inside a 30 cm × 40 cm × 36 cm chamber of the Lindberg 12500 W electric furnace and heated to the set temperature.Flux and reductant additions, ranging from 1 wt% to 20 wt%, were measured out and then combined with the correct amount of converter slag while maintaining the 400-gram of total weight.Once the slag and additives were combined, the ingredients were mixed vigorously and loaded into the furnace.Silica and lime were chosen as fluxes while reductants were chosen based on a previous work reported by Wolf and Mitrašinović [17] where various factors were taken in consideration, such as density, melting temperature relative to converter slag, ability to reduce molten FeO to molten metallic iron, availability and cost.Major components of converter slag are FeO and SiO 2 that are forming a fayalite type slag (2FeO-SiO 2 ) with a melting temperature close to its congruent point and therefore operational temperature could be further reduced below 1200 o C. The pseudo-binary system of the FeO-SiO 2 system is given in Figure 2. Flux and reductant additions together were also tested to observe how their combination would affect metal recovery.A summary of all tests is shown in Table 2.
After settling tests were complete the settled matte was separated from the slag.The separation was achieved by first, chipping away the alumina crucible with a chisel and hammer, second the matte and slag phases were separated by a combination of hammering and wedging the interface apart with a flathead screwdriver.Once the matte and slag phases were disjoined, samples of each phase were sent to Vale Technical Services Limited (VTSL) for chemical assaying.

results
Copper Cliff slag comprises of magnetite embedded in fayalite matrix.While the slag's microstructure is consistent physical appearance vary from smooth surface to some very porous particles.Residues of converter matte that are found in slag vary in size from 1mm round shape to very fine 5 µm thick elongated matte prills.Figure 3 shows a micrograph of the matte and slag after one hour treatment at 1300oC without additive addition.The matte consist of Cu-S phase (Fig. 3, region 2) embedded in Ni-S matrix (Fig. 3, region 1).The EDS examines showed that the region 2 in Fig. 3 is Cu2S-FeS while region 1 corresponds to Ni3S2-FeS phase.In previous work it was found that Lamella-like structures in matte is more pronounced for shorted settling times [20].
The amount of matte, slag and crust produced after testing and the normalized chemical assays of nickel, copper, cobalt, iron, sulphur and oxygen is given in Table 3. Settling trials (Tests 1-3, Table 3) conducted on converter slag without flux or reductant indicated satisfactory tests repeatability.Settling at 1300 o C at one hour recovered about 60 % of all copper, 50 % of all nickel and about 6 % of all cobalt while keeping iron recovery at 0.3 % and sulphur recovery at about 10 %.

Effect of silica and lime fluxes
The matte yield and partition coefficients of nickel, copper, cobalt, iron and sulphur from each test is summarized in Table 4. Partition coefficients were calculated based on the matte assays obtained by VTSL.No data is available for the tests in which a chemical assay was not conducted.When comparing tests with added silica to the converter slag (Tests 10-14) to those of without flux (Tests 1-3), the recoveries and partition coefficient of nickel, copper and cobalt are higher while the iron and sulphur transition is unchanged for tests with silica addition.
It is known that additions of up to 20wt% lime to converter slag will lower the melting temperature from about 1200ºC to less than 1100 ºC.However, the addition of 20wt% lime resulted in no matte collection, in fact, the matte particles were simply observed to descend to the bottom portion of the crucible but as independent prills as seen in crosssection macrograph in Figure 4.It was also shown that a 5wt% addition of lime to converter slag prior to settling would lower both metal recoveries and partition coefficients relative to settling with no additions.For this reason, no further investigation was carried out on the additions of lime to converter slag.

Carbon containing reductants
Employing pig iron as reductant (Tests 19-25) revealed two conclusions: a) tests with pig iron settles less matte than settling without additives and b) the recovery of nickel and cobalt increases while the copper and sulphur recoveries decreases.Comparing the results for pig iron (Tests 19-25) with the settling without additives (Tests 1-3) it is clear that coalescing is greater for nickel, copper and cobalt while unchanged for iron and lower for sulphur.This indicates the matte recovered from the addition of 5wt% pig iron to converter slag is of a higher grade than of matte recovered settling without additives.Furthermore, an addition of 2.5wt% pig iron settled only about half the matte than settling without additives, and hence had lower recovery while tests with 7.5wt% pig iron addition did not produce any coalescing as the matte formed metallic layers that were intertwined within the slag at the bottom portion of the crucible.Using 5 wt% coke as reductant did not produce any matte.Instead, two distinct slag regions were found inside the crucible.The upper half of the crucible was a hard, porous slag that contained small (< 1 mm) metallic prills.While the bottom half of the crucible was virtually identical to the slags that formed in all other tests.These two phases are presented in Figure 5.

Si alloys reductants
The addition of 5 wt% 90wt%Cu-10wt%Si maintained copper recovery in the same range as settling without additives but decreased coalescing of both nickel and cobalt significantly.Figure 6 shows an EDS analysis of matte formed during a 5 wt% 70wt%Cu-30wt%Si addition (Test4-7).The addition of 5 wt% 70wt%Cu-30wt%Si alloy increased the recoveries and partition coefficients of nickel and cobalt while at the same time lowered the sulphur recovery and kept the iron and copper recovery in the same range as settling without additives.
It is possible to increase the recoveries of nickel and cobalt while at the same time lowering copper, iron and sulphur recoveries by the addition of 5 wt% ferrosilicon to converter slag (Tests 26-28, Tables 3  and 4).Examining the high partition coefficients of nickel and cobalt and low sulphur partition coefficients indicates that ferrosilicon is an effective reducing agent.Unfortunately, the partition for copper decreases in the ferrosilicon tests and the iron partition doubles indicating lower matte grade.The partition coefficients for the four examined metals elements are around 80 for copper, 80 for nickel, five for cobalt and 0.2 for iron.

discussion
Slags originated from nonferrous metals extraction processes impose a great environment attentive.In developed countries strict regulations already force large nickel and copper producers to employ separate cleaning processes for slags with high heavy metals content.Therefore, with the goal to achieve less than one percent of heavy metals in disposed slags with lower process disruptions [19] a better understanding of thermodynamic and kinetic parameters of settling process should be done and research on the development of new additives (reductants and/or fluxes) [20,21].
A better understanding of the assimilation mechanism of new additives is key factor allowing further advancements in process efficiencies.The faster an additive can dissolve into solution, the process capacity increases and the chances for losses are lower.Therefore, the most favorable path of the additive assimilation is its melting followed by simultaneous exothermic reaction and assimilation process.Typical industrial additives are ferrocarbon alloys such as pig iron.This type of additive has a melting point above temperature of the liquid bath and does not exhibit any exothermic chemical reaction.The assimilation mechanism for ferrocarbon is defined by the formation of the solid layer of slag around the additive that subsequently has to melt and thereafter ferrocarbon A. Wolf   dissolves into the slag.Acquired results confirmed that copper-silicon reductants significantly improved valuable metals coalescence from Copper Cliff converter slag because its melting point is below the temperature required in the extraction process and the existence of exothermic chemical reactions [22].Hence, the assimilation mechanism of the copper-silicon additive is defined by concurrent melting and exothermic chemical which further significantly ease maintaining the operating temperature inside the converter [17,22].An interesting result could be obtained by adding reductants and fluxes in combination with one another where exothermic and micro exothermic effects of selected reductants add up on the increased coalescence of nickel, copper, cobalt and other precious metals.

conclusion
An investigation on the coalescence of nickel, copper and cobalt in fayalite slag was conducted in order to understand the behaviors of these metals and how the addition of selected reductants and/or fluxes will affect the metal recovery from Copper Cliff converter slag.Silica was added to converter slag to lower the Fe/SiO 2 ratio of the slag from 2.3 to 2. Such additions were found to lower the recoveries of nickel, copper and cobalt but increase the partition coefficients of copper and nickel as compared to settling with no additions.It can be concluded that silica additions will recover a higher grade matte when compared to the matte recovered from tests with no additions.Reductants that slightly increased metal recovery were: 90 wt%Cu-10wt%Si, FeS and coke.For 90wt%Cu-10wt%Si there was an insufficient amount of silicon to reduce the converter slag, coke was too light and only reduced the upper portion of the slag.Nickel, copper and cobalt recoveries and partition coefficients were increased if 5 wt% each of pig iron, ferrosilicon (15 wt% Si) and 70wt%Cu-30wt%Si were added individually to converter slag prior to melting.Each of these three reductants favored the formation of a highly metallized matte.After comparing the nickel, copper A. Wolf  and cobalt recoveries data it was concluded that ferrosilicon facilitated the most effective coalescence of cobalt while pig iron and ferrosilicon are equally effective in recovering nickel and 70wt%Cu-30wt%Si is the most effective for copper recovery.

Figure 4 .
Figure 4. Cross-sectioned crucible with the converter slag sample after adding 20 wt% lime and settling at 1300 o C for one hour.

Figure 5 .
Figure 5.Upper and lower part of converter slag after addition of 5 wt% coke and treatment at 1300 o C for one hour

ackNoWledgMeNtS
Authors are grateful to Prof. Torstein Utigard for all of his guidance and suggestions.

Table 2 .
Summary of conducted trials showing additive (flux or reductant) used and the total mass of the sample before melting.All settling tests were performed at 1300 o C for 1 hour.

Table 3 .
Normalized chemical assays of copper, nickel, cobalt, iron, sulphur and oxygen for matte and slag formed after one hour treatment at 1300 o C. Characteristic components of matte and slag after one hour treatment at 1300 o C without additive addition.1)Ni 3 S 2 -FeS, 2) Cu 2 S-FeS, 3) Ag particle, 4) FeO, 5) FeO-SiO 2 and 6) entrained matte prill.

Table 4 .
and A.M. Mitrašinović / JMM 52 (2) B (2016) 143 -150 Summary of all tests done showing used additive and the weight of the collected matte and slag with partition coefficients for copper, nickel, cobalt, iron and sulphur after coalescing for one hour at 1300 o C. *Crust is the part of sample that remained attached to the crucible