CHROMIUM DISTRIBUTION BETWEEN LIQUID SLAG AND MATTE PHASES
R Hurman Eric
School of Chemical and Metallurgical Engineering
University of the Witwatersrand, Private Bag 3, WITS 2050 Johannesburg, South Africa Keywords: Chromium, partition/distribution, slag, matte,
Abstract
The distribution of chromium between liquid silicate slags and copper-iron-nickel matte phases encountered in electric smelting of PGM containing South African sulphide concentrates were experimentally studied under controlled partial pressures of oxygen and sulphur. The reported experiments were conducted under silica saturation through the use of silica crucibles. Seven representative slag compositions were equilibrated with a typical sulphur deficient matte containing 18% Ni, 11% Cu, 42% Fe and 29% S. The slag constituents varied in the following ranges: SiO2: 42-58%, Al2O3: 3.5-9.0%, Fe2O3: 13-21%, MgO: 15.6-25%, CaO: 2-15%, Cr2O3: 0.2-3.5%. The slag and matte samples were synthetically prepared from pure components. The chromium content of the two phases was analysed chemically. According to the present available results of this ongoing research it was found that the partition of chromium to the matte phase decreased with an increase in the partial pressures of both oxygen and sulphur where the value of the distribution coefficient of chromium between the matte and the slag phase varied from as low as 0.07 to as high as 5.5.
Introduction
The copper-nickel and PGM (Platinum Group Metals) bearing sulphide concentrates produced in South Africa are smelted in electric furnaces. The slags obtained in the smelting of these low grade concentrates have relatively high MgO contents, which as a consequence of their high electrical resistivity renders them suitable for sufficient heat generation needed for the smelting process. The other main slag constituents are SiO2, FeO, Al2O3 and CaO. Originally the electric furnace smelters were designed to utilize the sulphide concentrates derived from the Marentsky Reef of the Bushveld Complex, which contained insignificant amounts of chromium oxide in the ore body resulting in very low concentrations of chromium in the slags mentioned above. As the reserves of the Marentsky Reef decreased, the smelters started to utilize another reef known as UG2, which contained similar PGM and Cu, Ni contents but was richer in chromium oxides. In fact the tailing stream of the concentration process for the UG2 reef is significantly rich in chromite and is actually used as a fine sized chromite concentrate in ferrochromium smelters using DC open arc furnaces. The relatively high chromite content of the UG2 sulphide concentrate employed in PGM smelters results in slags with much higher chromium contents.
Thus chromium is regarded as an element of high significance in the smelting of these sulphide concentrates especially from the UG2 Reef because its presence is deleterious to the process. The solubility of chromium oxide in the typical PGM smelting slags at smelting temperatures of
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
around 1300oC normally utilized is low and hence remains as solid oxide. The slag becomes a solid-liquid mixture with increased effective viscosity which will cause potential problems of settling of matte droplets through the slag, decreased mass transfer between slag and matte as well as flow and tapping problems. Solid chromium oxide also has a higher density then the slag and the matte and will tend to settle at the hearth of the furnace eventually decreasing the working volume and hence the smelting capacity of the furnace.
In general sulphide smelting slags are either saturated or are close to saturation with respect to both silica and magnetite and hence the oxygen potential in the system is near FeO-Fe3O4
equilibrium at the temperatures employed. The solid chromium oxide can react with solid magnetite (if exists due to saturation) and form spinels with densities intermediate between that of the slag and the matte and can form false bottoms at the slag-matte interface. This will stop slag-matte interactions and reactions at these locations and affect negatively the smelting operation.
The solubility of chromium in these slags at the given temperatures can only be increased by operating under more reducing lower oxygen potentials where Cr3+ is reduced to Cr2+ (CrO in molecular notation). While operating under more reducing conditions enhances the solubility of chromium in the slag and hence decreasing the magnitude of potential problems, there will be the possibility of increased transfer of chromium into the matte, which could significantly complicate the downstream processes of recovering Cu, Ni and PGMs. Another way to increase the solubility of chromium in the current slags is to increase temperature of operation. At higher temperatures the viscosity of the matte phase decreases considerably and tends to penetrate into any cracks or porosities of the refractories of the hearth region which can cause severe operational problems. At present the South African smelters use both of these approaches;
decreasing the oxygen potential by creating more reducing conditions through charging some carbonaceous materials and increasing temperatures up to 1500oC by increasing the power. Thus the partitioning of chromium between liquid silicate slags and liquid sulphide matte phases is obviously of importance in the processing of these ores.
There is extremely limited information in literature on slag-matte equilibrium and distribution of chromium between these phases under controlled conditions pertinent to the PGM smelting conditions mentioned above although numerous studies can be seen most of which fail to ensure correct equilibrium conditions and correct gas composition control. The partition of transition elements in a basic silicate melt with sulphide phases has been examined by Al’mukhamedov and Medvedev1, and they determined a partition coefficient of chromium of 1.9 to 2.0 between the sulphide and silicate phases. However, the experiments were done in an induction furnace with helium gas in the presence of carbon, indicating that, for their system, the partial pressures of oxygen and sulphur were indeterminate. Some preliminary work was done at the Council for Mineral Technology (Mintek) of South Africa2. This work involved the use of UG2 concentrate in contact with carbon and mixtures hydrogen and carbon dioxide gas. It was found that chromium partitions strongly into the matte phase under reducing conditions and that the distribution coefficient of chromium in the matte and the slag phases gradually decreases with an increase in the partial pressure of oxygen. However these experiments were performed under non-equilibrium conditions, since sulphur was escaping from the system throughout the runs.
These tests were supplemented by experiments performed at 1450oC under controlled partial pressures of sulphur and oxygen at sulphide saturation using synthetic mixtures of oxides and four natural rock types approximating the slag compositions2. The samples in the form of pellets were fixed to platinum loops and due to sulphide saturation and adjusted conditions only a very
small amount of sulphide melt was formed. Thus the partitioning of the chromium was determined by electron microprobe. The use of platinum wire loops to hold the samples resulted in iron loss to the platinum (which was compensated based on an approximate calculation procedure) and the studied system was somewhat far from actual metal slag interactions in the PGM smelting processes. Nevertheless the partition results although highly variable indicated that the chromium in sulphide melts increased with a decrease in the partial pressure of oxygen.
Experimental Procedure Starting Materials
A total of seven slag samples were prepared from reagent grade powdered oxides; SiO2, Al2O3, Fe2O3, MgO, CaO and Cr2O3, three of which approximating UG2 (designated as U) slags and three representing Marentsky (designated as M) slags. The main difference between the two types of slags is their Cr2O3 concentration. While the chromium oxide concentration of Marentsky slags is less than 0.5%, the UG2 slags usually contain over 2.5% Cr2O3 reaching up to 3.5%. The slag designated as 9UM can be considered to represent the case where the two ores are blended. The pure oxides were first dried in an oven at 105oC followed by mixing in the appropriate proportions in an agate mortar under a liquid blanket of acetone until all the acetone vaporized and again being bone dried in an oven. The powder mixtures were then pressed into disk shaped pellets followed by sintering and homogenization by heating to 1200oC for 12 hours in a muffle furnace. After cooling they were kept in closed bottles inside a desiccator. The initial slag compositions are summarized in Table I. The Cr to Fe ratio of the slags varied in the range 0.015 to 0.2 and their basicity were in the range 0.376 to 0.729. The basicity was defined as:
B= (CaO% + MgO%) / SiO2%. (1) The PGM smelting matte compositions do not vary much; Ni concentration is in the 16% to 18%
range, Cu concentration in the 9% to 11% range, Fe concentration in the 38% to 42% range and S concentration in the 26% to 28% range. Thus only one single matte sample was prepared from pure electrolytic grade Ni, Cu and Fe and triple distilled pure sulphur. The metallic powders and sulphur powder were carefully weighed and mixed and were then put in quartz tubes of 20mm in diameter which were first filled with argon gas and then evacuated by a mechanical-diffusion pump couple and sealed under vacuum by a hydrogen torch. The quartz sample tubes were heated to 1200oC to homogenize the composition and form the matte phase. After homogenization quartz tubes were carefully broken and the sulphide matte samples were recovered and kept in desiccators.
Equilibration Procedure
A gas-tight vertical molybdenum resistance furnace has been assembled and employed for the study of slag-matte equilibrium. Molybdenum wires were protected from oxidation by the use of ammonia gas, which cracks into hydrogen and nitrogen at high temperatures. Actual sample temperatures in the furnace were measured with a Pt-6 %Rh/Pt-30%Rh (B-type) thermocouple.
In the slag-matte distribution experiments performed at 1450oC quartz crucibles were used
saturating the slag with respect to silica similar to actual industrial conditions. The slag to matte mass ratio was kept constant at 1.5 while the total sample mass was 10 grams.
Table I. Initial slag sample compositions
M- Marentsky type, U: UG2 type, UM blend of UG2 and Marentsky, All composition values are in mass %
The appropriate fixed partial pressures of oxygen and sulphur were achieved by mixing CO, CO2
and SO2 gases. A manometer type of gas mixing apparatus employing calibrated capillary flowmeters was used to mix the three gases in the correct volume proportions. The calibration of the capillary flowmeters was done by the bubble-flowmeter technique where special care was taken to ensure that the soap solution was saturated especially in the calibration of the SO2
capillary flowmeter. The PO2 calibration was tested by the use of iron-wustite reaction at 1303oC and a value of logPO2 (atm) = -10.79 was recorded as comparison to the theoretical value of - 10.72. The current experimental value is also in very good agreement with previous data2, 3. The PS2 calibration was tested by the use of pressed pellets of ruthenium and tests the presence of ruthenium sulphide by microscopic techniques. At 1127oC, the equilibrium between ruthenium and ruthenium sulphide was found to be at logPS2 (atm) = -2.56, compared to the theoretical calculated value of -2.35.
No SiO2 Al2O3 Fe2O3 MgO CaO Cr2O3 Basicity Cr/Fe
1M 42.00 6.00 21.00 15.60 15.0 0.40 0.729 0.019
5U 49.0 3.50 17.00 25.00 2.00 3.50 0.551 0.201
7U 50.50 9.00 13.00 20.00 5.00 2.50 0.495 0.188
8U 51.25 5.50 16.25 17.50 7.50 2.00 0.488 0.120
9UM 52.00 6.50 15.25 16.25 8.50 1.50 0.476 0.096
15M 58.00 7.00 13.00 18.50 3.30 0.20 0.376 0.015
12M 54.20 7.50 15.00 20.00 3.0 0.3 0.424 0.020
The quartz crucibles containing the samples were introduced to the furnace from the bottom and the furnace tube was immediately sealed through the use of brass flanges while the sample is still at the bottom flange area. The reaction tube of the furnace was flushed for about 15 minutes with spectrographically pure argon before the actual gas atmosphere was employed. After the actual gas mixture was turned on the sample was raised to the hot zone by the use of a quartz rod and pedestal and equilibrated for 24 hours and then fast quenched by speedy extraction through the bottom brass flange and immersion into ice water. The slag and matte samples separated into two distinct liquid layers which were mostly preserved during quenching and solidification. Slag and matte were recovered by breaking the quartz crucible, the reaction between the quartz crucible and the slag was not extensive. Both samples were analyzed for their chromium content by the ICP technique. Duplicate analyses were performed on each sample.
Experimental Results and Discussion
It was observed that in spite of all the care and precautions taken to ensure adequate equilibration and homogenization of the samples some variation in the partition values of chromium occurred.
In certain instances, the discrepancy between the chromium contents of the phases and the starting materials is possibly due to the formation of varying amounts of chromium containing spinels at the slag matte interface and also due to some inconsistencies in the chemical analysis procedure employed. Another potential source of error is the inhomogeneous nature of some of the sulphide melts, which may show exsolution of an oxide phase2 possibly during quenching although this is thought to be occurring rarely. It must however be mentioned that this is an ongoing research and the results presented here are preliminary and initial in character.
Nevertheless the results indicate strong tendencies of partition values of chromium with respect to partial pressure of oxygen and sulphur. The available results are collected in Table II and are illustrated in Figures 1 and 2. As can be seen the partition/distribution ratio of chromium defined as:
PCr = (Mass percent of Cr in the matte) / (Mass percent of Cr in the slag) (2) increases very rapidly with decreasing partial pressure of oxygen even when the sulphur partial pressure is decreasing for all types of slags at 1450oC. The change of partition ratio of chromium with changing partial pressure of sulphur will definitely require further investigation.
Chromium prefers to dissolve in the sulphide matte phase under reducing conditions. This is a result of increased proportions of chromium existing as CrO in the silicate slag and, as a consequence, more chromium partitions into the sulphide matte phase, which is similar to the behavior of iron2. It is apparent that the oxygen potential in the system is the more dominant parameter in determining the behavior of chromium.
The plotted partition ratios of chromium for each slag displayed linear relationships and the regression equations had very high correlation coefficients as shown below in equations (3) to (9) as a function of the partial pressure of oxygen and in equations (10) to (16) as a function of the partial pressure of sulphur.
PCr = -1.292 logPO2 – 10.900 (1M, r2 = 0.992) (3)
PCr = -2.524 logPO2 – 2.129 (5U, r2 = 0.981) (4)
Table II. Experimental Results
Sample No log PO2 Iog PS2 Cr in slag
(mass %)
Cr in matte (mass %)
Pcr =(Cr)M/(Cr)S 1M
B=0.729 Cr/Fe=0.019
-8.5 -1.2 0.410 0.030 0.07
-9.0 -1.4 0.357 0.251 0.70
-9.5 -1.7 0.107 0.161 1.50
-10.0 -2.1 0.205 0.390 1.90
-10.5 -2.5 0.193 0.521 2.70
5U B= 0.551 Cr/Fe=0.201
-8.5 -1.2 1.360 0.150 0.11
-9.0 -1.4 2.431 3.841 1.58
-9.5 -1.7 2.481 6.962 2.81
-10.0 -2.1 2.321 8.113 3.50
-10.5 -2.5 1.706 9.315 5.46
7U B= 0.495 Cr/Fe=0.188
-8.5 -1.2 1.650 0.660 0.40
-9.0 -1.4 2.853 3.538 1.24
-9.5 -1.7 2.113 3.783 1.79
-10.0 -2.1 2.051 5.107 2.49
-10.5 -2.5 2.021 6.559 3.24
9UM B= 0.476 Cr/Fe=0.096
-8.5 -1.2 0.827 0.612 0.74
-9.0 -1.4 0.648 0.713 1.10
-9.5 -1.7 0.713 1.062 1.49
-10.0 -2.1 0.476 0.881 1.85
-10.5 -2.5 0.373 0.880 2.36
15M B= 0.376 Cr/Fe=0.015
-8.5 -1.2 0.071 0.035 0.44
-9.0 -1.4 0.084 0.110 1.31
-9.5 -1.7 0.056 0.112 2.00
-10.0 -2.1 0.064 0.166 2.59
-10.5 -2.5 0.053 0.178 3.36
8U B= 0.488 Cr/Fe=0.120
-9.5 -2.6 0.0993 0.685 0.69
-10.0 -3.0 1.286 1.980 1.54
-10.5 -3.5 0.623 1.320 2.12
12M B= 0.424 Cr/Fe=0.0.20
-9.5 -2.6 0.144 0.151 1.09
-10.0 -3.0 0.076 0.1401 1.84
-10.5 -3.5 0.071 0.183 2.57
PCr = -1.386 log PO2 – 11.335 (7U, r2 = 0.9967) (5)
PCr = -0.798 log PO2 – 6.073 (9UM, r2 = 0.995) (6)
PCr = -1.424 log PO2 – 11.588 (15M, r2 = 0.996) (7)
PCr = -1.430 log PO2 – 12.850 (8U, r2 = 0.988) (8)
PCr = -1.481 log PO2 – 12.967 (12M, r2 = 0.999) (9)
PCr = -1.918 log PS2 – 2.041 (1M, r2 = 0.969) (10)
PCr = -3.757 log PS2 – 3.995 (5U, r2 = 0.963) (11)
PCr = -2.062 log PS2 – 1.836 (7U, r2 = 0.977) (12)
PCr = -1.196 log PS2 – 0.620 (9UM, r2 = 0.990) (13)
PCr = -2.081 log PS2– 0.176 (15M, r2 = 0.971) (14)
PCr = -1.571 log PS2 – 3.316 (8U, r2 = 0.971) (15)
PCr = -1.637 log PS2 – 3.137 (12M, r2 = 0.995) (16)
The findings of this work are in reasonably good agreement with the work done by De Villiers and Kleyenstuber2; both studies indicate that the distribution of chromium to the liquid sulphide phase is more favorable under lower oxygen partial pressures and that linear relationships are observed between the partition ratio of chromium and partial pressure of oxygen. Despite also varying the sulphur partial pressure no clear tendency were observed in their study2. The range of the partition ratio of chromium in their work was narrower changing from about 0.27 to 3.16 but also their sulphur partial pressure range was narrower. It must again be emphasized that their work is not a true slag matte equilibrium; and the authors2 themselves expressed that there was also doubts that equilibrium may not have been fully reached. The partition ratio of chromium can, in principle, be related to the following chemical exchange reaction: CrO + ½ S2 = CrS + ½ O2 (17)
K = (PO2 / PS2)1/2 (aCrS / aCrO) (18) Due to limited data on Standard Gibbs Free Energy of formation of CrO and CrS and lack of data on activities and activity coefficients of both CrO and CrS at low concentrations in the slag and
Figure 1. The variation of the partition ratio of chromium with the partial pressure of oxygen.
Figure 2. The variation of the partition ratio of chromium with the partial pressure of Sulphur.
matte compositions used in this study, this approach was not pursued. However equation 18 clearly indicates that at a given temperature when the equilibrium constant is fixed, the partition ratio of chromium, here the activity ratio of CrS to CrO, will increase by decreasing partial pressure of oxygen qualitatively in accord with the findings of this work.
Conclusions
The partitioning/distribution ratios of chromium between silicate slags and sulphide mattes encountered in electric smelting of PGM containing Cu-Ni concentrates of South Africa have been studied at 1450oC under silica saturation and under controlled partial pressures of oxygen and sulphur. The available results of this ongoing research presented here are somewhat variable but definitely show a strong trend of increasing the chromium content in the oxide slag phase with an increase in the partial pressure of oxygen while the partial pressure of sulphur also increases. The partition coefficients determined changed from a low value of 0.07 to a high value of 5.5.
References
1. Al’ Mukhamedov, A. I., and Medvedev, A.Y. An experimental investigation of the interaction of a basic silicate melt with sulphur containing phases. Geochem. Int., Vol.16, no.1. 1980. pp 6-16.
2. De Villiers, J.P.R., and Kleyenstuber, A.S.E. The partitioning of chromium between sulphide and silicate melts at controlled partial pressures of oxygen and sulphur. Mintek Report, No. M139D, 1984.
3. Huebner, J.S. Oxygen fugacity values of furnace gas mixtures. Am. Mineral., Vol.60.
1975. pp. 815-823.
