THE DISTRIBUTION RULES OF ELEMENT AND COMPOUND OF COBALT/IRON/COPPER IN THE CONVERTER SLAG OF COPPER
SMELTING PROCESS
Hongxu Li1,2, Ke Du1,2, Shi Sun1,2, Jiaqi Fan1,2, Chao Li1,2
1 School of metallurgical and ecological engineering, University of science and technology, 30#
Xueyuan Road, Beijing, 100083, China
2The Beijing Key Laboratory of Recycling and Extraction of Metals (REM), University of science and technology, 30# Xueyuan Road, 100083, Beijing China
Keywords: converter slag, distribution rule, isomorphism, mineralogy
Abstract
In ISA copper smelting process, recovery Co from the converter slag usually through reduction and vulcanization method. While Co usually exists in fayalite and iron oxide, in the form of isomorphism-phase by replace of Fe. By analyses different micro areas of the converter slag using SEM and EDS, the distribution trends of Fe and Co, Cu and Co were acquired. The results indicate that the percentage of compositions of Fe and Co present positive correlation, while those of Cu and Co present negative correlation. According to the distribution trends, the distribution curves of Fe ~ Co and Cu ~ Co are fitted, and the mechanism has been studied based on the oxidation order of the three metal sulfide as FeS > CoS > Cu2S, and the similarity of the element property of Cu and Co such as atom outer shell electron distribution and ionic radius, which will provide necessary theoretical reference for effective recovery of cobalt by reduction smelting process.
Introduction
The ISA smelting has become a very widely applied process in copper smelters in the world these years. The content of cobalt in the burden of ISA furnace is 0.1~0.4%, and based on the calculation during ISA smelting, about 60% cobalt comes into matte, and goes to converting process with matte; in converting process, about 50% cobalt comes into converter slag, whose content of cobalt reaches 0.8~2.5%. In converting process, almost all the iron, cobalt and several amount of copper is oxidized coming into the converter slag, and generates compounds with gangue which have complex composition and structure, while the distribution rules of Fe~Co and Cu~Co appeal to be different. Commonly, the recovery of Co from converter slag is through reduction and vulcanization method, based on which moderate vulcanizing agent is added with the reducing agent to guarantee the reduced metal Fe, Cu and Co can easily dissolved in matte forming Fe-Cu-Co alloy; and next, the system cooling process is choose to guarantee the nucleation and growth of the alloy crystal; at last, the matte is crushed and Co is separated and recovered by magnetic separation and hydrometallurgy method. Through the reduction and vulcanization method, the recovery rate of Co is able to reach 95%.
Cobalt is a kind of significant strategic metal, which has excellent physical and chemical performance and mechanical property, and whose minerals and compounds have a wide range of applications in material field such as ceramics, glass, and enamel etc. In the late 20th century,
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
cobalt and its alloy began to be widely used in areas of machine, chemical industry, aeronautics and astronautics etc., the consumption of which rises every year. Therefore the high amount of cobalt in the converter slag has a high recovery value [1-3], and grasping the distribution rules among iron, copper and cobalt will definitely contribute to the recovery of cobalt.
Composition and distribution rules of Fe/Cu/Co in the converter slag The elements composition of the converter slag sample
The result of the chemical analysis of the converter slag samples provided by Chambishi Copper Smelter LTD (CCS) is shown in Table I.
Table I. The content of the main elements in the converter slag samples /%
Si S Cu Co Fe
Converter slag sample 1 8.54 0.65 8.56 2.84 48.55
Converter slag sample 2 10.03 0.58 8.76 1.97 48.97
Average 9.29 0.62 8.66 2.41 48.76
From Table I we know that the converter slag samples have very high content of cobalt which can reach 2.41%, besides, iron and copper also have relatively high contents, which are 48.76%
and 8.66%.The result of the XRD analysis of the converter slag sample is shown in Figure 1.
Figure 1. The XRD spectrum diagram of the converter slag sample
As shown in Figure 1, in the converter slag, iron, copper and cobalt can combine with each other generating complex compounds. As for iron, it is not only able to generate iron oxide and fayalite, but also to generate CuFe2O4 and CoFe2O4 with copper and cobalt; while copper has not generated compound with cobalt.
The correlation of iron, copper and cobalt in the slag
The SEM energy spectrum analysis is conducted upon the converter slag sample to analyze the composition of the 38 selected micro areas, the images and date obtained from the test are shown in Figure 2 and Table II respectively.
Figure 2. The SEM image of the converter slag sample
Table II. The composition of the 38 selected micro areas of the converter slag sample /%
Fe Cu Co Si O S
1 75.53 1.06 5.3 0.87 19.93 0.35
2 54.44 6.05 2.31 10.67 23.01 2.9
3 43.9 8.87 1.58 12.24 27.34 4.22
4 21.61 49.62 0.65 3.37 8.74 15.56
5 60.38 1.19 2.59 10.73 24.01 0.66
6 52.89 4.93 2.81 10.92 24.62 3.5
7 69.66 3.03 3.98 1.26 21.95 1.12
8 56.61 2.51 2.57 11.71 24.7 1.37
9 83.69 6 5.05 1.47 2.89 0
10 76.6 8.31 6.01 3.85 6.39 1.81
11 45.6 9.9 1.08 8.76 20.75 12.59
12 49.27 0.74 1.35 15.69 31.79 0.16
13 12.75 59.12 0.45 2.28 8.47 16.5
14 15.07 52.67 0.91 3.24 6.79 10.24
15 7.95 61.04 0.4 2.36 9.11 17.05
16 58.11 1.19 2.28 13.63 24.17 0.31
Continued from Table II
Fe Cu Co Si O S
17 50.98 0.94 1.85 14.14 30.3 0.36
18 55.33 1.29 3.56 13.8 25.21 0.55
19 49.21 2.1 1.66 13.72 30.77 1.26
20 12.8 62.25 0.33 2.32 7.93 13.65
21 69.72 0.69 3.64 0.83 25.63 0
22 53.45 2.41 3.66 13.7 25.6 0.97
23 29.83 31.22 1.5 8.08 19.92 8.71
24 33.01 44.13 1.17 2.02 6.44 6.97
25 24.45 39.65 1.04 6.5 18.29 9.68
26 60.07 3.43 1.59 6.15 25.8 1.2
27 61.37 2.96 2.36 10.82 19.92 1.65
28 51.85 8.71 1.24 9.78 23.65 3.62
29 66.41 7.4 3.45 8.8 10.99 1.42
30 49.11 2.49 1.02 14.64 30.27 1.22
31 51.26 2.57 2.64 14.64 27.19 1.41
32 53.02 0 2.02 15.67 28.53 0
33 47.52 2.1 1.01 18.62 24.17 1.14
34 51.1 1.72 1.11 15.39 28.44 0.65
35 52.53 1.25 1.25 14.14 29.28 0.29
36 52.11 1.9 3.9 10.93 28.65 1.54
37 74.8 1.01 7.01 1.27 20.3 0.3
38 71.59 6.6 6.6 4.24 12.56 1.22
Average 50.147 13.291 2.393 8.770 20.645 3.846
As shown in Figure 2 and Table II, the main compositions of the slag are iron oxide and fayalite which distributed widely appearing masses and plates; the distribution of Co is dispersive and the independent cobalt phase has not been observed, and in the areas such as 13, 14, 20, which has high content of Cu, the content of Co is low relatively. In the areas of 18, 22, 24, 29, 36 and 38, the contents of Fe, Co and O are relatively high, while that of Si is low, which indicates that the cobalt exists in the iron oxide phase in above areas; in the areas of 17, 18, 22 and 31, the contents of Fe, Co and Si are relatively high, and that of O is moderate, which indicates that the cobalt exists in the fayalite phase in above areas. The tendency of the distribution correlation of Fe~Co and Cu~Co are shown in Figure 3.
(a) (b)
Figure 3. The distribution correlation of Fe~Co (a) and Cu~Co (b) in the slag
Figure 4. The surface distributions of Fe, Co and Cu in the converter slag sample
The Figure3 (a) and (b) clearly show that the distributions of iron and cobalt present positive correlation, the content of cobalt rises as that of iron rises, which indicates cobalt is liable to gather with iron in iron oxide and fayalite; while the distributions of copper and cobalt present negative correlation: after the point of 10% content of Cu, the content of cobalt falls with that of copper rises, which is because the gradually reducing iron reduces the content of cobalt gathering
with iron. The result of the SEM Surface scanning of the converter slag sample is shown in Figure 4, which further confirms the similarity of the distribution of iron and cobalt and the difference between that of copper and cobalt.
The mechanism of Fe/Cu/Co mineralogy formation during converting The oxidation order of the sulfide of iron, copper and cobalt in converting process
There are two stages in the converting of matte, which are slag making stage and copper making stage. In the stage of slag making, almost all the iron and a part amount of sulfur in the matte are oxidized, and the iron oxide comes into the slag combining with SiO2 from gangue and flux; In the stage of copper making, part amount of the Cu+ is oxidized to Cu2+, and the following cross reaction of them occurs, which generates Cu. In the converting process, almost all the FeS, CoS and a little amount of Cu2S are oxidized to oxide coming into the converter slag, the related reactions are shown below [4-6].
2/3FeS + O2 = 2/3FeO + 2/3SO2 G = -303340 - 5.27T J/mol (1) 2/3Cu2S + O2 = 2/3Cu2O + 2/3SO2 G = -268190 + 81.17T J/mol (2) 2/3CoS + O2 = 2/3CoO + 2/3SO2 G = -299217 +51.52T J/mol (3) The reactions above are all based on 1mol O2, from which the trends of the reactions can be compared. The relationship of G-T of reaction (1) is minimum, (3) is moderate, and (2) is maximum, which indicates FeS is the most liable to be oxidized, and the next is CoS and Cu2S.
In fact, even if the Cu2S and CoS are oxidized to Cu2O and CoO, their oxide will also be reduced by FeS if there is FeS remaining; at the same time, the reaction of Cu2O and CoS can occur which generate CoO and Cu2S. The reactions related are shown below [7-8].
CoO + FeS = FeO + CoS G = -37978 + 8.87T J/mol (4) Cu2O + FeS = Cu2S + FeO G = -105440 + 85.48T J/mol (5) Cu2O + CoS = Cu2S + CoO G = -31027 – 29.65T J/mol (6) As the reactions show, in the stage of slag making, iron is firstly oxidized, cobalt is oxidized gradually with the rise of oxidation potential and almost all the cobalt comes into the slag when close to the end-point of converting process. And when in the stage of copper making, only a little amount of copper oxide enters the slag. The metal oxides combine and generate complex compounds with gangue and flux in the slag. In fact, the slag loss of copper also includes physical loss, namely some matte particles mixed in the slag which have little sizes, and are not liable to gather and settle. The amount of physical loss is roughly equal to that of the chemical loss.
The mechanism of isomorphism in the gathering of iron and cobalt
In nature, cobalt exists in forms of compounds and isomorphism which have different crystal chemical properties in cobalt minerals. The compounds mainly exist in sulfide, arsenide, selenide and etc., such as linnaeite and smaltite. And the isomorphous cobalt optionally enters the lattice of oxide and saline material as the accessory constituent of carrier mineral [9-10]. For instance, cobalt usually replaces the iron existing in fayalite and iron oxide as isomorphism in the converter slag. Cobalt belongs to the first transitional element, and has similar properties with
iron which both have the transitivity of oxyphile and sulfophile. Some element parameters of iron and cobalt are shown in Table III.
Table III. Element parameters of iron and cobalt
Fe Co
Atomic weight 55.85 58.93
Atomic volume(cm3•mol-1) 7.1 7.1
Atomic number 26 27
Element period 4 3
Element group Ⅷ Ⅷ
Outer shell electron distribution 3d64S2 3d74S2
Crystal texture face-centered cubic, body-centered cubic face-centered cubic
Valence +6、+3、+2 +3、+2
High spin radius of divalent ion 0.76 0.75
Low spin radius of divalent ion 0.64 0.65
The Table III shows the very similar element parameters of iron and cobalt which necessarily have very similar properties. Especially, the outer shell electron distribution of iron is 3d64S2, and that of cobalt is 3d74S2 which are exactly similar and based on which they are both liable to lose their two outermost electrons to generate divalent ion; in addition, (R2 - R1) R2 = 0%~6.6%
< 10%~15% (the R2 is the bigger radius, R1 is the smaller radius), which indicates the two ions have nearly equal radiuses. Therefore, the similarity of element properties determines cobalt is liable to replace the iron as isomorphism which exists in different kinds of iron oxide and iron silicate, and their compositions have infinite solubility [11-13]. In fact, the content of cobalt existing in the form of compounds in different minerals is just a few percent of the total content of cobalt on earth, and most of cobalt exists freely as isomorphism in different kinds of dark iron silicate (pyroxene, hornblende, and biotite etc.) and magnetite. Moreover, cobalt is also able to enter in the ores of sulfide, arsenide, selenide and telluride containing iron as isomorphism, such as pyrite (FeS2), chalcopyrite (CuFeS2), arsenopyrite (FeAsS), symplesite (FeAs2), achavalite (FeSe2) and durdenite (FeTe2) etc., whose contents of cobalt are all in the scope of 0.n~1, and which can be exploited as the main cobalt deposits[14]. The above distribution rules and their mechanism will provide necessary theoretical reference for the effective recovery of cobalt. For instance, cobalt replaces iron as isomorphism, therefore in the reduction smelting process, the system temperature must be high enough which is above 1623K to break the Fe-O and Co-O bond and release Co as much as possible; the reduction order is Cu > Co > Fe, therefore the amount of reducing agent must be enough to guarantee the total recovery of Cu and the very low content of Fe in the slag, which will insure the high reduction rate of Co.
Conclusion
The converter slag samples provided by CCS have very high content of cobalt which can reach 2.41%. In the samples, iron is not only able to generate iron oxide and fayalite, but also to generate CuFe2O4 and CoFe2O4 with copper and cobalt; while copper cannot generate compound with cobalt. The distribution rules are acquired by the analysis of the composition of the 38 selected micro areas of the slag, which is that the distributions of iron and cobalt present positive
correlation, and that of copper and cobalt present negative correlation, and which indicates cobalt is liable to gather with iron in iron oxide and fayalite. In the converting process, almost all the FeS, CoS and a little amount of Cu2S are oxidized coming into the slag, and the oxidation order is FeS >
CoS > Cu2S; the slag loss of copper also includes physical loss, whose amount is roughly equal to that of the chemical loss. The similarity of element properties such as outer shell electron distribution and ionic radius determines cobalt is liable to replace the iron as isomorphism which exists in different kinds of iron oxide and iron silicate. For the effective recovery of cobalt, the system temperature must high enough which is above 1623K, and amount of reducing agent must be enough to guarantee the high reduction rate of Co.
Acknowledgments
The authors gratefully acknowledge the financial support of the National Science Foundation of key funds PRC for the research project (No.51234008), also the financial support of Beijing technical development project (No. 00012132) and the development of science and technology fund supported by CCS.
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