CA1055710A

CA1055710A - Nickel matte refining by oxidation

Info

Publication number: CA1055710A
Application number: CA231,602A
Authority: CA
Inventors: Ramamritham Sridhar
Original assignee: Vale Canada Ltd
Current assignee: Vale Canada Ltd
Priority date: 1975-07-16
Filing date: 1975-07-16
Publication date: 1979-06-05
Also published as: FR2318233A1; FR2318233B1

Abstract

Abstract of the Disclosure A process for substantially lowering the levels of elements such as cobalt and iron, and in addition lead contained in a molten nickel matte by selective oxidation in the presence of a molten slag mixture. The metal oxides dissolve in the slag mixture which is subsequently removed from contact with the matte. The slag is cleaned by reduction and returned for further matte refining.

Description

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The invention is a process for substantlally lowering the level of elements such as cobalt and iron, and in addition lead, contained in a molten nickel matte by selective oxidation in the presence of a molten slag mixture. The transfer to the slag of elements which form more stable oxides than nickel and have a higher affinity for the slag purifies the nickel mattè.
The slag is then subjected to a reduction ~reatment to remove the metal oxides and thereafter returned for further matte refining.
In the pyrometallurgical processing of nickel-containing sulfide concentrates, the concentrate is roasted and smelted to produce a furnace matte. This matte contains about 15% to 45%
iron and is converted by the addition of a silicious flux, which serves to oxidize and slag the iron, thereby providing a matte suitable for further processing. Following this conversion the matte usually contains about 0.8~ iron, about 0.8~ cobalt~ and about 0.05% lead as well as any copper present from the furnace matte. However, it is highly desirable to reduce further the levels of these elements. For example, since cobalt is physi-cally and chemically akin to nickel, it is difficult to separate this element from nickel, but for a number of applications~it is of great importance to keep cobalt at its lowest practicable level. Furthermore, the presence of iron, even in small ~
quantities, can adversely affect the quality of nlckel electro-plate and also, it is an undesirable constituent in some high - -nickel alloys. Similarly, the deleterious effect of lead on '~ the weldability of nickel alloys is well documented. Thus, for these and other reasons, it is advantageous to limit the levels of impurity elements in the mat e from which the final form of 0 nickel is produced.

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The process of the present invention is intended to reduce the levels of iron, cobalt, lead and elements that form more stable oxides than nickel at an earlier stage of the refining process. Early removal is intended to eliminate the need for separate refining steps later in the operation such as those required in the electrorefining of nickel.
Known pyrometallurgical processes, concerned with removal of elements such as coba:Lt and iron at an early stage of the nickel refining operation, differ substantially from the method of the present invention although they too make use of the principle of selective oxidation. One of these methods is concerned with the affinity of three metals, i.e., Fe, Ni, Co, for oxygen and uses an oxide slag for refining nickel-containing matte. The slag, consisting mainly of iron and cobalt oxides, is formed in situ by preferential o~idation of some of the metallic components of the matte at temperatures of about 1100C to 1200C. The slag from the process which is enriched in the valuable element cobalt and also contains a substantial amount of nickel, is subsequently discarded. The major disadvantage of this process is that the cobalt and nickel values, contained within the discarded slag, cannot be economi-cally recovered.
Another prior art process makes use of a slag containing nickel oxides, oxides of metals less noble than nickel, silica , and lime. This slag is brought into contact and reacted with a molten bath containing nickel and impurity elements. The impurity elements are preferentially oxidized, transferred to another location in the reaction vessel, reduced and removed.
This process is, of necessity, operated at a temperature in ' 30 excess of the melting temperature of nickel, above about : ' .
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1455~C, and such a high temperature is known to cause deterio-ration of refractories and in addition requires substantial quantities of fuel and hence is costly. The high operating temperature of this process represents a major drawback.
It has now been discovered that -the levels of elements that form oxides more readily than nickel, such as iron and cobalt, can be substantially reduced in a molten nickel matte by selectively oxidizing the matte while in contact with a molten slag mixture. The molten slag mixture is principally composed of alkali metal oxides, silica and boron oxide but can also contain alumina and alkaline earth metal oxides. Oxides of elements more reactive than nickel transfer by gravity separation to and dissolve in the oxide slag layer. Also, surprisingly the lead content of the nickel matte is substan-tially lowered during treatment. After a suitable contact period, the slag mixture is removed from contact with the matte -and subjected to a reduction step wherein the contained impurity metal oxides are substantially removed from the slag by a reduction operation. The clean slag is then returned for refining a new batch of nickel matte.
It is an object of the process of this invention to provide a means for removing undesired elements, such as cobalt and/or iron from nickel matte.
It is another Gbject of this invention to provide a ~ more efficient method than presently used for removing one or J' more elements such as iron, cobalt and lead from a nickel ma~te.
~, Yet another object of this invention is to use advan~
tageously low operating temperatures which provide substantial J, savings in energy and, in addition, improve refractory life.

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Generally speaking, the presentinvention contemplates a process for substantially lowering the levels of elements that form oxides more stable than nickel and capable of sub-stantially lowering the lead content of an impure nickel matte and comprises contacting an impure nickel matte with a slag mixture containing alkali metal oxides, silicon dioxide and boron oxide at temperatures of about 650C to about 1350C
- while selectively oxidizing said impure nickel matte suf-ficiently to convert at least one elemental impurity having greater affinity for oxygen than nickel, such as cobalt or iron, contained therein to a metallic oxide form and dis-solving said metallic oxide form in said slag mixture; removing said slag mixture containing said metallic oxide form from contact with the purified nickel mattei reducing said slag mixture to convert said metallic oxide form to a separable ~orm; and removing said separable form from said slag mixture.
Although the process may be used at temperatures as high as 1350C, it is preferred that the upper temperature be limited to about 9S0C. Operation at temperatures below 950C
provides considerable savings in fuel and also serves to extend refractory life.
In practice, it is preferred that the slag mixture be in continuous contact with the nickel matte and that its flow direction be counter-current to the flow direction of the nickel matte since impurities can thereby be removed to a lower level.
- This may be accomplished by continuously removing unclean slag ~;~
and purified nickel matte from opposite ends of a reaction .
vessel while charging impure matte and clean slag. The counter-current flow provides maximum contact of the slag mixture and .

the molten nickel matte, thus improving the rate at which OXlC es . . ' .

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of impurities are removed from the nickel matte. By using a continuous process, it has been found that only about 3% of the nickel is transferred to the slag and a cobalt removal of 98~ can be achieved whereas only about ~0% of the cobalt is removed in a single-stage batch operation. The continuous process offers the advantage of increased throughput, lower heat losses and better efficiency in materials handling.
The slag mixture is composed of a combination of ingredients selected to provide a melting temperature roughly equal to or preferably 50C, above that of the melting tempera-ture of the nickel matte. An upper limit of 1350C is required to avoid corrosive attack of the refractories used in the reaction vessel. A useful slag mixture can contain from about 10% to about 50~ alkali metal oxides (e.g., Na2O, K2O), from about 5% to about 50~ silicon dioxide (SiO2), from about 20 to about 40% boron oxide (B2O3), from 0~ up to about 15~
alumina (A12O3), and from 0~ up to about 20% alkaline earth metal oxides (e.g., CaO, BaO). A preferred slag mixture contains from about 20% to about 35~ sodium oxide, from about 20% to about 35~ boron oxide and from about 10% to about 45~ silicon dioxide. The preferred slag can be prepared by melting and adding an appropriate mixture of sodium carbonate (Na2CO3?, borax (Na2B4O7.10 H2O) and silica (SiO2) to the molten matte.
Another factor which must be considered in selecting the proper combination of slag ingredients is the composition of the nickel matte and in particular the elements desired to be removed from it. The elements that will be selectively removed from the nickel matte will have reactivity greater ~ than that of nickel but the reactivity must be less than that t 30 of, for example, silicon, since otherwise, the silicon dioxide will be reduced and silicon will dissolye into the nickel matte.~ ~

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Surprisingly, the lead content of nickel mattes is reduced during the selective oxidation process even though thermodynamic data suggests that lead should not be removed since lead has a lesser affinity for oxygen than nickel.
The exact explanation for the observed phenomenon is not completely understood, however, and without being bound to any particular theory, it is thought that some lead oxide forms during the selective oxidation of the matte and separates to the slag mixture. Subsequently, the lead oxide vaporizes and is thereby removed from the slag. The vaporization of the lead oxide from the slag, in turn, provides a driving force for additional lead oxidation in the impure nickel matte.
In the preferred continuous process, the degree of purification is controlled by the rates at which slag and nickel matte are processed and oxidized. A slag may be given several cleaning treatments and reused with the same batch of nickel matte or contacted several times with the same batch of nickel matte without a cleaning step.
The selective oxidation of the nickel matte may be accomplished in a number of ways. One means consists of adding a controlled amount of nickel oxide to the oxide slag mixture.
At the interface of the slag and the nickel matte, the nickel -oxide reacts with the metal sulfides having greater affinity for oxygen than nickel, giving up oxygen to these elements which then transfer and dissolve in the slag layer.
Another means for oxidizing the nickel matte is to introduce air or oxygen through a lance or tuyeres under the ~
surface o~ the nickel matte. This leads, in part, to direct ~ -formation of oxides of elements having greater affinity~for oxygen than nickel and also, the formation of nickel oxide within the nickel matte which in turn also reacts with the contained metal su1fides having gxeater afinity or oxygen.

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The slag layer is preferably removed from the vessel where it has been in contact with the impure nickel matte, and transferred to a separate vessel for temporary storage to compile a suitable quantity for the reduction step; also, a continuous reduction process may be effected in the second vessel. The invention also contemplates the situation wherein the purified matte is removed in its entirety from the reaction vessel and the slag is reduced within the same vessel used for the initial oxidation step. The slag can be cooled,crushed and stored until such time as a sufficient quantity is available for reduction. When a sufficient quantity is obtained, carbon and/or sulfur can be blended with the slag prior to heating to the reduction temperature.
Although a continuous operation is preferred, the present invention can be carried out in conjunction with a conventional converting operation. The conventional converting operation is stopped when the matte contains about 5~ iron rather than the usual 0.8~. In the conventional Bessemer converter or top-`
blown rotary converter, the iron silicate slag can be removed ~0- and replaced by the alkali metal oxide, silica, boron oxide mixture. The selective oxidation of the matte is then carriéd out by introducing oxygen or air through submerged tuyeres or by top-lancing or by the addition of metal oxides such as nickel oxide. During such an operation, it has been found that about 80% of the cobalt can be transferred from the matte to ~-the slag while only about 3~ of the nickel is transferred.
During a conventional refining treatment utilizing iron silicate slag, about 30% of the nickel would be transferred to the slag -- in order to obtain 80% cobalt removal from the matte~ Also, in the conventional operation, it is dif~icult to produce -:~ ' 10~7~Q

mattes which contain less than 0.5~ iron whereas with the process of the present invention, iron levels below 0.1% can be achieved. The refining can also be carried out in batches by multiple additions of clean slag with subseq~lent oxidation and slagging.
A suitable reduction process may consist of lancing the oxide slag with hydrogen sulfide. Sulfides of the metallic elements, such as cobalt and iron, form and sink to the bottom of the slag mixture due to their greater density. The purified --slag mixture is subsequently separated from the metal sulfides and returned to the first reaction vessel for further contact wlth the nickel matte.
Another means for purifying the slag mixture is carbo-thermic reduction. A fuel such as pulverized coal or ~aseous hydrocarbons may be introduced to the oxide slag in the second reaction vessel and used to reduce the metallic oxides contained -' therein to metallic form. The elemental metals separate to the bottom of the second reaction vessel where they are removed for additional refining operations and the purified oxide slag returned to the first reaction vessel.
A reactive element such as aluminum may be used to reduce ; the metallic oxides to elemental form. However, this type of - :
addition agent must be chosen with care since the oxides that are formed may contaminate the oxide slag, possibly resulting in increased slag melting temperature. Another element useul for ~ this purpose is silicon. Since silicon dioxide is one of the -i compounds used to formulate the oxide slag, it would not serve ~:
as a contaminant, however, it would be necessary to adjust ~' the composition of the slag to allow for dilution by silica.
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Another means for purifying ~he oxide slag contemplated by this invention is electrolytic reduction. With this method, the metallic contaminants are removed by plating on a suitable electrode.
In order to give those skilled in the art a better understanding of the invention, the following illustrative examples are given which demonstrate the process of purifying a nickel matte by selective oxidation and reaction with a molten slag mixture.
EX~PLE 1 A nickel matte containing 0.54% copper, 69.2% nickel, 3.5% cobalt, 2.2% iron and 21.9~ sulfur, shown in Table I, was treated with an oxide slag at 860C. This composition is representative of a matte derived from a lateritic ore in which the converting operation has been interrupted at the 2.2% iron level, rather than the more conventional 0.8~ level, and as a consequence also contains higher cobalt. The refining operation was accomplished in three stages in which oxygen was lanced into the matte for 45 minutes at a rate of 100 milliliters per minute. ~ -200 grams of matte were treated in this manner while in contact -with 400 grams of slag which contained 32~ Na2O, 35.5% B2O3 and 32.5% SiO2 and was prepared from a mixture of sodium carbonate, sodium borate and silica. The melting point of this slag was ~; about 650C and it is non-corrosive to fire-clay and chrome magnesite refractories up to about 1350~C. As shown in Table I, the copper content of the matte remained the same during the three stages of treatment since copper has less affinity for ;~-oxygen than nic}cel. The cobalt content of the matte was reduced from 3.5~ to 1.5% (57% removal) during the first stage of the operation. Simllarly, the iron content of the matte was reduced from 2.2% to 0.049~ (97.8~ removal).
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When 150 grams of the first stage matte was contacted with 300 grams of new slag, the cobal~ content was further reduced to 0.33~ (about 35% additional removal) and the iron to 0.01%. In the third stage, 100 grams of nickel matte from the second stage was placed in contact with 200 grams of the new oxide slag. The cobalt content was reduced further to 0.014%. The overall efficiency for cobalt removal for the three stage process was 99.6% and for iron, 99.5%.

TABLE I

REFINING NICKEL MATTE WITH CLEAN OXIDE SLAG
: gm : % Cu : % Ni : % Co : % Fe : % S
Stage I . : : : : :

Matte In : 200: 0.54 : 69.2 : 3.5 : 2.2 : 21.9 Slag In : 400: - : 4.0 : 0.4 : 0.1 Matte Out: : 0.55 : 73.5 : 1.55 : 0.049: 23.0 Slag Out : : 0.008: 3.88: 1.15 : 1.02 : - :-.
Stage II :

` Matte In : 150: 0.55 : 73.5 : 1.55 : 0.049: 23.0 Slag In : 300: - :` 2 : 0.02 :

Matte Out: : 0.54 : 74.0 : 0.33 : 0.01 : 23.4 Slag Out : : 0.01 : 2.79: 0.65 : 0.071:
Stage III: : : : : : .

Matte In : 100: 0.54 : 74.0 : 0.33 : 0.01 : 23.4 Slag In : 200:
Matte Out: : 0.58 : 73.4 : 0.014: 0.01 : 24.0 ' Slag Out : : 0.01 : 2.3 : 0.13 : 0.05 :

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A slag to matte ratio of 1:1 was used in this example at a reaction temperature of 840C. The o~ide sla~ composition was the same as that described in Example I. The same slag was kept in contact with the nickel matte for two hours without intermediate purification treatment. Oxygen was blown through the matte at a rate of 100 ml/min. Samples were withdrawn from the nickel matte and the oxide s]ag layer at four intervals of 30 minutes. Chemical analysis of the nickel matte and oxide slag are shown in Table II. It was found that, as in Example 1, the copper content of the matte remained the same during the -: entire two hour period of contact. Cobalt content of the matte was reduced substantially from 3.08% to 0.65% (78.9% removal).
The iron content of the matte was lowered from 0.19% to 0.014%
t92-6~ removal). The lead content of the matte was reduced from 0.059% to 0.038% (35.6% removal). This example illustrates the need to allow sufficient time ~or oxidation and removal of the impurity elements to the oxide slag layer.

TABLE II

REFINING NICREL MATTE BY PROLONGED CONTACT WITH A SLAG MIXTURE
: Weight Percent : Cu : Ni : Co . Fe : S : Pb Initial Matte : 0.58 : - : 3.08 : 0.19 : 22.4 : 0.059 Matte 30 min. : 0.58 : - : 1.96 : 0.026 : 22.0 : 0.050 Slag 30 min. : 0.029 : 1.10 : 1.06 : 0.31 : : 0.012 -Matte 60 min. : 0.58 : - : 1.26 : 0.019 : 22.9 : 0.046 , Slag 60 min. : 0.015 : 2.05 : 1.55 : 0.26 : : 0.016 Matte 90 min. : 0.59 : - : 0.88 : 0.017 : 23.2 : 0.043 i~ Slag 90 min. : 0.019 : 3.35 : 1.90 : 0.26 : : 0.019 30Matte 120 min.: 0.60 ~ - : 0.65 : 0.014 : 23.9 : 0.038 Slag 120 min.: 0.038 : 6.00 : 2.00 : 0.33 : : 0.024 '` :

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A synthetically prepared nickel matte containing 0.50~
copper, 71.8% nickel, 1.51% cobalt, 0.32~ iron and 26.7% sulfur, shown in Table III, was treated wlth a slag prepared from a mixture of sodium carbonate, sodium borate and silica. This composition was equivalent to 34% Na2O, 23% B2O3 and 43% SiO2.
The 6000 gram matte was treated continuously for five hours at temperatures from 800C to 950C with the 3000 gram slag mixture.
Oxygen was lanced into the matte at a flow rate of 2.5 liters per minute. The slag mixture and matte were kept molten with a natural gas burner.
During treatment, the copper content of the matte remained essentially unchanged and there was no pick-up of this element by the slag mixture. The cobalt content of the matte was reduced by 76% and the iron content by 97% while the nickel content of the slag mixture was increased by only 3.84% during the five hour oxidation treatment.

TABLE III

REFINING NICKEL MATTE BY PROLONGED CONTACT WITH A SLAG MIXTURE
Time:Temp.,: Matte Composition, ~ : Slag Composition, %
hr.: C : Cu : Ni : Co : Fe : S : Cu : Ni : Co : Fe :
0 : 800 :0.50:71.8:1.51:0.32:26.7: ~ : 0 : 0 : 0

2 : 800 : :72.2:0.35:0.09:27.7:<0.005 : 2.98: 1.33: 0.54

3 : 840 : :72.6:0.45:0.0~:27.5:~0.005 : 3.07: 1.53: 0.58

4 : 920 : :73.0:0.44:0.09:26.6:C0.005 : 3.64: 1.95: 0.~7

5 : 950 :0.54:73.4:0.36:0.01:27.9:~0.005 : 3.84: 2.13: 0.65 . ~ , .

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An 80 gram slag containing 33% Na2O, 23% B2O3 and 44%
SiO2, was used to treat a 400 granl nickel-copper matte. This matte contained 27.7% copperr 47.~% nickel, 0.719~ cobalt, 0.94%
iron, 22.4% sulfur and 0.052% lead as sho~n in Table IV. Air was introduced to the matte through a lance at a rate of 0.1 liters per minute. The matte was treated for one hour at a temperature of 950C. During this treatment, 59% of the cobalt, 87% of the iron and 54% of the lead were removed from the matte while the slag mixture only picked up 3.0% nickel.
TABLE IV
REFINING OF NICKEL-COPPER MATTE
Matte Composition, ~ : Slag Composition, %
Cu : Ni : Co : Fe : S : Pb : Cu : Ni : Co : Fe : Pb 27.7: 47.8: 0.71: 0.94: 22.4: 0.052: 0 : 0 : 0 : 0 : 0 27.7: 46.9: 0.29: 0.12: 23.0: 0.024: 0 : 3.0: 1.8: 4.2: 0.048 -This example shows that nickel oxide dissolved in the oxide slag can be used as an oxidant instead of air or oxygen lanced into the matte. As shown in Table V, when an oxide slag containing 3.38% nickel was contacted with molten nickel matte, --the nickel level of the slag was lowered. As a consequence, the cobalt level of the slag increased from 1.15% to 1.8% and the iron content of the slag lncreased from 1.02% to 2.06%.
This treatment was performed at a temperature of 850C with a slag to matte ratio of 2:1.
TABLE V
OXIDATION OF NICKEL MATTE BY NICKEL OXIDE
_ ADDITION TO OXIDE SLAG
-~ 30 : - Weight, %
Cu : Ni Co : Fe : S
- Matte In 0.54 :69.2 3.5 : 2.22 : 21.9 Slag In :0.008: 3.38: 1.15: 1.02 :
; Matte In :0.55 :74.1 : 2.23: 0.07 : 22.4 ~ Slag Out :0.009: 2.13: 1.80: 2.06 , ~ .
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E~LE 6 This example illustrates a method used to remove metallic oxide from a non-metallic oxide slag. A 330 ~ram char~e of an oxide slag containing 24.6% Na2O, 10.0% SiO2, 35.4% B2O3, 10%
A1203 and the balance metallic oxides was heated to 900C and hydrogen sulfide was introduced therein at a flow rate of 100 cubic centimeters per minute. As shown in Table VI, slag samples taken every hour during treatment exhibited a gradual reduction of nickel, cobalt and iron contents so that after six hours of treatment, the nickel content was lowered from 3.6% to 0.11% (96.9% removal efficiency), the cobalt content was lowered from 0.99% to 0.038% (96.2% removal efficiency) and the iron content was lowered from 0.20% to 0.055% (72.5% removal efficiency). The resultant oxide slag was considered suf-ficiently clean at this point and suitable for additional processing of impure nickel matte. A 25 gram matte recovered from the cleaned oxide slag contained 31.3% sulfur, 45.0% nickel, 14% cobalt and 0.041% iron from which the nickel and cobalt values could be readily recovered by conventional techniques.

TABLE VI

REDUCTION OF OXIDE SLAG LAYER TO REMOVE METAL OXIDE~

Time, E~r. : % Cu : % Ni : ~ Co : % Fe 0 : 0.016 : 3.60 : 0.99 : 0.20 1 : 0.0075 : 0.59 : 0.58 : 0.21 ' 2 : 0.012 : 0.056 : 0.084 : 0.13 3 : 0.012 : 0.24 : 0.067 : 0.25 4 : 0.016 : 0.71 : 0.20 : 0.14 S : 0~035 : 0.25 : 0.070 : 0.081

6 : 0.004 : 0.11 : 0.038 : 0.055 ~' .' - ' ' 14 0557~

Slag mixtures that had been used for purification of nickel ~atte were treated by three carbothermic reduction techniques as shown in Table VII. In treatment A, a 2.9~, by weight, coal addition was added to a 200 gram charge at 1500C.
After holding for one hour, chemical analysis showed that the slag had been sufficiently cleaned to be suitable for recycling to the refining operation. In treatment B, an addition of 3%
coal plus 6% sulfur was made to 200 grams of the used slag.
This charge was held at 1200C for one hour in a magnesia crucible and this treatment was also found to be effective for reprocessing the used slag. Treatment C consisted of 3~ coal plus 12.3% pyrrhotite (FeS) addition to a 200 gram used slag sample. The slag was also effectively reprocessed by this treatment at 1200C for one hour. The mattes that resulted from the three treatments were well suited for subsequent refining operations in which the nickel and cobalt values would -be recovered.
TABLE VII
CLEANING OF A SLAG MIXTURE
Treat-:Identity: Cu : Ni : Co : Fe : S :Si0 :Na 0:B 0 ment : 2 2 2 3 :Used : : : : : : : :
: Slag : 0.08: 5.41: 1.94: 0.58: :42.8:21.3:24.5 A :Sulfide : 1.0 :66.0 : 27 : 5.4 :
- :Cleaned :
: Slag : 0.01: 0.28: 0.15: 0.18: :45.9:25.7:29.6 B :Sulfide : 0.8 :50.4 :13.5 : 2.3 :32.9 :
`~ :Cleaned : : : : : : : : `
: Slag :0.004: 0.40: 0.38: 0.50: 0.59:39.8:25.1:28.9 C :Sulfide :1.4 :37.1 : 9.2 :21.1 :31.0 :
, :Cleaned : : : ~ : : : : :
Slag :0.045: 0.75: 0.69: 9.41: 1.15:38.2:21.6:24.8 .
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10557~0 Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for reducing the levels of elemental impurities in an impure nickel matte comprising: contacting an impure nickel matte with a slag mixture containing, by weight, about 10 - 50% alkali metal oxides, about 5 - 50%
silicon dioxide, and about 20 - 40% boron oxide at temperatures from about 650°C to about 1350°C while selectively oxidizing said impure nickel matte sufficiently to convert at least one elemental impurity having greater affinity for oxygen than nickel contained therein to a metallic oxide form and dissolv-ing said metallic oxide form in said slag mixture; removing said slag mixture containing said metallic oxide form from contact with the purified nickel matte; reducing said slag mixture to convert said metallic oxide form to a separable form; and removing said separable form from said slag mixture.

2. A process as defined in claim 1 wherein said elemental impurities are elements having greater affinity for oxygen than nickel.

3. A process as defined in claim 2 wherein said elements having greater affinity for oxygen than nickel are cobalt and iron.

4. A process as defined in claim 1 wherein said elemental impurity is lead.

5. A process as defined in claim 2 wherein said slag mixture is continuously used to remove said metallic oxide form from said impure nickel matte.

6. A process as defined in claim 5 wherein said slag mixture flows counter-currently to said impure nickel matte flow.

7. A process as defined in claim 1 wherein said slag mixture further contains at least one oxide selected from the group consisting of alumina and alkaline earth metal oxides.

8. A process as defined in claim 7 wherein said slag mixture contains, by weight, up to about 15% alumina and up to about 20% alkaline earth metal oxides, said slag mixture being selected to have a melting temperature above about 650°C
and to be noncorrosive to refractories up to temperatures of about 1350°C.

9. A process as defined in claim 1 wherein said slag mixture contains, by weight, about 20 - 35% sodium oxide, about 20 - 35% boron oxide and the balance, in an amount of about 10 - 45%, essentially silicon dioxide.

10. A process as defined in claim 1 wherein said impure nickel matte and said slag mixture are maintained at temperatures below about 950°C.

11. A process as defined in claim 1 wherein oxygen is blown through said impure nickel matte to oxidize said im-pure nickel matte.

12. A process as defined in claim 1 wherein air is blown through said impure nickel matte to oxidize said impure nickel matte.

13. A process as defined in claim 1 wherein nickel oxide is added to said slag mixture to oxidize said impure nickel matte.

14. A process as defined in claim 1 wherein said separable form is an elemental metal resulting from a carbothermic re-duction of said slag mixture.

15. A process as defined in claim 1 wherein said separable form is a metal sulfide formed by reacting hydrogen sulfide with said slag mixture.

16. A process as defined in claim 1 wherein said separable form is a mixture of a metal and a metal sulfide formed by reacting a mixture of carbon and sulfur or a mixture of carbon and metal sulfide with said slag mixture.

17. A process as defined in claim 1 wherein said separable form is an elemental metal formed by reacting aluminum with said slag mixture.

18. A process as defined in claim 1 wherein said separable form is an elemental metal formed by electrolytic reduction of said slag mixture.