CA2893406A1 - Smelting of low grade chromite concentrate fines - Google Patents
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Abstract
A chrome product by smelting a feed material which contains a chromite ore under conditions in which gangue, in the feed, provides the flux for the chrome product.
Description
SMELTING OF LOW GRADE CHROMITE CONCENTRATE FINES
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to the smelting of chromite.
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to the smelting of chromite.
[0002] Chromite ores are typically evaluated on the basis of the ratio Cr:Fe and on the Cr2O3 content of the ore. Usually, the Cr2O3 content is maximised prior to smelting. This often results in a relatively high loss of chromium during an ore dressing phase. This negative outcome applies to a metallurgical system in which the establishment of a higher grade is subject to a constraint on what grade is regarded as acceptable. It is not feasible to maximise grade and recovery simultaneously. Consequently, a trade-off between grade and recovery is necessary if a target is set. However, if the grade constraint is relaxed, the recovery can be improved.
[0003] Pure chromite is a double oxide of iron and chromium with the formula FeO.Cr203. Naturally occurring chromite is described by the formula (Fe,Mg)0.(Cr,AI,Fe)203, for, in the naturally occurring chromite, the iron is partially replaced by magnesium, and the chromium is partially replaced by aluminium and ferric iron. Conventionally, in the processing of a chromite ore, an objective is to produce a concentrate by removing as much as possible of the minerals that diluted the more valuable chromite, in other words an objective is to maximize grade (mass %Cr203) and the Cr:Fe ratio of the final concentrate product.
[0004] By way of example only, in the case of UG2 chromite, Figure 1 shows in simplified flowsheet form, a conventional chromite beneficiation process.
[0005] A flotation plant, not shown, used to recover PGMs, produces a slime 10, referred to as flash flow tails, which contains UG2 chromite. The slime 10 is fed to a de-sliming cyclone 12 to produce an overflow 16, which is directed to an overflow spiral 18, and an underflow 20 comprising solids. A mixture 22 is prepared from the underflow by adding clean water 23, typically in a ratio of 30% to 70%. The mixture is directed to a set of rougher spirals 24, from there, to a set of cleaner spirals 26 and then to a set of re-cleaner spirals 28 in order to separate the mixture into resulting products.
[0006] A chemical-grade product 30 and a metallurgical-grade product 32 are produced from the Cr203 outflow of the spirals 28. The chemical-grade product typically has about 42% to 43% Cr203 and less than 1% silica. The metallurgical-grade product can have up to 2.5% silica.
[0007] In each product 30, 32 the concentration of Cr203, of the order of 42%
to 43%, is deemed saleable. In further processing, when this concentrate is smelted, a furnace operator typically adds flux (often silica and limestone) to the feed material for slag modification.
to 43%, is deemed saleable. In further processing, when this concentrate is smelted, a furnace operator typically adds flux (often silica and limestone) to the feed material for slag modification.
[0008] An object of the present invention is to provide a process wherein the requirement for the separate addition of a flux, when the concentrate is smelted, is reduced or eliminated. Another object is to increase the combined recovery of chrome by reducing the requirement for upgrading of the ore. A further object is to be able, to effectively utilise previously discarded low-grade material on its own or via co-smelting with high-grade chromite concentrate.
SUMMARY OF INVENTION
SUMMARY OF INVENTION
[0009] In accordance with the invention a feed material which contains chromite concentrate is smelted in a furnace under conditions in which flux requirements for the production of ferrochromium are provided primarily by gangue components present in the feed material.
[0010] The gangue components may be diverse but typically include one or more of Si02, A1203, MgO, and CaO.
[0011] Thus, in terms of the invention, during the smelting process the requirement for the separate addition of flux is substantially reduced or eliminated as the feed material is, essentially, self-fluxing.
[0012] The invention finds particular application when the feed material contains chromite at a grade which is lower than a commonly accepted metallurgical grade.
Although "a commonly accepted metallurgical grade" (as used herein) may vary from application to application it is dependent on a variety of factors. Typically the invention can be used effectively when the feed material includes chromite concentrates containing less than 38% mass Cr203 %. Although this numerical value is generally indicative of a condition under which the invention can be used effectively it is not necessarily restrictive for other factors may come into play. For example, the distance between a production site and a smelting site, the cost of transport, type or origin of ore and the like, can influence this numerical value.
Although "a commonly accepted metallurgical grade" (as used herein) may vary from application to application it is dependent on a variety of factors. Typically the invention can be used effectively when the feed material includes chromite concentrates containing less than 38% mass Cr203 %. Although this numerical value is generally indicative of a condition under which the invention can be used effectively it is not necessarily restrictive for other factors may come into play. For example, the distance between a production site and a smelting site, the cost of transport, type or origin of ore and the like, can influence this numerical value.
[0013] The grade of the chromite in the feed material may be as low as 25 to mass `)/0 Cr203.
[0014] The feed material may be drawn from a conventional chrome recovery process as an intermediate product or from stockpiled discard tailings.
[0015] Preferably smelting takes place in a DC open-arc furnace.
[0016] In essence the invention provides for the smelting of a chromite ore that contains its own flux in the gangue which is not separated in an upstream process but which is retained in the ore. By way of contrast, in a conventional recovery process, the gangue is removed via an appropriate mineral processing method and flux is separately added during smelting. The DC furnace allows for the processing of lower-grade ores as the composition of the slag is not as critical as when processing in a conventional AC submerged-arc furnace.
[0017] The invention also extends to a method of producing one or more chrome products which includes the step of smelting a feed material which contains a chromite ore under conditions in which flux requirements for the production of the chrome product are provided primarily by gangue components present in the feed material.
[0018] The feed material may be of the kind described hereinbefore. It is however possible to prepare a suitable feed material by means of an appropriate blending process, or by using a suitable minerals processing technique.
[00191The feed material may be fed to a de-sliming cyclone to produce an underflow comprising solids from which a mixture is prepared by adding clean water, typically in a ratio of 30% to 70%.
[0020]The mixture may be directed to a set of rougher spirals, from there, to a set of cleaner spirals and then to a set of re-cleaner spirals in order to separate the mixture into resulting products.
[0021]The resulting products may include a chemical-grade product and a metallurgical-grade product.
[0022]During an initial processing stage a proportion of the feed material which is passed through at least a part of the set of rougher spirals may be passed to a secondary set of cleaner spirals.
[0023] The secondary set of cleaner spirals may produce an intermediate product which has a chromite content of about 34%.
[0024] The intermediate product may have a significantly higher gangue content than the metallurgical-grade product and is suitable for chromite smelting, particularly in a DC open arc furnace.
[0025] The higher gangue content may be achieved through the use of an appropriate preceding process or by blending tailings with high grade material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention is further described by way of example with reference to Figure 2 of the accompanying drawings which illustrates a modification to the conventional UG2 chromium recovery process shown in Figure 1 which has been discussed in the preamble hereto.
DESCRIPTION OF PREFERRED EMBODIMENT
[0027] Figure 2 illustrates in block diagram form a UG2 chromium recovery process for the production of a chromite ore of metallurgical grade, according to the invention.
The process in Figure 2 has substantial similarities to the process shown in Figure 1 and for this reason a detailed description of the process in Figure 2 is not given.
Reference numerals are used in Figure 2 which are the same as the reference numerals used in Figure 1 to designate components which are the same as the components in Figure 1.
[0028] During an initial processing stage a proportion of the feed material 22 passing through at least a part of the set of rougher spirals 24 is directed to a secondary set of cleaner spirals 34. The remainder of the feed material 22 is processed in the manner described in connection with Figure 1 i.e., it passes sequentially through the set of cleaner spirals 26 and then to the re-cleaner spirals 28 to produce a chemical-grade product 30 and a metallurgical-grade product 32.
[0029] The secondary set of cleaner spirals 34 produces an intermediate product 40 which has a chromite content of 34%.
[0030] The product 40, compared to the product 32 which is customarily deemed to be saleable and which has a chromite content of about 42% or higher, has a significantly higher gangue content.
[0031] This is illustrated in Table 1 which compares the assayed content of the material 40 with a metallurgical-grade product 32 which has a chromite content of 42%. Notably the Si02 content is 13.9% in the product 40 while it is reduced to 1.0%
in the commercially acceptable product 32.
Table 1: Composition of chromite ores (mass %) Intermediate Metallurgical Description Product grade UG2 Designation 34% feed 42% feed Cr203 33.7 41.7 FeO 23.8 27.7 A1203 13.1 16.0 CaO 0.95 0.09 MgO 12.8 9.2 Si02 13.9 1.0 TiO2 0.67 0.82 NiO 0.18 0.17 MnO 0.23 0.22 V205 0.27 0.37 0.014 0.005 Mass ratios Cr/Fe 1.25 1.33 (Ca0+mg0)/Si02 1.02 7.60 MgO/A1203 0.98 0.58 [0032] Computer modelling has shown that the inherent or natural composition of the gangue materials in the product 40 is suitable for chromite smelting, particularly in a DC open arc furnace. The use of this type of furnace, instead of a submerged-arc furnace, displays advantages which include the following:
1. the DC furnace can be used for the smelting of chromite fines directly ¨
this eliminates the need for agglomeration;
2. anthracite and coal can be used as reducing agents in place of metallurgical coke and char, which are relatively expensive;
3. there is a high chromium recovery with residual chromium oxide in discard slag typically ranging from 3% to 5%;
4. the electrode consumption, in the smelting step, is lower; and 5. the DC furnace is flexible in operation - this allows various alloy compositions to be produced. It is noted that the product grade of metal produced from a smelting furnace, utilising a low-grade ore, depends on the Cr:Fe ratio of the ore and not on the grade (Cr203 content). It is therefore feasible to utilise the process of the invention, as described, and to achieve a secondary benefit by adding either Fe or Cr, depending on which product is targeted. It is therefore possible to modify the alloy grade which is produced, by means of suitable additives which are added either directly to the furnace or post-taphole, to create a specialised chromium alloy. For example, scrap iron can be added to dilute the chromium content, or high-grade fines, recovered for example from chromium slag dumps, can be added to upgrade the Cr:Fe ratio of the feed and thus of the end product. Si content of the alloy can be minimised or increased . ..
via normal control of the slag composition. Typically increased slag basicity (defined as the mass ratio of (Ca0+Mg0)/Si02 in the slag) results in low Si content in the alloy.
[0033] The slag requires only a small modification, if any, when compared to the metallurgical-grade concentrate of commercial quality of the kind referred to.
[0034] Table 2 summarises predicted slag and alloy compositions for the product 40 and the commercial product 32. In each case this is for 94% Fe recovery in the alloy produced from the furnace. The energy requirement per ton of chrome product produced for the product 40 is lower than for the commercial product 32.
Table 2: Model results for 34% and 42% chromite ores Nominal chromite grade 34% feed 42% feed Cr:Fe mass ratio (concentrate) 1.25 1.33 Fe recovery 94.0% 94.0%
Cr recovery 92.6% 92.6%
Anthracite, % of chromite 19.4 24.0 Quartzite, % of chromite 4.0 18.0 Limestone, % of chromite 10.0 kg slag/100 kg chromite 47.8 51.0 kg metal/100 kg chromite 42.2 54.4 kg gas/100 kg chromite 33.3 43.4 Operating temperature, C 1720 1720 Estimated slag liquidus, C 1677 1677 Slag composition, mass %
Cr203 5.2 5.7 FeO 2.7 2.8 SiO2 33.5 32.2 CaO 2.0 9.7 MgO 25.6 16.4 A1203 28.7 31.2 Alloy mass ratio Cr:Fe ratio 1.69 1.77 Slag mass ratio (Ca0+Mg0)/Si02 0.823 0.808 Metal composition, mass %
Cr 50.6 51.9 Fe 41.5 40.2 Si 0.4 0.4 7.5 7.5 0.01 0.01 0.03 0.03 Cr:Fe mass ratio 1.22 1.29 Energy requirement kWhit of chromite 1.30 1.62 kWh/t metal product 3.08 3.18 kWhit Cr product 6.09 6.13 [0035] Table 3 reflects calculated energy requirements and flux addition requirements for the product 40 and for the commercial product 32.
Table 3: Summary of case studies ¨ models solved for identical Cr production 42% Met-grade Type of chromite 34 % Low-grade concentrate concentrate Chromite concentrate fed 200 000 tpa 161 596 tpa Chromite concentrate plus fluxes 208 000 tpa 206 843 tpa Total feed 246 700 tpa 245 691 tpa Fe recovery 94.0% 94.0%
Cr recovery 92.6% 92.6%
Cr:Fe mass ratio concentrate 1.25 1.33 Alloy product 84 460 tpa 82 349 tpa % Cr (alloy) 50.6 % 51.6 %
Quartzite 8 000 tpa 29 089 tpa Limestone - tpa 16 160 tpa Anthracite 38 700 tpa 38 848 tpa Cr balance:
Cr in alloy 42 701 tpa 42 701 tpa Cr in discard slag 3 413 tpa 3 404 tpa % of chromite Metal make 42.23 50.96 Slag make 47.84 54.37 Energy requirement (SER)* MWhit MWhit feeds 1.25 1.27 MWhit chromite 1.30 1.62 MWhit alloy 3.08 3.18 Assumptions for furnace design Smelting intensity 400 kW/m2 400 kW/m2 Availability 94 % 94%
Thermal efficiency 90 % 90 %
Average year 365.28 days 365.28 days Power requirement¨ & estimated furnace diameter Power (assumed design basis) 35 MW 35 MW
Diameter of furnace 10.6 m 10.6 m Nominal diameter 11m 11 m * Energy requirement excludes thermal efficiency (smelting energy only "SER=specific energy requirement") ** Power requirement includes thermal efficiency factor # Pre-heating of chromite ore only; flux and reductant assumed fed at 25T
$ Feed = chromite plus flux (excludes reductant) [0036] The aforementioned tables show that there are significant benefits with regard to flux requirement and energy consumption for the self-fluxing ore (the product 40) specifically compared to the metallurgical-grade concentrate 32.
It is noticeable that the power requirement and furnace design for both scenarios are identical.
[0037] The components of the gangue in the self-fluxing product 40 are complementary, in the context of chromite smelting, thus allowing for a decrease in, and even elimination of, the requirement to feed slag modifiers such as limestone and silica to the furnace. The extent to which flux addition can be reduced is proportional to the degree to which the concentrate was upgraded (chromium grade).
Calculations show that it may be possible to optimize the grade of the concentrate to eliminate flux addition altogether, while retaining the capability to process practically any type of concentrate in the furnace. By eliminating, or dramatically reducing, the addition of limestone, a significant energy saving can be achieved.
[0038] A significant increase in the overall Cr recovery (across the combined concentrator and smelter) can be expected if the grade of the product to be smelted is dropped from 42% to 34% for example, especially if the concentrator and smelter functions are operated as an integrated plant. Further benefits lie upstream of the smelter. Simplistically put: in a conventional process, flux (as gangue) in the feed material is removed and the chromite is then transported to an appropriate smelting plant. If the transport is significant then the cost saving in removing the flux prior to transport taking place, is notable. However, if it is possible to smelt a lower-grade material from which the gangue has not been removed, at a location which is close to the processing plant, so that transport costs are not unduly increased, then the benefit of the invention immediately becomes apparent. The invention also allows for the use of discard chromium tailings, typically stockpiled in tailings dams.
Reclaiming these chromium units, typically at a very low cost, will reduce the cost of smelting for such an operation dramatically.
EXPERIMENTAL TESTING
[0039] A demonstration-scale DC furnace smelting campaign processing low-grade chromite concentrate (34% Cr203) was undertaken. The material consisted of 42%
metallurgical grade chromite and low-grade fines (with about 25% chromium oxide content), pre-blended and dried via a rotary kiln prior to processing. The demonstration-scale smelting testwork was conducted on a 2.5 m DC electric arc furnace facility. Auxiliary equipment included an ore feed system, water-cooling circuits, an offgas handling system which included a bag filter plant, and a control system.
[0040] Metallurgical stability and control were achieved and the process was demonstrated over a period of about two weeks.
[00411 The smelting recipe for the low-grade chromite material (not optimized) was 22.7% anthracite and 13.0% limestone (mass ratio relative to chromite processed), and resulted in a metal with a weighted average composition of 50.5% Cr, 41.8%
Fe, 0.11% Si and 8.1% C. The corresponding slag contained a weighted average of 4.4% Cr203 with a basicity ratio of 1.45.
, [0042] Two shorter conditions were also carried out, namely reduction of fluxes resulting in a period of ultra-low flux addition (5%) as well as a period of "fluxless"
smelting. These conditions further investigated the effect of lower flux additions on the Si deportment to the metal. Both conditions demonstrated a marked increase in the Si deportment to the metal phase and, as a result of dilution, an alloy containing just under 50% Cr. Neither condition was optimized for either recovery or alloy grade, but the results corroborated the general predictions from the modelling work.
[0043] Overall accountabilities of all major elements were good and a consistently high Cr deportment to the metal was demonstrated. The overall furnace availability for the test work campaign was 94% which is excellent for the scale of operation, thus further highlighting that the low-grade material does not adversely affect operability. The average energy consumption (neglecting thermal efficiency and availability) was determined to be 1.37kWh/kg chromite for low grade material.
[0044] Electrode consumption was fairly typical for this scale of operation at 1.6 to 1.8kg/MWh, which agrees well with tests conducted with metallurgical grade feed stocks. The furnace lining was found to be in good condition at the end of the campaign and a significant build-up of frozen slag was found on the sidewalls, indicative of the freeze-lining that was present during operation.
[0045] In Table 4 a comparison of typical metallurgical grade smelting operations (per unit of Cr produced) shows a significant reduction in the fluxing requirement for the smelting of low grade chromite (a reduction of 46%) while the reductions in energy requirement and reductant addition are small, as is expected.
[0046] Overall the results successfully demonstrated the production of ferrochromium metal from the chromite tested, whilst consistently achieving slag with < 5% Cr203 by mass, equivalent to between 92 and 93% Cr recovery to the metal phase.
[0047] The test work was successful from not only a metallurgical but also an operational point of view in that DC smelting of low-grade, fine chromite ore on the whole was seen to be flexible, efficient and robust.
Table 4: Experimental results for 34% and 42% chromite ores Nominal chromite grade 34% feed 42% feed Cr:Fe mass ratio (concentrate) 1.28 1.40 Fe recovery 97.6 96.9 Cr recovery 93.0 92.1 Anthracite, % of chromite 22.7 27.9 Quartzite, % of chromite 8.6 Limestone, % of chromite 13.0 20.5 kg slag/100 kg chromite 49.3 52.1 kg metal/100 kg chromite 43.6 51.2 kg gas/100 kg chromite Operating temperature, C 1690 1705 Estimated slag liquidus, C 1698 1683 Slag composition, mass %
Cr203 4.4 4.2 FeO 2.4 2.2 Si02 26.7 25.9 CaO 14.8 19.8 MgO 23.8 17.4 A1203 26.9 29.5 Alloy mass ratio: Cr:Fe ratio 1.63 1.65 Slag mass ratio: (Ca0+Mg0)/S102 1.45 1.44 Metal composition, mass %
Cr 50.5 51.7 Fe 41.8 39.2 Si 0.1 0.1 C 8.5 8.1 0.01 0.02 0.03 0.03 Cr:Fe mass ratio 1.21 1.32 Energy requirement kWhit of chromite 1.37 1.60 kWhit metal product 3.12 3.24 kWhit Cr product 6.18 6.27
[00191The feed material may be fed to a de-sliming cyclone to produce an underflow comprising solids from which a mixture is prepared by adding clean water, typically in a ratio of 30% to 70%.
[0020]The mixture may be directed to a set of rougher spirals, from there, to a set of cleaner spirals and then to a set of re-cleaner spirals in order to separate the mixture into resulting products.
[0021]The resulting products may include a chemical-grade product and a metallurgical-grade product.
[0022]During an initial processing stage a proportion of the feed material which is passed through at least a part of the set of rougher spirals may be passed to a secondary set of cleaner spirals.
[0023] The secondary set of cleaner spirals may produce an intermediate product which has a chromite content of about 34%.
[0024] The intermediate product may have a significantly higher gangue content than the metallurgical-grade product and is suitable for chromite smelting, particularly in a DC open arc furnace.
[0025] The higher gangue content may be achieved through the use of an appropriate preceding process or by blending tailings with high grade material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention is further described by way of example with reference to Figure 2 of the accompanying drawings which illustrates a modification to the conventional UG2 chromium recovery process shown in Figure 1 which has been discussed in the preamble hereto.
DESCRIPTION OF PREFERRED EMBODIMENT
[0027] Figure 2 illustrates in block diagram form a UG2 chromium recovery process for the production of a chromite ore of metallurgical grade, according to the invention.
The process in Figure 2 has substantial similarities to the process shown in Figure 1 and for this reason a detailed description of the process in Figure 2 is not given.
Reference numerals are used in Figure 2 which are the same as the reference numerals used in Figure 1 to designate components which are the same as the components in Figure 1.
[0028] During an initial processing stage a proportion of the feed material 22 passing through at least a part of the set of rougher spirals 24 is directed to a secondary set of cleaner spirals 34. The remainder of the feed material 22 is processed in the manner described in connection with Figure 1 i.e., it passes sequentially through the set of cleaner spirals 26 and then to the re-cleaner spirals 28 to produce a chemical-grade product 30 and a metallurgical-grade product 32.
[0029] The secondary set of cleaner spirals 34 produces an intermediate product 40 which has a chromite content of 34%.
[0030] The product 40, compared to the product 32 which is customarily deemed to be saleable and which has a chromite content of about 42% or higher, has a significantly higher gangue content.
[0031] This is illustrated in Table 1 which compares the assayed content of the material 40 with a metallurgical-grade product 32 which has a chromite content of 42%. Notably the Si02 content is 13.9% in the product 40 while it is reduced to 1.0%
in the commercially acceptable product 32.
Table 1: Composition of chromite ores (mass %) Intermediate Metallurgical Description Product grade UG2 Designation 34% feed 42% feed Cr203 33.7 41.7 FeO 23.8 27.7 A1203 13.1 16.0 CaO 0.95 0.09 MgO 12.8 9.2 Si02 13.9 1.0 TiO2 0.67 0.82 NiO 0.18 0.17 MnO 0.23 0.22 V205 0.27 0.37 0.014 0.005 Mass ratios Cr/Fe 1.25 1.33 (Ca0+mg0)/Si02 1.02 7.60 MgO/A1203 0.98 0.58 [0032] Computer modelling has shown that the inherent or natural composition of the gangue materials in the product 40 is suitable for chromite smelting, particularly in a DC open arc furnace. The use of this type of furnace, instead of a submerged-arc furnace, displays advantages which include the following:
1. the DC furnace can be used for the smelting of chromite fines directly ¨
this eliminates the need for agglomeration;
2. anthracite and coal can be used as reducing agents in place of metallurgical coke and char, which are relatively expensive;
3. there is a high chromium recovery with residual chromium oxide in discard slag typically ranging from 3% to 5%;
4. the electrode consumption, in the smelting step, is lower; and 5. the DC furnace is flexible in operation - this allows various alloy compositions to be produced. It is noted that the product grade of metal produced from a smelting furnace, utilising a low-grade ore, depends on the Cr:Fe ratio of the ore and not on the grade (Cr203 content). It is therefore feasible to utilise the process of the invention, as described, and to achieve a secondary benefit by adding either Fe or Cr, depending on which product is targeted. It is therefore possible to modify the alloy grade which is produced, by means of suitable additives which are added either directly to the furnace or post-taphole, to create a specialised chromium alloy. For example, scrap iron can be added to dilute the chromium content, or high-grade fines, recovered for example from chromium slag dumps, can be added to upgrade the Cr:Fe ratio of the feed and thus of the end product. Si content of the alloy can be minimised or increased . ..
via normal control of the slag composition. Typically increased slag basicity (defined as the mass ratio of (Ca0+Mg0)/Si02 in the slag) results in low Si content in the alloy.
[0033] The slag requires only a small modification, if any, when compared to the metallurgical-grade concentrate of commercial quality of the kind referred to.
[0034] Table 2 summarises predicted slag and alloy compositions for the product 40 and the commercial product 32. In each case this is for 94% Fe recovery in the alloy produced from the furnace. The energy requirement per ton of chrome product produced for the product 40 is lower than for the commercial product 32.
Table 2: Model results for 34% and 42% chromite ores Nominal chromite grade 34% feed 42% feed Cr:Fe mass ratio (concentrate) 1.25 1.33 Fe recovery 94.0% 94.0%
Cr recovery 92.6% 92.6%
Anthracite, % of chromite 19.4 24.0 Quartzite, % of chromite 4.0 18.0 Limestone, % of chromite 10.0 kg slag/100 kg chromite 47.8 51.0 kg metal/100 kg chromite 42.2 54.4 kg gas/100 kg chromite 33.3 43.4 Operating temperature, C 1720 1720 Estimated slag liquidus, C 1677 1677 Slag composition, mass %
Cr203 5.2 5.7 FeO 2.7 2.8 SiO2 33.5 32.2 CaO 2.0 9.7 MgO 25.6 16.4 A1203 28.7 31.2 Alloy mass ratio Cr:Fe ratio 1.69 1.77 Slag mass ratio (Ca0+Mg0)/Si02 0.823 0.808 Metal composition, mass %
Cr 50.6 51.9 Fe 41.5 40.2 Si 0.4 0.4 7.5 7.5 0.01 0.01 0.03 0.03 Cr:Fe mass ratio 1.22 1.29 Energy requirement kWhit of chromite 1.30 1.62 kWh/t metal product 3.08 3.18 kWhit Cr product 6.09 6.13 [0035] Table 3 reflects calculated energy requirements and flux addition requirements for the product 40 and for the commercial product 32.
Table 3: Summary of case studies ¨ models solved for identical Cr production 42% Met-grade Type of chromite 34 % Low-grade concentrate concentrate Chromite concentrate fed 200 000 tpa 161 596 tpa Chromite concentrate plus fluxes 208 000 tpa 206 843 tpa Total feed 246 700 tpa 245 691 tpa Fe recovery 94.0% 94.0%
Cr recovery 92.6% 92.6%
Cr:Fe mass ratio concentrate 1.25 1.33 Alloy product 84 460 tpa 82 349 tpa % Cr (alloy) 50.6 % 51.6 %
Quartzite 8 000 tpa 29 089 tpa Limestone - tpa 16 160 tpa Anthracite 38 700 tpa 38 848 tpa Cr balance:
Cr in alloy 42 701 tpa 42 701 tpa Cr in discard slag 3 413 tpa 3 404 tpa % of chromite Metal make 42.23 50.96 Slag make 47.84 54.37 Energy requirement (SER)* MWhit MWhit feeds 1.25 1.27 MWhit chromite 1.30 1.62 MWhit alloy 3.08 3.18 Assumptions for furnace design Smelting intensity 400 kW/m2 400 kW/m2 Availability 94 % 94%
Thermal efficiency 90 % 90 %
Average year 365.28 days 365.28 days Power requirement¨ & estimated furnace diameter Power (assumed design basis) 35 MW 35 MW
Diameter of furnace 10.6 m 10.6 m Nominal diameter 11m 11 m * Energy requirement excludes thermal efficiency (smelting energy only "SER=specific energy requirement") ** Power requirement includes thermal efficiency factor # Pre-heating of chromite ore only; flux and reductant assumed fed at 25T
$ Feed = chromite plus flux (excludes reductant) [0036] The aforementioned tables show that there are significant benefits with regard to flux requirement and energy consumption for the self-fluxing ore (the product 40) specifically compared to the metallurgical-grade concentrate 32.
It is noticeable that the power requirement and furnace design for both scenarios are identical.
[0037] The components of the gangue in the self-fluxing product 40 are complementary, in the context of chromite smelting, thus allowing for a decrease in, and even elimination of, the requirement to feed slag modifiers such as limestone and silica to the furnace. The extent to which flux addition can be reduced is proportional to the degree to which the concentrate was upgraded (chromium grade).
Calculations show that it may be possible to optimize the grade of the concentrate to eliminate flux addition altogether, while retaining the capability to process practically any type of concentrate in the furnace. By eliminating, or dramatically reducing, the addition of limestone, a significant energy saving can be achieved.
[0038] A significant increase in the overall Cr recovery (across the combined concentrator and smelter) can be expected if the grade of the product to be smelted is dropped from 42% to 34% for example, especially if the concentrator and smelter functions are operated as an integrated plant. Further benefits lie upstream of the smelter. Simplistically put: in a conventional process, flux (as gangue) in the feed material is removed and the chromite is then transported to an appropriate smelting plant. If the transport is significant then the cost saving in removing the flux prior to transport taking place, is notable. However, if it is possible to smelt a lower-grade material from which the gangue has not been removed, at a location which is close to the processing plant, so that transport costs are not unduly increased, then the benefit of the invention immediately becomes apparent. The invention also allows for the use of discard chromium tailings, typically stockpiled in tailings dams.
Reclaiming these chromium units, typically at a very low cost, will reduce the cost of smelting for such an operation dramatically.
EXPERIMENTAL TESTING
[0039] A demonstration-scale DC furnace smelting campaign processing low-grade chromite concentrate (34% Cr203) was undertaken. The material consisted of 42%
metallurgical grade chromite and low-grade fines (with about 25% chromium oxide content), pre-blended and dried via a rotary kiln prior to processing. The demonstration-scale smelting testwork was conducted on a 2.5 m DC electric arc furnace facility. Auxiliary equipment included an ore feed system, water-cooling circuits, an offgas handling system which included a bag filter plant, and a control system.
[0040] Metallurgical stability and control were achieved and the process was demonstrated over a period of about two weeks.
[00411 The smelting recipe for the low-grade chromite material (not optimized) was 22.7% anthracite and 13.0% limestone (mass ratio relative to chromite processed), and resulted in a metal with a weighted average composition of 50.5% Cr, 41.8%
Fe, 0.11% Si and 8.1% C. The corresponding slag contained a weighted average of 4.4% Cr203 with a basicity ratio of 1.45.
, [0042] Two shorter conditions were also carried out, namely reduction of fluxes resulting in a period of ultra-low flux addition (5%) as well as a period of "fluxless"
smelting. These conditions further investigated the effect of lower flux additions on the Si deportment to the metal. Both conditions demonstrated a marked increase in the Si deportment to the metal phase and, as a result of dilution, an alloy containing just under 50% Cr. Neither condition was optimized for either recovery or alloy grade, but the results corroborated the general predictions from the modelling work.
[0043] Overall accountabilities of all major elements were good and a consistently high Cr deportment to the metal was demonstrated. The overall furnace availability for the test work campaign was 94% which is excellent for the scale of operation, thus further highlighting that the low-grade material does not adversely affect operability. The average energy consumption (neglecting thermal efficiency and availability) was determined to be 1.37kWh/kg chromite for low grade material.
[0044] Electrode consumption was fairly typical for this scale of operation at 1.6 to 1.8kg/MWh, which agrees well with tests conducted with metallurgical grade feed stocks. The furnace lining was found to be in good condition at the end of the campaign and a significant build-up of frozen slag was found on the sidewalls, indicative of the freeze-lining that was present during operation.
[0045] In Table 4 a comparison of typical metallurgical grade smelting operations (per unit of Cr produced) shows a significant reduction in the fluxing requirement for the smelting of low grade chromite (a reduction of 46%) while the reductions in energy requirement and reductant addition are small, as is expected.
[0046] Overall the results successfully demonstrated the production of ferrochromium metal from the chromite tested, whilst consistently achieving slag with < 5% Cr203 by mass, equivalent to between 92 and 93% Cr recovery to the metal phase.
[0047] The test work was successful from not only a metallurgical but also an operational point of view in that DC smelting of low-grade, fine chromite ore on the whole was seen to be flexible, efficient and robust.
Table 4: Experimental results for 34% and 42% chromite ores Nominal chromite grade 34% feed 42% feed Cr:Fe mass ratio (concentrate) 1.28 1.40 Fe recovery 97.6 96.9 Cr recovery 93.0 92.1 Anthracite, % of chromite 22.7 27.9 Quartzite, % of chromite 8.6 Limestone, % of chromite 13.0 20.5 kg slag/100 kg chromite 49.3 52.1 kg metal/100 kg chromite 43.6 51.2 kg gas/100 kg chromite Operating temperature, C 1690 1705 Estimated slag liquidus, C 1698 1683 Slag composition, mass %
Cr203 4.4 4.2 FeO 2.4 2.2 Si02 26.7 25.9 CaO 14.8 19.8 MgO 23.8 17.4 A1203 26.9 29.5 Alloy mass ratio: Cr:Fe ratio 1.63 1.65 Slag mass ratio: (Ca0+Mg0)/S102 1.45 1.44 Metal composition, mass %
Cr 50.5 51.7 Fe 41.8 39.2 Si 0.1 0.1 C 8.5 8.1 0.01 0.02 0.03 0.03 Cr:Fe mass ratio 1.21 1.32 Energy requirement kWhit of chromite 1.37 1.60 kWhit metal product 3.12 3.24 kWhit Cr product 6.18 6.27
Claims (18)
1. A feed material, containing chromite concentrate, which is to be smelted in a furnace under conditions in which flux requirements for the production of ferrochromium are provided primarily by gangue components present in the feed material.
2. A feed material according to claim 1 wherein the gangue components include one or more of SiO2, Al2O3, MgO, and CaO.
3. A feed material according to claim 1 or 2 which contains chromite at a grade which is lower than a commonly accepted metallurgical grade.
4 A feed material according to claim 3 which includes chromite concentrates containing less than 38% mass Cr2O3 %
5. A feed material according to claim 3 or 4 wherein the grade of the chromite is between 25 to 34 mass % Cr2O3
6 A feed material according to any one of claims 1 to 5 which is drawn from a conventional chrome recovery process as an intermediate product or from discard tailings or which is prepared by means of a blending process or which is prepared by a minerals processing technique.
7 A feed material according to any one of claims 1 to 6 wherein the furnace is a DC open-arc furnace.
8. A method of producing a chrome product which includes the step of smelting a feed material which contains a chromite ore under conditions in which flux requirements for the chrome product are provided primarily by gangue components in the feed material.
9. A method according to claim 8 wherein the gangue components are inherently in the feed material or are added as required to the chromite ore.
10. A method of producing at least one chrome product which includes the step of directing a feed material according to any one of claims 1 to 7 to a de-sliming cyclone to produce an underflow comprising solids from which a mixture is prepared by adding clean water, typically in a ratio of 30% to 70%.
11. A method according to claim 10 wherein the mixture is sequentially directed to a set of rougher spirals, a set of cleaner spirals and a set of re-cleaner spirals to separate the mixture into resulting products.
12. A method according to claim 11 wherein the resulting products include a chemical-grade product and a metallurgical-grade product.
13. A method according to claim 12 wherein the metallurgical grade product has a chromite content of at least 42% and an SiO2 content of about 1.2%.
14. A method according to claim 11, 12 or 13 wherein a proportion of the mixture which is passed through at least a part of the set of rougher spirals is then directed to a secondary set of cleaner spirals to produce an intermediate product which has a chromite content of 34% and SiO2 content of 13.2%.
15. A method according to claim 14 wherein the intermediate product is smelted in a DC open arc furnace and anthracite or coal are used as a reducing agent during the smelting process.
16. A method according to claim 15 which includes the step of modifying an alloy grade produced by the smelting process by means of at least one suitable additive which is added directly to the furnace or post-taphole.
17. A method according to claim 15 wherein scrap iron is added to the furnace to dilute the chromium content of the chrome product.
18. A method according to claim 15 wherein high-grade fines are added to upgrade the Cr:Fe ratio of the feed material and thus of the chrome product.
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| CA (1) | CA2893406C (en) |
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109046744A (en) * | 2018-07-20 | 2018-12-21 | 周全民 | A kind of cupro-nickel multi-metal tailings comprehensive recycling process |
| WO2020188379A1 (en) * | 2019-03-20 | 2020-09-24 | Arxo Metals (Pty) Ltd. | Recovery of chromite fines |
| WO2022168015A1 (en) * | 2021-02-05 | 2022-08-11 | SIBANYE GOLD LIMITED t/a SIBANYE-STILLWATER | Plant layout and process for extracting fine chrome |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
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| JPS5816053A (en) * | 1981-07-21 | 1983-01-29 | Nippon Kokan Kk <Nkk> | Manufacture of ferrochromium |
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2015
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- 2015-06-01 AP AP2015008650A patent/AP2015008650A0/en unknown
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109046744A (en) * | 2018-07-20 | 2018-12-21 | 周全民 | A kind of cupro-nickel multi-metal tailings comprehensive recycling process |
| WO2020188379A1 (en) * | 2019-03-20 | 2020-09-24 | Arxo Metals (Pty) Ltd. | Recovery of chromite fines |
| WO2022168015A1 (en) * | 2021-02-05 | 2022-08-11 | SIBANYE GOLD LIMITED t/a SIBANYE-STILLWATER | Plant layout and process for extracting fine chrome |
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| AP2015008650A0 (en) | 2015-08-31 |
| CA2893406C (en) | 2018-02-06 |
| ZA201503921B (en) | 2016-03-30 |
| EA201590864A1 (en) | 2016-03-31 |
| EA033243B1 (en) | 2019-09-30 |
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