WO2016139461A1

WO2016139461A1 - Process

Info

Publication number: WO2016139461A1
Application number: PCT/GB2016/050533
Authority: WO
Inventors: Animesh Jha; Sergio SANCHEZ-SEGADO; Lidia ESCUDERO-CASTEJON
Original assignee: University of Leeds; University of Leeds Innovations Ltd
Current assignee: University of Leeds; University of Leeds Innovations Ltd
Priority date: 2015-03-04
Filing date: 2016-03-01
Publication date: 2016-09-09
Anticipated expiration: 2017-09-04
Also published as: CN107406904B; GB201503636D0; CN107406904A

Abstract

The present invention relates to a process for recovering a primary metal residue from a metal-containing composition.

Description

Process

Naturally-occurring reactive metal oxides of titanium, chromium and aluminium commonly combine chemically with transition metal oxides such as ferrous, ferric, vanadium, niobium and tantalum oxides and rare earth oxides to form complex geological minerals. Similar and dissimilar ionic substitutions lead to complex multi-phase systems including (for example) solid solutions and highly complex crystal structures. The recovery of metal oxides by physicochemical separation is therefore challenging. Similar challenges arise in recovering metal oxides from perovskite-type deposits which are rich in MgO and CaO. Moreover current industrial processes for recovering metal oxides are energy intensive and produce (for example) chromite ore process residue (COPR) from chromate processes. Thus there is a constant demand for improvements in efficiency, whilst economic, environmental, safety and energy considerations are paramount.

Chromite ores ((Fe, Mg)(Cr, Al)₂0₄) are the main source of chromium metal and chromium chemicals widely used in industries like metallurgy, chemistry and refractory. Processes such as soda-ash roasting, acid leaching, alkali leaching and alkali fusion have been developed for processing chromite ores to produce sodium chromate (Na₂Cr0₄) which is the primary product for the manufacture of chromium chemicals. However economic and technical constraints mean that only soda-ash roasting has been used worldwide. Due to its toxicity, chromium (particularly Cr(VI)) in waste products raises numerous environmental concerns. During processing of chromium mineral by soda-ash roasting, chromium present in the chromite ore as Cr³⁺ is oxidized to Cr⁶⁺ in the form of water soluble sodium chromate. After extraction of the sodium chromate, the residue including environmentally hazardous Cr⁺⁶ is placed in landfill.

The development of a lime-based process was one of the main advances in this field. The process produces water soluble sodium chromate by alkali roasting with soda ash, lime and dolomite in an oxidizing atmosphere. The lime neutralizes silica as silicates which reduces the amount of alkali consumed and enhances oxygen diffusion during roasting. However the process is hampered by the formation of highly water soluble calcium chromate (CaCrC ) which is a source of toxic Cr⁺⁶ pollution and has a negative effect on the yield of sodium chromate. Considerable research has been carried out to optimize the process parameters of soda-ash roasting of chromite ore {eg composition of the ore, time and temperature) and to study the kinetics and mechanism of the roast. However it is impossible to completely avoid the formation of residual Cr⁺⁵.

A lime-free technology is now adopted in most developed countries as an alternative to a dolomite-based process. However it is only practicable when silica content in the initial ore is less than 3wt due to the negative effect that silica has on the oxygen transport at the reaction interface. With low silica chromites the maximum yield of sodium chromate achieved by the lime-free process is less than 90%.

The present invention seeks to provide an improved process for recovering a primary metal residue {eg oxide) from a metal-containing composition in a high concentration. In particular, the process permits the recovery of metal value-containing precipitates in consistently high concentrations which reduces the levels of waste and usage of raw materials.

Thus viewed from a first aspect the present invention provides a process for recovering a primary metal residue from a metal-containing composition comprising:

(a) preparing a feedstock of the metal-containing composition and an alkali salt;

(b) reductively roasting the feedstock at a roasting temperature for a roasting period to produce a roast;

(c) cooling the roast to produce a roasted mass containing metallic iron or an alloy or compound thereof and soluble metal oxides;

(d) adding an aqueous medium to the roasted mass to form a substantially insoluble product and a first solution of soluble metal oxides;

(e) acid leaching the substantially insoluble product or a fraction thereof to produce a leach residue and a second solution of soluble metal oxides;

(f) roasting the leach residue in the presence of a bisulphate or bicarbonate of an alkali metal or alkaline earth metal to produce a roasted residue; and

(g) hydrometallurgically extracting from the roasted residue the primary metal

residue and a third solution of soluble metal oxides.

The presence of a bisulphate or bicarbonate of an alkali metal or alkaline earth metal assisted by any residual iron advantageously promotes the formation of soluble metal compounds {eg double salts and sulphates) which can be extracted in step (g) into the third solution of soluble metal oxides. Step (a) is typically carried out by mixing (eg homogeneously mixing) the metal- containing composition with the alkali salt to produce the feedstock.

The metal-containing composition may be present in the feedstock in an amount in excess of 40wt%.

The alkali salt may be present in the feedstock in an amount in excess of 20wt%.

The alkali salt may be an alkali metal salt or alkaline earth metal salt. The alkali salt is typically decomposable at less than 900°C, preferably less than 800°C, particularly preferably less than 750°C. The alkali salt may be an alkali metal halide, carbonate, bicarbonate, hydrogen sulphide, hydrogen sulphate, nitrate, chlorate, perchlorate or sulphate. Preferably the alkali salt is a carbonate, hydroxide, bicarbonate or sulphate of a group IA or group IIA metal or a mixture thereof. For example, the alkali salt may be selected from the group consisting of Na₂C0₃, K2CO3, Na₂S0₄, K₂S0₄, NaOH, NaHS0₄, KHS0₄, KHCO3, NaHC0₃ and KOH.

The weight ratio of metal-containing composition: alkali salt is typically in the range 1:0.1 to 1:2.

Preferably step (a) is preparing a feedstock of the metal-containing composition, the alkali salt and an iron-containing (eg ferrous or ferric) material. The iron-containing material may be iron oxide (eg Fe20₃), ferrochrome waste or a recycled magnetic fraction (eg the magnetic fraction from step (d2) described below).

Preferably step (a) is preparing a feedstock of the metal-containing composition, the alkali salt and an aluminium-containing material. The aluminium-containing material may be alumina or Cr-AI rich material. The Cr-Al rich material may have Al₂0₃ in excess of 70wt%. The Cr-AI rich material may have Cr₂0₃ in excess of 10wt%.

In step (a), the feedstock may be ground to a mean particle diameter of 106 microns or less, preferably 90 microns or less.

Step (b) may be carried out reductively in the presence of carbon (eg charcoal, graphite, coal or coke) in a flow of an inert gas such as nitrogen or argon (eg at a flow rate of 1-10 l/min). Carbon may be present in an amount of 20wt% or more. The weight ratio of metal-containing composition: carbon is typically in the range 1:0.01 to 1:0.5.

Step (b) may be carried out in a rotary furnace or static furnace (eg kiln). Step (b) may be carried out in a reductive atmosphere or air in a tubular or chamber kiln using a crucible. The roasting temperature may be in the range 600°C to 1500°C, preferably 800°C to 1300°C, more preferably 875°C to 1100°C (eg about 1050°C for reductive roasting).

The roasting period may be in the range 0.1 to 4 hours.

In step (c), cooling may take place (for example) in a rotary cooling drum. The cooling period is typically in excess of 60 minutes.

In step (c), the roasted mass may be ground to a mean particle diameter of 106 microns or less, preferably 90 microns or less.

In step (c), the soluble metal oxides may be or include aluminium oxide or vanadium oxide.

In step (d), the aqueous medium may be water or an alkali (eg dilute KOH, NaOH or Na₂C03). Preferably in step (d) the aqueous medium is water.

Step (d) may be accompanied by water leaching. Typically the aqueous medium is water at an elevated temperature. Step (d) may be carried out at an elevated temperature (eg 25 to 70°C). Step (d) may be carried out over a period of 0.5 to 5 hours.

Step (d) may form a slurry.

Step (e) may be carried out at an elevated temperature (eg a temperature in the range 25-100°C). Step (e) may be carried out for a period of 1 to 10 hours. Step (e) may be carried out in air or in a reducing agent (eg H₂) and/or an inert gas (eg Ar). Step (e) may be carried out at a pH of 4 or less.

The acid leachant may be an inorganic acid such as hydrofluoric acid, hydrochloric acid, nitric acid, sulphuric acid, an acidic oxide and mixtures thereof or a weak organic acid such as formic, oxalic, ascorbic, citric, lactic or acetic acid. Sulphuric acid is preferred.

Ascorbic acid and oxalic acid are preferred. Sulphuric acid and oxalic acid are preferred.

In step (f), the temperature may be in the range 400°C to 750°C. Step (f) may be carried out for a period of 0.1 to 4 hours. Step (f) may be carried out in air. The weight ratio of the bisulphate or bicarbonate to leach residue may be in the range 0.2:1 to 3:1.

Preferably the bisulphate or bicarbonate of an alkali metal or alkaline earth metal is a bisulphate.

Preferably the bisulphate or bicarbonate of an alkali metal or alkaline earth metal is a bisulphate or bicarbonate of an alkali metal, particularly preferably of Na.

The bisulphate or bicarbonate of an alkali metal or alkaline earth metal may be one or more of NaHS0₄, KHS0₄, NaHCOs or KHC0₃. Preferably the bisulphate or bicarbonate of an alkali metal or alkaline earth metal is NaHS0₄.

The metal-containing composition may be synthetic or natural.

The metal-containing composition is typically a mixture of simple and/or complex metal oxides. The metal-containing composition may include one or more of the group consisting of AI2O3, V₂0₅, Ti0₂, Fe₂0₃, FeO, Fe₃0_4/ CaO, Na₂0, MgO, MnO and Si0₂.

The metal-containing composition may be a metal-containing by-product of an industrial process such as a metal-containing slag.

The metal-containing composition may be a mixture or solid solution of metals such as an alloy (eg ferrochrome).

The metal-containing composition may be a mixture of metal compounds (eg oxides, sulphides and/or silicates) such as an ore, ore concentrate, mineral or mixture thereof.

An ore concentrate may be prepared from an ore by one or more physical (eg mechanical) processes such as crushing and gravitational, magnetic or electrical separation or by chemical processes such as leaching or high temperature reduction (eg slag formation).

Typically the metal-containing composition is iron-containing. The metal-containing composition may be iron-rich. For example, the metal-containing composition may have an amount of iron up to 75wt . The metal-containing composition may include Fe, FeO, Fe₂0₃ or Fe₃0₄. For example, the metal-containing composition may be chromite.

In the metal-containing composition, it is preferred that the amount of any alumina is minimal {eg 15wt% or less).

In the metal-containing composition, it is preferred that the amount of any silica is minimal (eg 5wt% or less).

Step (g) may include one or more steps of the group consisting of leaching, precipitation, separation (eg chemical separation or physical separation such as magnetic separation), thickening, evaporation, washing, drying and reduction. Each of these steps may be carried out at ambient or elevated temperature or pressure. For example, a leaching step may be carried out in a reaction vessel such as a stirred tanker reactor or a pressure vessel such as an autoclave.

Preferably step (g) comprises: (gl) adding to the roasted residue an aqueous medium to produce the third solution of soluble metal oxides and the primary metal residue; and

(g2) separating the primary metal residue from the third solution of soluble metal oxides.

The aqueous medium may be water or an alkali solution (eg a dilute solution of KOH, NaOH or Na₂C03). Typically water is used at an elevated temperature. The hot water may be at a temperature in the range 25 to 80°C. Step (gl) may be carried out in hot water for 20 to 300 minutes.

The process may further comprise recovering one or more metal value-containing precipitates. The one or more metal value-containing precipitates may contain oxides or hydroxides of Ca, Na, Mg, Ti, Mn, Al, Ni, Zn, Ca, Fe or V [eg V₂0₅, Ti0₂, Fe₂0_3/ FeO, CaO, Na₂0, Al₂0₃, MgO, MnO, NiO or ZnO) or hydrates thereof.

Preferably the process further comprises: recovering one or more metal value- containing precipitates from the first solution of soluble metal oxides. The one or more metal value-containing precipitates may contain V, Al or Na values.

Preferably the process further comprises: recovering one or more metal value- containing precipitates from the second solution of soluble metal oxides. The one or more metal value-containing precipitates may contain Al values.

Preferably the process further comprises: recovering one or more metal value- containing precipitates from the third solution of soluble metal oxides. The one or more metal value-containing precipitates may contain Na, Ti, Ca, Mn, Mg, Fe or V values.

The recovery of metal value-containing precipitates from a solution of soluble metal oxides may be carried out by basification. For example the recovery of metal value- containing precipitates may be carried out by the addition of an inorganic base (such as an inorganic base selected from the group consisting of potassium hydroxide, sodium hydroxide or sodium carbonate and mixtures thereof) or of a weak organic base.

The recovery of metal value-containing precipitates from a solution of soluble metal oxides may be carried out by acidification. For example the recovery of metal value- containing precipitates may be carried out by the addition of an inorganic acid such as an inorganic acid selected from the group consisting of hydrofluoric acid, hydrochloric acid, nitric acid, sulphuric acid, an acidic oxide and mixtures thereof or of a weak organic acid such as formic, oxalic, ascorbic, citric, lactic or acetic acid and/or C0₂ (eg by C0₂ bubbling). In a preferred embodiment, the metal-containing composition is chromium-rich. Typically the chromium-containing composition has an amount of chromium oxide in excess of 35wt% (eg in the range 40-50wt%).

Preferably the chromium-containing composition is a chromite ore concentrate.

Preferably the primary metal residue is chromium (III) oxide (Cr₂03) (eg metallurgical grade chromium (III) oxide). Cr₂03 may be used downstream for manufacturing sodium chromate (Na2Cr0₄), sodium dichromate ( a2Cr₂07), chrome metal or other chromium- based chemicals.

The presence of a bisulphate or bicarbonate of an alkali metal or alkaline earth metal advantageously promotes the formation of soluble metal compounds (such as Na₂ g(S0₄)₂ and Na₂Ca(S0₄)₂) at the expense of insoluble calcium and magnesium compounds which would otherwise contaminate the primary metal residue.

In the preferred embodiment, Cr₂C>3 may be present in the primary metal residue in a concentration in the range 85 to 90wt .

In the preferred embodiment, step (e) serves primarily to remove Si, Al, Mg and Na. Step (e) is typically carried out in the presence of sulphuric acid and oxalic acid. Step (e) may be carried out at an elevated temperature {eg 40 to 60°C). Step (e) may be carried out for 1 to 10 hours (eg 1.5 hours).

Particularly preferably the process further comprises: recovering one or more aluminium values (eg A Oi) from the first solution of soluble metal oxides. The one or more aluminium values may be recovered in a concentration in the range 80 to 85wt%.

Particularly preferably the process further comprises: recovering one or more metal value-containing precipitates from the second solution of soluble metal oxides. The one or more metal value-containing precipitates may contain Na, Al, Si and Mg values.

Particularly preferably the process further comprises: recovering one or more metal value-containing precipitates from the third solution of soluble metal oxides. The one or more metal value-containing precipitates may contain Na, Ca, Mg and Fe values.

(1) In a first preferred embodiment, step (b) is: reductively roasting the feedstock at a roasting temperature for a roasting period to produce a roast.

In the first preferred embodiment, the process preferably further comprises:

(d2) magnetically separating from the substantially insoluble product a magnetic fraction and a substantially non-magnetic fraction, wherein step (e) is: acid leaching the substantially non-magnetic fraction to produce a leach residue and a second solution of soluble metal oxides.

The iron is predominantly present in the magnetic fraction as metallic iron or an alloy thereof and may be advantageously recovered.

Step (d2) is typically carried out by wet magnetic separation.

The efficiency of step (d2) may be dependent on the particle size of the metallic iron. If iron particles are insufficiently large, they might fail to cluster and separate from sodium chromite particles which decreases the efficiency of the separation. Thus controlling the particle size of metallic iron by grinding serves to optimise efficiency.

In the first preferred embodiment, the process preferably further comprises:

(al) grinding the feedstock.

In the first preferred embodiment, the process preferably further comprises:

(cl) grinding the roasted mass.

In the first preferred embodiment, step (d) typically forms a slurry.

Particularly preferably the process further comprises:

(d2a) water leaching the substantially non-magnetic fraction to produce an alkaline solution. The alkaline solution is advantageously added to the first solution.

Particularly preferably the process further comprises:

(d3) smelting the magnetic fraction to produce a steel residue and a non-magnetic slag.

Step (d3) may be carried out in the presence of about lwt% carbon. Step (d3) may be carried out for 1 to 3 hours. Step (d3) may be carried out in a flow of an inert gas (eg argon). Step (d3) may be carried out at a temperature in excess of 1400°C.

The steel residue may be recovered with a concentration of iron of 95wt% or more.

More preferably the process further comprises:

(d3a) acid leaching the non-magnetic slag to produce a slag leach residue and a fourth solution of soluble metal oxides;

(d3b) roasting the slag leach residue in the presence of a bisulphate or bicarbonate of an alkali metal or alkaline earth metal to produce a roasted slag leach residue; and

(d3c) hydrometallurgically extracting from the roasted slag leach residue a secondary metal residue and a fifth solution of soluble metal oxides. Step (d3a) may be carried out at an elevated temperature (eg a temperature in the range 25-100X). Step (d3a) may be carried out for a period of 1 to 10 hours. Step (d3a) may be carried out in air. The acid leachant may be an inorganic acid such as hydrofluoric acid, hydrochloric acid, nitric acid, sulphuric acid, an acidic oxide and mixtures thereof or a weak organic acid such as formic, oxalic, ascorbic, citric, lactic or acetic acid. Sulphuric acid is preferred.

In step (d3b), the temperature may be in the range 600°C to 750°C. Step (d3b) may be carried out for a period of 1 to 2 hours. Step (d3b) may be carried out in air. The weight ratio of the bisulphate or bicarbonate to slag leach residue may be in the range 0.2:1 to 3:1.

Step (d3c) may be carried out in a similar manner to step (g).

The recovery of secondary metal residue may be 85wt% or more. The secondary metal residue is typically the same as the first metal residue.

Even more preferably the process further comprises:

recovering one or more metal value-containing precipitates from the fifth solution of soluble metal oxides. The one or more metal value-containing precipitates may contain Na, Ti, Ca, Fe or V values.

In the first embodiment, the process may further comprise:

(el) heating the leach residue to remove carbon.

Step (el) may be carried out at a temperature in the range 900 to 1000°C. Step (el) may be carried out for 1 to 2 hours.

The present invention will now be described in a non-limitative sense with reference to Examples and the accompanying Figures in which:

Figure 1 illustrates an embodiment of the process of the invention;

Figure 2 is the predominance diagram of the system Na-Fe-Cr-O-C at 1323K (1050°C) calculated by Fact-Sage 6.4 software;

Figure 3 illustrates schematically the cold water stream assisted magnetic separation carried out in step C3 of the embodiment of the invention;

Figure 4 is the SEM elemental mapping of the magnetic fraction from the embodiment of the invention;

Figure 5 illustrates XRD patterns of (a) the roasted mass, (b) magnetic fraction and (c) nonmagnetic fraction of the embodiment of the invention; Figure 6 illustrates XRD patterns of the Cr₂0₃-rich residue of the embodiment of the invention (a) before and (b) after washing;

Figure 7 illustrates XRD patterns for the reduced material, magnetic and non-magnetic fractions of Example 2;

Figure 8 illustrates backscattered SEM images of the magnetic fraction of Example 2;

Figure 9 illustrates XRD patterns of reduced samples and magnetic fractions in Example 3; Figure 10 illustrates SEM images of the magnetic fraction in Example 3; and

Figure 11 illustrates XRD patterns of acid leached residues in Example 4.

Example 1

An embodiment of the process of the invention is illustrated in Figure 1. In this embodiment, Cr₂0₃ is recovered from a chromite concentrate of a South African mineral ore (40-50wt% Cr₂0₃, 25-30wt% Fe₂0₃, 7-10wt% MgO, 7-10wt% Al₂0₃ and 3-5wt% Si0₂).

A3. The as-received concentrate was thoroughly mixed with Na₂C0₃ and charcoal/coal in the weight ratio concentrate:Na₂C0₃:C = 1:1:0.3 to produce a feedstock.

B3. The feedstock was reduced isothermally for 2.5 hours at 1050°C in an inert (argon or nitrogen) atmosphere. The roast was then cooled to room temperature to form a roasted mass and the roasted mass was ground to less than 90μητ The alkaline reduction taking place is based on the Na-Fe-Cr-O-C predominance area diagram shown in Figure 2 where it is observed that metallic Fe co-exists with sodium chromite (NaCr0₂). The iron present in the spinel phase is reduced to metallic Fe and chromium reacts with Na₂C0₃ and carbon to form NaCr0₂ as shown below in reaction (1). Aluminium oxide reacts with Na₂C0₃ to form water soluble sodium aluminate (NaAl0₂) as shown in reaction (2). Part of the sodium carbonate reacts with magnesium and silica to form complex sodium magnesium silicates and with calcium to form calcium carbonate (CaC0₃).

C3. The ground roasted mass was weighed, made up into a slurry and subjected to wet magnetic separation in the arrangement shown in Figure 3. The ground roasted mass was gently poured onto a magnetic trough 100 with a flow of cold water 102 from a nearby tap to wash-off non-magnetic components and dissolve water-soluble components. The ground roasted mass was scrubbed with a brush to aid separation and a non-magnetic fraction 1 suspended in a first solution 4 was collected in a beaker 101 for detailed chemical, physical and microstructural analyses. A magnetic fraction 12 was retained in the magnetic trough 100. The non-magnetic fraction 1 containing mainly sodium chromite, silicates and calcium carbonate was allowed to settle and the first solution 4 was decanted. The compositions of the magnetic fraction 12 and non-magnetic fraction 1 analysed by XRF are shown in Table 1 below. Sodium aluminate was dissolved in the first solution 4. The magnetic fraction 12 contained some residual sodium chromite. SEM analysis (Figure 4) showed that this is due to entrapment of Fe (bright phase) in IMaCr0₂ particles (grey phase) and sintering between both phases. X-Ray diffraction patterns of (a) the roasted mass, (b) the magnetic fraction 12 and (c) the non-magnetic fraction 1 are shown in Figure 5.

D3. The non-magnetic fraction 1 was leached in water at 40-60°C for 1.5 hours to

remove as much sodium as possible from sodium chromite into an alkaline solution 21. Any residual sodium aluminate from step C3 was dissolved in the alkaline solution 21. The residue was a leachate 22.

E3. The leachate 22 was leached in oxalic and sulphuric acid at 40-60°C for 1.5 hours with the aim of removing the remaining sodium from the partially leached sodium chromite (Nai-_xCr0₂) and the sodium magnesium silicates. Some of the remaining alumina in the form of complex silicates was also removed. The outcome was a Si, Na, Al and Mg-containing second solution 11 and a leach residue 23 rich in Cr₂0₃ (75- 80wt% Cr₂0₃).

F3. The leach residue 23 was then roasted with NaHS0₄ at 650°C for 1 hour in air to

produce a roasted residue 41. The remaining Ca, Mg, Fe and the added sodium form double sulphates as shown in reactions (3) to (7).

G3. The roasted residue 41 was water leached for 1.5 hours at 40°C to 60°C to produce a primary metal residue 25. Water soluble sulphates were removed into a third solution 24. The third solution 24 can be then treated to recover sodium carbonate for recycling. The primary metal residue 25 was washed in 0.5M HCI to remove water insoluble Na₃Fe(S0₄)₃ 26 and was at least 85-90wt% Cr₂0₃. Figure 6 illustrates XRD patterns of the primary metal residue 25 (a) before and (b) after washing. H3. Carbon dioxide was bubbled through a combination of the first solution 4 and the alkaline solution 21 at 40-60°C for 1.5 hours to recover an alumina-rich precipitate 5 (80-85wt% Al₂0₃ and 5-10wt% Si0₂).

13. A solution 29 separated after step H3 was heated to evaporate the water and to

crystallise sodium carbonate which can be recycled. The purity of the sodium

carbonate recovered was above 90wt%.

J3. The magnetic fraction 12 was smelted above 1460°C to obtain steel 27. Residual

chromium can be recovered from the slag. Part of the magnetic fraction 12 may be recycled into step B3 as it might help to: a) increase the ratio of iron in the feedstock to help growth of the iron particles and improve the efficiency of magnetic

separation; and b) recover some of the sodium chromite lost in the magnetic fraction 12.

Table 1 - XRF analysis of magnetic and non-magnetic fractions

Cr₂0₃ Fe₂0₃ Na₂0 MgO Al₂0₃ S1O2 CaO

Magnetic fraction 21.1 71.2 1.6 1.99 1.85 0.45 0.592

Non-magnetic fraction 60.1 5.01 6.43 10.3 10.1 3.87 2.33

Reactions

FeCr₂0₄ + Na₂C0₃ + 2C = Fe + 2NaCr0₂ + 3CO(g) (1)

AI2O3 + Na₂C0₃ + C = 2NaAl0₂ + 2CO (2)

CaO + 2NaHS0₄ = Na₂Ca(S0₄)₂ + H₂0 (3)

MgO + 2NaHS0₄ = Na₂Mg(S0₄)₂ + H₂0 (4)

2Fe + 4NaHS0₄ + 0₂(g) = 2FeS0₄ + 2Na₂S0 + 2H₂0 (5)

2Fe + 6NaHS04 + 1.50₂(g) = Fe₂(S0₄)₃ + 3Na₂S0₄ + 3H₂0 (6)

Fe₂(S0₄)₃ + 3Na₂S0 = 2Na₃Fe(S0₄)₃ (7)

Example 2

An as-received chromite ore (48.80% Cr₂0₃, 31.30% Fe₂0₃, 7.03% MgO, 7.15% Al₂0₃, 3.45% Si0₂ and 0.54% CaO) is thoroughly mixed with sodium carbonate (Na₂C03), activated charcoal and pure iron oxide. The mixture is placed in an alumina crucible and isothermally reduced in an electrically heated stainless-steel tube furnace with a temperature-controlling device. Reduction is carried out in an argon atmosphere at a flow rate of 2.0 L/min. Reduction conditions are shown in Table 2.

Table 2

Reduced samples are ground and subjected to wet magnetic separation. The magnetic and the non-magnetic fractions were dried and characterised by different analytical techniques.

RESULTS

X-ray powder diffraction (XRD) patterns for the reduced material, magnetic and non-magnetic fractions are presented in Figure 7. XRD shows that chromium from chromite ore combines with sodium to form sodium chromite (NaCr0₂) which is an insoluble phase where chromium is in the +3 state. Iron is reduced to metallic iron and excess sodium reacts with aluminium to form sodium aluminate. After wet magnetic separation, metallic iron is the main phase present in the magnetic fraction, while sodium chromite is preferentially found in the nonmagnetic fraction. Although this cannot be observed in the XRD pattern of the non-magnetic fraction due to partial leaching of sodium from NaCr0₂ during wet magnetic separation, X-ray fluorescence (XRF) results presented in Table 3 clearly indicate that chromium is more concentrated in the non-magnetic fraction.

Table 3

Chromium is still present in a significant quantity in the magnetic fraction as NaCr0₂. This is due to physical entrapment of metallic iron in sodium chromite particles. As a consequence, part of the NaCr0₂ is carried along with metallic iron particles to the magnetic fraction. This is observed in the backscattered scanning electron microscopy (SEM) images of the magnetic fraction in Figure 8. The role of Fe₂0₃ is to promote the growth of metallic iron particles so that the efficiency of magnetic separation is increased. An improvement is observed with respect to the results obtained after reduction without addition of Fe₂0₃ but further optimization is needed in order to minimise the chromium content in the magnetic fraction.

Example 3

An as-received chromite ore (48.80% Cr₂0₃, 31.30% Fe₂0₃, 7.03% MgO, 7.15% Al₂0₃, 3.45% Si0₂ and 0.54% CaO) is thoroughly mixed with sodium carbonate (Na₂C0₃), activated charcoal and pure alumina or Cr-AI rich material. The mixture is placed in an alumina crucible and isothermally reduced in an electrically heated stainless-steel tube furnace with a temperature-controlling device. Reduction is carried out in argon atmosphere at a flow rate of 2.0 L/min. Reduction conditions are shown in Table 4.

A first experiment was carried out with addition of alumina. A second experiment was carried out with addition of Cr-AI. The percentage of alumina/Cr-Al was 2.5% with respect to the chromite ore as shown in Table 4. The alumina is pure AI2O3 and the Cr-AI is a material with an approximate composition of 75% Al₂0₃, 14% Cr₂0₃ and smaller quantities of K, Fe, Na, Mg and Si.

Table 4

RESULTS

XRD patterns of reduced samples and magnetic fractions are presented in Figure 9. The phases present in the reduced samples from both experiments are mainly metallic Fe, NaCr0₂, NaAl0₂ and MgO. SEM images in Figure 10 show that a substantial part of the NaCr0₂ is found in the magnetic phase and the efficiency of the magnetic separation needs to be improved.

Reduction in the presence of alumina/Cr-AI seems to enhance the formation of NaCr0₂.

Example 4

A chromite ore sample is alkali reduced for 2.5 hours at 1050°C with Na₂CC>3 and activated charcoal. The reduced sample is subjected to magnetic separation. The non-magnetic fraction is leached in water at 60°C for 1.5 hours with a soiidrliquid ratio of 1:100 g/mL and with continuous stirring. Water leaching is carried out in order to solubilise sodium which can be later recovered as Na₂C03 and recycled back into the process. Any residual sodium aluminate that does not dissolve during magnetic separation is also leached out at this stage.

A second leaching step is carried out using acid. Samples are leached in acid at 60°C for 1.5 hours with continuous stirring. Soiidrliquid ratio and acid concentration vary from 3:50 g/mL to 3:300 g/mL and 0.05M to 0.5M respectively. Firstly sulfuric acid is used and the effect of acid concentration and soiidrliquid ratio are studied. Secondly oxalic acid is used and the results are compared with sulfuric acid leaching. Acid leached residues are finally heated at 800°C for 1 hour in order to burn any remaining carbon. The Cr₂03-rich residues obtained are characterised by X-ray powder diffraction and X-ray fluorescence.

RESULTS

Leaching with H2SO4 The effect of acid concentration is studied by carrying out leaching experiments with four different concentrations of sulfuric acid (0.05M, 0.1M, 0.25M and 0.5M). Experiments are performed at 60°C during 1.5 hours with a solidrliquid ratio of 3:200 g/mL and the residues obtained are analysed by XRF (Table 5). It can be observed that the purity of Cr₂03 increases with increasing acid molarity.

Table 5

wt% Cr₂0₃ Fe₂0₃ Na₂0 MgO Al₂0₃ Si0₂ CaO

0.05M H₂S0₄ 62.30 3.31 5.39 7.16 8.40 4.32 3.43

0.1M H2SO4 74.20 3.72 4.16 1.58 6.04 0.77 2.18

0.25M H2SO4 71.30 3.70 6.59 1.65 5.15 0.48 2.64

0.5M H2SO4 79.30 1.22 3.75 0.95 5.17 0.41 1.19

Different solid:liquid ratios (g/mL) are tested during leaching experiments using sulfuric acid of concentration 0.5 M. Experiments are carried out at 60°C during 1.5 hours and the residues obtained are analysed by XRF. The chemical composition of the acid leached residues are presented in Table 6. It can be seen that the wt% of Cr₂0₃ in the residue increases with increasing solidrliquid ratio. An 83.6 wt% Cr₂0₃ residue is achieved using a solidrliquid ratio of 3:300 g/mL.

Table 6

wt% Cr₂0₃ Fe₂0₃ Na₂0 MgO Al₂0₃ Si0₂ CaO

3:50 g/mL 46.70 6.17 5.02 14.30 12.10 7.25 5.85

3:100 g/mL 64.20 7.85 2.68 2.70 10.50 3.74 3.20

3:150 g/mL 76.40 3.30 2.15 2.57 8.89 0.78 2.38

3:200 g/mL 79.30 1.22 3.75 0.95 5.17 0.41 1.19

3:300 g/mL 83.60 2.66 1.65 1.49 4.79 0.53 2.25

Leaching with oxalic acid

Oxalic acid is tested for acid leaching of water leached residues and the results obtained are compared with sulfuric acid leaching. The concentration and solidrliquid ratio used for experiments with sulfuric and oxalic acids are 0.5 M and 3:300 g/mL respectively. XRD patterns and composition of acid leached residues can be seen in Figure 11 and Table 7.

Table 7

wt. Cr₂0₃ Fe₂0₃ Na₂0 MgO Al₂0₃ S1O2 CaO

Non-mag

50.20 6.16 7.42 13.30 7.01 4.34 6.63 fraction

Water leached

68.30 5.30 1.83 9.68 6.04 3.64 3.08 residue

Sulfuric leached

83.60 2.66 1.65 1.49 4.79 0.53 2.25 residue

Oxalic leached

81.50 2.17 1.41 1.82 6.07 0.27 4.77 residue

It is observed that the purity of the Cr₂0₃-rich residue is very similar for sulfuric and oxalic acid (83.60 wt% and 81.50 wt% Cr₂0₃ respectively). This means that it is possible to use oxalic acid in place of sulphuric acid.

Claims

1. A process for recovering a primary metal residue from a metal-containing composition comprising:

(d) adding an aqueous medium to the roasted mass to form a substantially

insoluble product and a first solution of soluble metal oxides;

(g) hydrometal!urgically extracting from the roasted residue the primary metal residue and a third solution of soluble metal oxides.

2. A process as claimed in claim 1 wherein the bisulphate or bicarbonate of an alkali metal or alkaline earth metal is a bisulphate or bicarbonate of an alkali metal.

3. A process as claimed in claim 1 or 2 wherein the bisulphate or bicarbonate of an alkali metal or alkaline earth metal is NaHS0₄.

4. A process as claimed in any preceding claim further comprising: recovering one or more metal value-containing precipitates from the first solution of soluble metal oxides.

5. A process as claimed in any preceding claim further comprising: recovering one or more metal value-containing precipitates from the second solution of soluble metal oxides.

6. A process as claimed in any preceding claim further comprising: recovering one or more metal value-containing precipitates from the third solution of soluble metal oxides.

7. A process as claimed in any of claims 1 to 6 wherein the metal-containing composition is chromium-rich.

8. A process as claimed in claim 7 wherein the primary metal residue is Cr₂03.

9. A process as claimed in claim 7 or 8 further comprising: recovering one or more aluminium values from the first solution of soluble metal oxides.

10. A process as claimed in any of claims 8 or 9 further comprising: recovering one or more metal value-containing precipitates from the second solution of soluble metal oxides.

11. A process as claimed in any of claims 8 to 10 further comprising: recovering one or more metal value-containing precipitates from the third solution of soluble metal oxides.

12. A process as claimed in any preceding claim wherein step (b) is: reductively roasting the feedstock at a roasting temperature for a roasting period to produce a roast.

13. A process as claimed in claim 12 further comprising:

(d2) magnetically separating from the substantially insoluble product a magnetic fraction and a substantially non-magnetic fraction,

wherein step (e) is: acid leaching the substantially non-magnetic fraction to produce a leach residue and a second solution of soluble metal oxides.

14. A process as claimed in claim 12 or 13 further comprising:

(cl) grinding the roasted mass.

15. A process as claimed in any of claims 12 to 14 further comprising:

(d2a) water leaching the substantially non-magnetic fraction to produce an alkaline solution.

16. A process as claimed in any of claims 12 to 15 further comprising:

17. A process as claimed in claim 16 further comprising:

(d3c) hydrometallurgically extracting from the roasted slag leach residue a secondary metal residue and a fifth solution of soluble metal oxides.

18. A process as claimed in claim 17 further comprising: recovering one or more metal value-containing precipitates from the fifth solution of soluble metal oxides.