US20070122325A1

US20070122325A1 - Production of titania

Info

Publication number: US20070122325A1
Application number: US11/522,651
Authority: US
Inventors: Alan Stuart; Graham Reynolds; John Lawson
Original assignee: Individual
Current assignee: BHP Innovation Pty Ltd
Priority date: 2004-03-18
Filing date: 2006-09-18
Publication date: 2007-05-31
Also published as: EA010600B1; CN1961086A; ZA200607893B; TW200538397A; BRPI0508889A; EA200601725A1; EP1733063A4; WO2005090619A1; EP1733063A1

Abstract

A process for producing titania from a solid, iron-containing titaniferous material (such as ilmenite) is disclosed. The process includes a first step of treating iron-containing titaniferous material under conditions that reduce ferric ions to ferrous ions in the titaniferous material. Thereafter the process include steps of leaching treated titaniferous material and forming a leach liquor that includes an acidic solution of titanyl sulfate and iron sulfate and recovering titania from the leach liquor.

Description

The present application is a continuation-in part application of and claims priority to PCT/AU2005/000387 published in English on Sep. 29, 2005 as PCT WO 2005/090619 and from AU 2004901444 filed Mar. 18, 2004, the entire contents of each are incorporated herein by reference in their entirety.

BACKGROUND

The present invention relates to a process for producing titania from a titaniferous material.
The term “titaniferous” material is understood herein to mean any titanium-containing material, including by way of example, ores, ore concentrates, and titaniferous slags.
The present invention relates particularly to a process for producing titania from a solid titaniferous material that can be described in general terms as a sulfate process.
The term “sulfate” process is understood herein to mean a process for producing titania from a titaniferous material that comprises treating a solid feed material and at least substantially dissolving the feed material into solution and thereafter recovering titania from solution.
The applicant has been carrying out research and development work in relation to the sulfate process. The work has resulted in two options for the sulfate process.
One process option is described in US 2006/0177363 (International application PCT/AU03/01386 published as WO 2004/035843) and the other process option is described in US 2005/180903 (International application PCT/AU2004/001421 published as WO 2005/038060).
Both process options include the steps of:

- (a) leaching solid, iron-containing titaniferous material, such as ilmenite, with a leach solution containing sulfuric acid and forming a leach liquor that includes an acidic solution of titanyl sulfate (TiOSO₄) and iron sulfate (FeSO₄);
- (b) separating titanyl sulfate from the leach liquor, for example by solvent extraction as described in International application PCT/AU03/01386 or precipitation as described in International application PCT/AU2004/001421; and
- (c) recovering titania from the titanyl sulfate.

The research and development work included leaching ilmenite in 400-750 g/l sulfuric acid. The work included adding iron in the form of scrap iron during the leaching step (a). The main advantage of adding iron is to accelerate the rate of leaching. The disadvantages of adding iron include increased amounts of iron sulfate as a by-product and increased process complexity (such that an industrial plant would need equipment for handling scrap iron and for dealing with hydrogen gas that is evolved as a consequence of iron addition). In addition, scrap iron, although not expensive currently, introduces an additional operating cost to the process. In addition, the leaching rate is such, even with the addition of iron, that it may still be necessary to grind ilmenite prior to the leaching step to improve the rate.
The applicant has found in further research and development work that it is possible to achieve leaching rates that are comparable to leaching rates achieved with the addition of scrap iron in the leaching step (a) by treating ilmenite prior to the leaching step under conditions that reduce ferric ions to ferrous ions in the ilmenite. The conditions include contacting the titaniferous material with a reducing gas. The ilmenite treatment step makes it possible to avoid or at least minimize scrap iron addition during the leaching step.

BRIEF SUMMARY

The present invention provides a method for producing titania from a solid, iron-containing titaniferous material (such as ilmenite) that includes the steps of: treating iron-containing titaniferous material under conditions that reduce ferric ions to ferrous ions in the titaniferous material, the conditions including contacting the titaniferous material with a reducing gas; leaching treated titaniferous material and forming a leach liquor that includes an acidic solution of titanyl sulfate and iron sulfate; and recovering titania from the leach liquor.
Additional features and benefits of the present invention are described and will be apparent from the accompanying drawings and detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate embodiments of the invention according to the practical application of the principles described and shown.
FIG. 1 shows the results of the treatment of Stradbroke ilmenite with a reducing gas in a thermogravimetric analysis apparatus.
FIG. 2 shows the results of the treatment of Stradbroke ilmenite in a Hot Reduction Rig.
FIG. 3 is a plot of titanium extraction versus leach time where the ilmenite was reduced under different temperature conditions.
FIG. 4 is a plot of titanium extraction versus leach time where the ilmenite was reduced under different partial pressures of CO at the same temperature.

DESCRIPTION

The applicant has found in further research and development work that it is possible to achieve leaching rates that are comparable to leaching rates achieved with the addition of scrap iron in the leaching step (a) by treating ilmenite prior to the leaching step under conditions that reduce ferric ions to ferrous ions in the ilmenite. The conditions include contacting the titaniferous material with a reducing gas. The ilmenite treatment step makes it possible to avoid or at least minimize scrap iron addition during the leaching step.
Accordingly, the present invention provides a process for producing titania from a solid, iron-containing titaniferous material (such as ilmenite) which includes the steps of:

- (a) treating iron-containing titaniferous material under conditions that reduce ferric ions to ferrous ions in the titaniferous material, the conditions including contacting the titaniferous material with a reducing gas;
- (b) leaching treated titaniferous material and forming a leach liquor that includes an acidic solution of titanyl sulfate and iron sulfate;
- (c) recovering titania from the leach liquor.

Preferably, step (a) comprises treating titaniferous material under conditions that result in ferrous ions being a predominant form of iron in the titaniferous material.
The term “predominant” is understood to herein to mean that the amount of ferrous ions is a major part of the total amount of iron in the treated material. In terms of the actual amount of ferrous ions in the treated material, this may mean that the amount of ferrous ions is at least 40%, typically at least 50%, by weight of the total weight of iron in the treated material. In terms of the increase in ferrous ions from the amount of ferrous ions in the titaniferous material prior to treatment step (a), this may mean that there is an increase in ferrous ions by at least 50% of the amount of ferrous ions in the titaniferous material prior to treatment step (a).
Preferably, step (a) comprises treating titaniferous material under conditions that do not result in substantial formation of rutile that is unleachable under the conditions of leaching step (b).
Preferably, step (a) comprises treating titaniferous material under conditions so that there is either no metallic iron formed in the step or a selected relatively small amount of metallic iron formed in the step.
Step (a) may be carried out in any suitable treatment apparatus such as a fluidized bed or a kiln.
Preferably, step (a) comprises treating titaniferous material by contacting titaniferous material with the reducing gas in a fluidized bed.
The relevant conditions for step (a) include by way of example the selection of (i) the composition of the reducing gas, (ii) the temperature of the reducing gas, and (iii) the contact time of the reducing gas and the titaniferous material.
The reducing gas may be any suitable gas, such as hydrogen, carbon monoxide, and mixtures thereof.
Preferably, the reducing gas is a mixture of (a) hydrogen and/or carbon monoxide and/or methane and (b) another suitable gas such as an inert gas and/or carbon dioxide.
Preferably, the inert gas is nitrogen.
In a situation in which the reducing gas comprises hydrogen and the inert gas comprises nitrogen, preferably the hydrogen comprises up to 25%, more preferably 3-25%, by volume of the reducing gas.
In a situation in which the reducing gas comprises hydrogen, preferably the temperature of the reducing gas is less than 700° C. to avoid substantial formation of rutile that is unleachable under the conditions of leaching step (b).
In a situation in which the reducing gas comprises hydrogen, preferably the temperature of the reducing gas is 450-550° C.
In a situation in which the reducing gas comprises carbon monoxide and the other gas comprises carbon dioxide, preferably the carbon monoxide comprises up to 60%, more preferably 30-60%, by volume of the reducing gas.
In a situation in which the reducing gas comprises carbon monoxide and the inert gas comprises nitrogen, preferably the temperature of the reducing gas is less than 700° C., more preferably less than 650° C.
In a situation in which the reducing gas comprises carbon monoxide and the inert gas comprises nitrogen, it is preferred particularly that the temperature of the gas be 600° C.
Preferably, the average contact time of the material and the reducing gas is less than 120 minutes, more preferably 20-120 minutes.
Steps (b) and (c) may be as described in US 2006/0177363 (International application PCT/AU03/01386 published as WO 2004/035843) and the other process option is described in US 2005/180903 (International application PCT/AU2004/001421 published as WO 2005/038060), the entire contents of each are incorporated herein by reference.
Specifically, the leaching step (b) may comprise a multiple leaching step involving (i) a first step of leaching the titaniferous material with the leach liquor and forming a process solution that includes an acidic solution of titanyl sulfate; (ii) separating the process solution and a residual solid phase; (iii) leaching the residual solid phase in a subsequent leach step with a leach liquor and forming a further process solution that includes an acidic solution of titanyl sulfate and iron sulfate; (iv) separating the process solution and a residual solid phase; and (v) supplying the separated process solution to the first leach step and/or mixing the separated process solution with the process solution from the first leach step for subsequent processing in reducing step (c).
Preferably, the leaching step (b) includes selecting and/or controlling the leaching conditions to avoid undesirable amounts of premature hydrolysis and undesirable amounts of premature precipitation.
Typically, the acid concentration in the leaching step (b) should be at least 350 g/l sulfuric acid throughout the leaching step when operating at a leach temperature in the range of from 95° C. to the boiling point in order to avoid premature hydrolysis.
Furthermore, typically, the acid concentration at the end of the leaching step (b) should be less than 450 g/l when operating at a leach temperature in the range from 95° C. to the boiling point in order to avoid an undesirable amount of premature precipitation of titanyl sulfate.
The acid concentration at the start of the leaching step (b) may be higher than the above desirable concentrations, typically as high as 700 g/l.
Specifically, step (c) of recovering titania from the leach liquor may comprise the steps of (i) separating iron sulfate from the leach liquor, (ii) separating titanyl sulfate from the leach liquor either after or before step (i), and (iii) recovering titania from the titanyl sulfate.
More specifically, step (ii) of separating titanyl sulfate from the leach liquor may comprise solvent extraction of titanyl sulfate from the leach liquor, as described in US 2006/0177363 (International application PCT/AU03/01386 published as WO 2004/035843).
A further, although not the only other possible option for step (ii) of separating titanyl sulfate from the leach liquor comprises precipitating titanyl sulfate from the leach liquor, as described in US 2005/180903 (International application PCT/AU2004/001421 published as WO 2005/038060).
Steps (b) and (c) are not confined to the steps described in US 2006/0177363 (International application PCT/AU03/01386 published as WO 2004/035843) and the other process option is described in US 2005/180903 (International application PCT/AU2004/001421 published as WO 2005/038060).
For example, the leaching step (b) may be in accordance with the standard sulfate process, which includes a 2-stage step with a first stage involving solid state sulfation of pre-treated titaniferous material from step (a) with concentrated sulfuric acid and a second stage involving dissolving the sulfated product in water/dilute acid and forming an acidic solution of titanyl sulfate and iron sulfate.
The research and development work carried out by the applicant in relation to treatment of ilmenite has focused on the use of hydrogen gas and carbon monoxide.
The following description relates to the research and development work carried out at the Newcastle Technology Centre of the applicant.
Overview of Feed Materials and Experimental Parameters.
Based on promising results in the experiments on Beenup ilmenite, a second set of treatment experiments was carried out using ilmenite from two batches of Stradbroke ilmenite. The following description discusses these experiments.
A further set of experiments including reducing gases containing carbon monoxide was carried out on Cruzor (trade mark) ilmenite. The following description discusses these experiments.
Stradbroke Ilmenite Experiments.
The experiments on the first batch of Stradbroke ilmenite concentrated on the impact of three variables on treatment of ilmenite. The variables were:

- (a) reducing gas temperature—450° C. and 550° C.;
- (b) gas composition—10% H₂in N₂and 20% H₂in N₂; and
- (c) reduction time—45 minutes and 90 minutes.

The experiments on the second batch of Stradbroke ilmenite concentrated on confirming the results of treatment on the first batch of Stradbroke ilmenite and generating comparative data to provide a basis to assess the treatment.
Particle size of Feed Materials.

Table 1 sets out the particle size distribution of the first batch of Stradbroke ilmenite.

TABLE 1


Sizing data for 1^stBatch of Stradbroke Ilmenite.

	Size (mm)	Weight (g)	%	% passing

1	0	0.000	100.000
0.5	0	0.000	100.000
0.425	0.1	0.023	99.977
0.3	0.8	0.187	99.790
0.25	3.7	0.865	98.925
0.18	43.4	10.143	88.782
0.125	172.8	40.383	48.399
0.109	85.9	20.075	28.324
0.09	88.4	20.659	7.665
0.063	32.4	7.572	0.093
0.053	0.2	0.047	0.047
0.038	0.1	0.023	0.023
Total:	219.2	100

Table 2 sets out the particle size distribution of the second batch of Stradbroke ilmenite.

TABLE 2


Sizing data for 2^NDBatch of Stradbroke Ilmenite.

	Size (mm)	Weight (g)	%	% passing

1	0	0.000	100.000
0.5	0	0.000	100.000
0.425	0	0.000	100.00
0.355	0.2	0.044	99.956
0.3	0.7	0.156	99.800
0.25	4.5	1.001	98.799
0.18	43.9	9.763	89.073
0.125	166.6	37.047	51.990
0.106	79.9	17.678	34.312
0.09	105.3	23.416	10.896
0.063	47.3	10.518	0.378
0.053	1.2	0.267	0.111
0.038	0.4	0.089	0.022
−0.038	0.1	0.022	0.000
Total:	449.7	100

The first and second batches of the Stradbroke ilmenite had similar particle size distributions and were finer in particle size than the Beenup ilmenite. Specifically, whilst the top size was smaller, the bottom size was the same as that of Beenup ilmenite.
Thermogravimetric Analysis Apparatus Tests.
The impact of treatment of the first batch of Stradbroke ilmenite with a reducing gas in accordance with the invention was evaluated first in a thermogravimetric analysis apparatus (“TGA”).
The full size fraction of the first batch of the Stradbroke ilmenite as set out in Table 1 above was selected for use in the TGA experiments, as the finest fraction was large enough to resist terminal velocity at 12 l/min gas flow.
In addition, the use of the “as received” size fraction also had a benefit of minimizing further beneficiation through sizing or crushing at either a mine site or plant scale production.
Eight experiments were completed on 10 g samples of the first batch of the Stradbroke ilmenite. The experiments evaluated the impact of the three variables of gas composition, temperature, and time, as described above.
The experiments monitored the weight loss of the samples during the course of the experiments.
A matrix of the variables is given in Table 3.

TABLE 3

Matrix of variables for TGA tests.

Temperature Time Gas Composition

Low

450° C. 45 10% H₂in N₂

High 550° C. 90 20% H₂in N₂

A summary of experimental conditions and the weight loss during the treatment in the TGA tests is shown in Table 4.

TABLE 4


Result summary for TGA Tests.

Test	Temp	Gas	Time	Weight Loss %

STR-IR0-L1	n/a	n/a	n/a	n/a
STR-IR1-L1	450	10% H₂in N ₂	45	1.35
STR-IR2-L1	450	10% H₂in N ₂	90	1.46
STR-IR3-L1	550	10% H₂in N ₂	45	1.63
STR-IR4-L1	550	10% H₂in N ₂	90	1.74
STR-IR5-L1	450	20% H₂in N ₂	45	1.37
STR-IR6-L1	450	20% H₂in N ₂	90	1.45
STR-IR7-L1	550	20% H₂in N ₂	45	1.68
STR-IR8-L1	550	20% H₂in N ₂	90	2.04

The results of the TGA experiments indicate that the treatment step partially reduced the ilmenite and did not completely reduce the ilmenite, i.e. to the point of formation of 100% metallic iron.
An example of the results in this regard is shown in FIG. 1. The figure shows that there was substantial weight loss in the first 20 minutes of the experiment and that thereafter the weight loss was at a constant, relatively shallow, gradient to the time that the experiment was stopped. The constant gradient weight loss is consistent with what would be expected with reduction of ferric ions to ferrous ions. If reduction had been completed, i.e. with formation of 100% metallic iron, it would have been expected that the weight loss plot would have plateaued at a lower level.
The trend shown in FIG. 1 was repeated in the other tests.
Leaching Tests on TGA Treated Ilmenite.
The treated ilmenite was removed from the TGA and thereafter leached (on the same day) in 431 g/l sulfuric acid at or just below 100° C. The leach stage lasted 5 hours total. The solids and liquids were separated by filtration and the filtrate was tested for free acid and titanium.

Table 5 summarizes the extraction of titanium from the ilmenite and the acid consumed in the experiments.

TABLE 5


Extraction and Acid Consumption Results

Reduction Conditions

Leach Results

Test	Temp	Gas	Time	Weight Loss	Extraction	Acid Consumption

STR-IR0-L1	N/a	n/a	N/a	n/a	5.25	43.11
STR-IR1-L1	450	10% H₂in N ₂	45	1.35	6.44	41.97
STR-IR2-L1	450	10% H₂in N ₂	90	1.46	22.20	28.33
STR-IR3-L1	550	10% H₂in N ₂	45	1.63	30.84	22.62
STR-IR4-L1	550	10% H₂in N ₂	90	1.74	35.00	18.79
STR-IR5-L1	450	20% H₂in N ₂	45	1.37	27.21	18.07
STR-IR6-L1	450	20% H₂in N ₂	90	1.45	32.19	16.94
STR-IR8-L1	550	20% H₂in N ₂	90	2.04	36.33	20.42
STR-IR13-L1	550	20% H₂in N ₂	45	1.66	48.87	15.80

Extraction was determined as the mass of titanium present in the liquid phase after leaching, with respect to the mass available from the solid phase at the commencement of the experiment.
Sample STR-IR13-L1 is a repeat of sample STR-IR7-L1 (550° C., 20% H₂in N₂for 45 minutes) because the original result, i.e. sample STR-IR7-L1, was much lower than expected. The results of sample STR-IR7-L1 are not presented here. There were two faults noted in the original experiment—the sample when removed from the TGA was hotter than expected (possibly causing reoxidation) and secondly the level of the water bath dropped significantly during the leach due to a poor seal between the water bath and the lid.
Evaluation—TGA Tests.
The results of leaching ilmenite treated in accordance with the invention in the TGA tests showed that, with the variables tested,
(a) increasing time or hydrogen concentration with the ranges tested improved the leaching extraction of titanium from ilmenite,
(b) a combination of the two variables did not significantly further improve the extraction, and
(c) increasing temperature within the range tested improved the extraction in all cases.
Hot Reduction Rig Test and Subsequent Leaching Test of Treated Material.
The impact of treatment of the first batch of the Stradbroke ilmenite in accordance with the invention was also evaluated in a Hot Reduction Rig (HRR).
The HRR is a tube furnace that comprises a tube passing through the centre of a Kanthal split furnace. The tube furnace has three main sections. The upper section has a cyclone to decelerate and remove particulates from the gas phase. A dip leg returns these solids to the bed. The sample section has a perforated plate that allows the solids bed to be fluidised by the flow of reaction gas. The sample size is usually around 500 g mass. The lower section below the sample is filled with stainless steel turnings. This area is designed to act as a heat exchanger to heat the reaction gas quickly to the required reaction temperature.
The HRR differs from the TGA as the entire sample is evenly exposed to a steady stream of reaction gas. The fluidised bed avoids mass transfer effects seen in the latter stages of TGA tests. Consequently, comparisons between the TGA tests and HRR tests can be difficult, as the time required to reach a certain reduction point in the HRR should be less than the required in the TGA.
500 g of the first batch of the unground Stradbroke ilmenite was treated in accordance with the invention for 90 minutes under 20% H₂in N₂at 550° C. in the HRR. At the end of the 90 minute treatment period, the treated sample was cooled under nitrogen to 40° C., removed and thereafter leached in sulfuric acid for 5 hours on the same day in a two-litre reactor kettle. The acid concentration was maintained at 400 g/l. The kettle was maintained at a temperature of 110°0 C. The solids loading was 400 g/l at the start of the leach.
The rate of titanium and iron dissolution exceeded any other bench test result of the applicant to that date. The speed at which the initial reaction occurred caused the acid levels to drop below the target acidity. The final solution containing 49 g/l titanium exceeds almost all tests thus far for concentration of titanium in solution with respects to solids loading, time and acid concentration. Extraction of titanium from the solids was 34% (42% when adjusted for sampling).
Comparative Performance Tests.
Experiments were carried out on the second batch of Stradbroke ilmenite to confirm the above-described results of treatment on the first batch of Stradbroke ilmenite and to compare the results of treatment against alternative process options.
A 500 g sample of the second batch of the Stradbroke ilmenite was treated for 20 minutes under 20% H₂at 550° C. in the HRR. At the end of the 20 minute treatment period, the treated sample was cooled under nitrogen to 40° C., removed and thereafter a 50 g sample was leached in sulfuric acid for 5 hours in a two-litre reaction kettle. The kettle was maintained at a temperature of 110° C. The acid concentration was maintained at 450 g/l. The solids loading at the start of the leach was 50 g/l.
A 50 g sample of the second batch of the Stradbroke ilmenite in the as-received form and a 50 g sample of the second batch of the Stradbroke ilmenite in a ground form were also leached in two-litre reaction kettles under the same conditions described in the preceding paragraph. In both cases, iron rods were added to the kettles.
Samples of the leach liquor were taken periodically during the 5 hour experiments and were analysed to determine the extraction of titania from the ilmenite into the liquor. The results are presented in FIG. 2, with the extraction expressed as g/l titanium metal in the liquor.
It is evident from FIG. 2 that the extraction of titania from the second batch of Stradbroke ilmenite that was treated in accordance with the invention and subsequently leached was:
(a) comparable to that achieved with ground ilmenite; and
(b) significantly better in terms of ultimate recovery and extraction rate than the unground ilmenite.
Cruzor Ilmenite Experiments.
500 g samples of Cruzor ilmenite were fluidised with N₂in a fluid bed reactor. The reactor was heated to a target temperature and the N₂atmosphere was replaced by reducing gas (combinations of CO, CO₂, H₂and CH₄). After a specified time the reducing gas was replaced with N₂and the reactor was allowed to cool.
The product from the reactor was leached immediately in 450 g/l sulfuric acid for 48 hours at 110° C. Samples were taken at specific intervals for analysis to determine leaching rate and extraction.
The results of experiments on Cruzor ilmenite with reducing gas selected to be CO are presented in FIGS. 3 and 4.
FIG. 3 is a plot of titanium extraction (%) versus leach time (hrs) for experiments on 4 samples that were reduced under different temperature conditions. The purpose of the Figure is to illustrate the impact of reduction temperature on titanium extraction.
The samples shown in FIG. 3 were reduced under the following conditions.

Reducing gas Temperature (° C.)

pp CO = 0.45 700

pp CO = 0.45 600

pp CO = 0.45 500

Blank - N₂only 600
FIG. 3 shows that optimum titanium extraction on leaching was obtained with the sample that was reduced at 600° C.
Specifically, the Figure shows that the titanium extraction on leaching the samples reduced at 500° C. and 700° C. was significantly lower than that of the 600° C. reduction sample.
FIG. 4 is a plot of titanium extraction (%) versus leach time (hrs) for experiments on 4 samples reduced under different partial pressures of CO in the reducing gas. The purpose of the Figure is to illustrate the impact of the partial pressure of CO on titanium extraction.
The samples shown in FIG. 4 were reduced under the following conditions.

Reducing gas Temperature (° C.)

pp CO = 0.6 600

pp CO = 0.45 600

pp CO = 0.3 600

Blank - N₂only 600
FIG. 4 shows that titanium extraction increased with partial pressure of CO in the reducing gas with the sample reduced with a CO partial pressure of 0.6 in the reducing gas achieved the highest titanium extraction.
The results of the above-described experiments indicate that treatment of ilmenite in accordance with the invention is a comparable alternative option for accelerating the leaching rate and achieving high recoveries of titania with low acid consumption than the current option of grinding and thereafter leaching ilmenite with scrap iron.
Many modifications may be made to the present invention described above

- without departing from the spirit and scope of the invention. While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.

Claims

1. A process for producing titania from a solid, iron-containing titaniferous material (such as ilmenite) which includes the steps of:

(a) treating iron-containing titaniferous material under conditions that reduce ferric ions to ferrous ions in the titaniferous material, the conditions including contacting the titaniferous material with a reducing gas;

(b) leaching treated titaniferous material and forming a leach liquor that includes an acidic solution of titanyl sulfate and iron sulfate;

(c) recovering titania from the leach liquor.

2. The process defined in claim 1 wherein step (a) comprises treating titaniferous material under conditions that result in ferrous ions being a predominant form of iron in the titaniferous material.

3. The process defined in claim 1 wherein step (a) comprises treating titaniferous material under conditions that do not result in substantial formation of rutile that is unleachable under the conditions of leaching step (b).

4. The process defined in claim 1 wherein step (a) comprises treating titaniferous material under conditions so that there is either no metallic iron formed in the step or a selected relatively small amount of metallic iron formed in the step.

5. The process defined in claim 1 wherein step (a) comprises treating titaniferous material by contacting titaniferous material with the reducing gas in a fluidized bed.

6. The process defined in claim 1 wherein the reducing gas comprises any suitable gas, such as hydrogen, carbon monoxide, and mixtures thereof.

7. The process defined in claim 1 wherein the reducing gas is a mixture of (a) hydrogen and/or carbon monoxide and/or methane and (b) another suitable gas such as an inert gas and/or carbon dioxide.

8. The process defined in claim 7 wherein the inert gas is nitrogen.

9. The process defined in claim 7 wherein, in a situation in which the reducing gas comprises hydrogen and the inert gas comprises nitrogen, the hydrogen comprises up to 25% by volume of the reducing gas.

10. The process defined in claim 1 wherein, in a situation in which the reducing gas comprises hydrogen, the temperature of the reducing gas is less than 700° C. to avoid substantial formation of rutile that is unleachable under the conditions of leaching step (b).

11. The process defined in claim 1 wherein, in a situation in which the reducing gas comprises hydrogen, the temperature of the reducing gas is 450-550° C.

12. The process defined in claim 7 wherein, in a situation in which the reducing gas comprises carbon monoxide and the other gas comprises carbon dioxide, the carbon monoxide comprises up to 60% by volume of the reducing gas.

13. The process defined in claim 7 wherein, in a situation in which the reducing gas comprises carbon monoxide and the inert gas comprises nitrogen, the temperature of the reducing gas is less than 700° C.

14. The process defined in claim 7 wherein, in a situation in which the reducing gas comprises carbon monoxide and the inert gas comprises nitrogen, the temperature of the gas is 600° C.

15. The process defined in claim 1 wherein the average contact time of the titaniferous material and the reducing gas in step (a) is less than 120 minutes.

16. The process defined in claim 1 wherein the leaching step (b) comprises a multiple leaching step involving (i) a first step of leaching the titaniferous material with the leach liquor and forming a process solution that includes an acidic solution of titanyl sulfate; (ii) separating the process solution and a residual solid phase; (iii) leaching the residual solid phase in a subsequent leach step with a leach liquor and forming a further process solution that includes an acidic solution of titanyl sulfate and iron sulfate; (iv) separating the process solution and a residual solid phase; and (v) supplying the separated process solution to the first leach step and/or mixing the separated process solution with the process solution from the first leach step for subsequent processing in reducing step (c).

17. The process defined in claim 1 wherein the leaching step (b) includes selecting and/or controlling the leaching conditions to avoid undesirable amounts of premature hydrolysis and undesirable amounts of premature precipitation.

18. The process defined in claim 1 wherein the acid concentration in the leaching step (b) is at least 350 g/l sulfuric acid throughout the leaching step when operating at a leach temperature in the range of from 95° C. to the boiling point in order to avoid premature hydrolysis.

19. The process defined in claim 1 wherein the acid concentration at the end of the leaching step (b) is less than 450 g/l when operating at a leach temperature in the range from 95° C. to the boiling point in order to avoid an undesirable amount of premature precipitation of titanyl sulfate.

20. The process defined in claim 1 wherein the acid concentration at the start of the leaching step (b) is less than 700 g/l.

21. The process defined in claim 1 wherein step (c) of recovering titania from the leach liquor comprises the steps of (i) separating iron sulfate from the leach liquor, (ii) separating titanyl sulfate from the leach liquor either after or before step (i), and (iii) recovering titania from the titanyl sulfate.

22. The process defined in claim 21 wherein step (ii) of separating titanyl sulfate from the leach liquor comprises solvent extraction of titanyl sulfate from the leach liquor.

23. The process defined in claim 21 wherein step (ii) of separating titanyl sulfate from the leach liquor comprises precipitating titanyl sulfate from the leach liquor.

24. The process defined in claim 1 wherein the leaching step (b) is in accordance with a standard sulfate process which includes a 2-stage step with a first stage involving solid state sulfation of pre-treated titaniferous material from step (a) with concentrated sulfuric acid and a second stage involving dissolving the sulfated product in water/dilute acid and forming an acidic solution of titanyl sulfate and iron sulfate.