OA18668A

OA18668A - Improved ilmenite smelting process.

Info

Publication number: OA18668A
Application number: OA1201800198
Authority: OA
Inventors: Kabwika Bisaka; Xolisa Goso; Itumeleng Thobadi
Original assignee: Mintek
Priority date: 2015-11-18
Filing date: 2016-11-18
Publication date: 2019-04-08

Abstract

A method of preparing a pre-reduced ilmenite ore for smelting, wherein métal oxides contained in the ore are selectively reduced in solid-state reactions over titanium oxide, the method including the step of prereducing carbon-based pellets of the ore.

Description

Mintek

IMPROVED ILMENITE SMELTING PROCESS

BACKGROUND OF THE INVENTION . .

This invention relates to a Consolidated process for the préparation of carbon-based ilmenite pellets, the soiid-state.....réduction thereof, and the subséquent smelting thereof in an eiectric furnace.

I i t I

The smelting of ilmenite consumes substantial quantifies of electrical energy. Additïonally, the operability of the furnace can be hindered due to frothing effects.

Feed to the smelting furnace is generaily made up of raw ilmenite ore and a solid, carbonaceous reductant. Raw ilmenite, in a particular process, is replaced by pre10 reduced ilmenite pellets. The*steps for the production process of the latter are td préparé ilmenite pellets using bentonite and to pre-reduce the pëilets in a rotary kiln in the presence of a solid, carbonaceous reductant in excess. The smelting of the pre-reduced ilmenite pellets is thus carried out in an AC furnace. The TiO₂ slag produced in this way is, however, contaminated .with the bentonite which,is an 15 inorganic binder.

An object of the présent invention is to provide an alternative process for prereducing an ore essentially targeting the métallisation of iron oxides contàined in the ore.

SUMMARY OF THE INVENTION

The invention provides a method of preparing a pre-reduced ilmenite ore for smelting, wherein métal oxides, such as iron, chromium and manganèse oxides contàined in the ore are selectively reduced in solid-state reactions over titanium oxide, the method including the step pf pre-reducing carbon-based pellets of the ore.

The métal oxides, other than titanium oxides, in the pellets may be pre-reduced to a 25 maximum extent i.e. essentially fully or they may be partially pre-reduced.

The pellets may be less than 6mm in size and preferably lie in the range of 2mm to 5mm.

The pellets may be prepared from a blend of required proportions of the ore, coal fines of -106 microns and a suitable organic binder.

The ratio of the coal to the metallic oxide content may be practically determined. For example a stoichiometric ratio for the full réduction of iron in the ore can be used.

The organic binder content may lie in the range of 0 to 1%. This content may be dictated by the physical properties of the resulting pellets principally the strength of the pellets in a green state and in an air-dried or indurated State. The pellets may be may be air-indurated for at least 4 days. This period is usually adéquate to ensure that the pellets are sufficiently strong to allow their safe and efficient handling to subséquent pre-reduction reactors. The mechanical strength of the pellets is preferably above 600N. The pellets should also hâve an acceptable behaviour in a hot reactor environment to avoid decrepitation due to excessive swelling.

A single binder or a mixture of binders may be used. The invention is not limited in this respect.

Pre-reduced pellets are evaluated based on the réduction extent of iron oxides contained in the ore. Preferably the iron oxide should be présent in a quantity of less than 10% from the initial content. However a consistent pre-reduction yield should be a main target during a normal and stable operation.

The pellets may be subjected to a thermal réduction process or to a hybrid, solidstate, réduction process.

The pellets, air-dried and indurated, may be heated in a fixed bed reactor at an optimal résidence time which may lie in a range of from 0.5 to 4 hours.

If a thermal pre-reduction step is adopted then the pellets may be heated at a température in the range of 1100 to 1200°C.

If the hybrid, solid-state, pre-reduction step is adopted then the pellets may be heated to a température in the range of 900 to 1000°C in a controlled atmosphère of a reducing gas.

The reducing gas may comprise one or more of the following: CO, syngas (CO + H₂), natural gas and hydrogen.

If a fixed bed reactor is employed then the reducing gas may be filtered through a hot burden in the reactor. The reducing gas flowrate should be selected to achieve an adéquate réduction yiéld of the iron oxides in the ore, as well as acceptable reactor operation performance.

The invention finds particular application in the préparation of pre-reduced, carbonbased, ilmenite micro-pellets which are to be smelted e.g. in a DC open arc furnace. However, the principes of the invention may be employed for the pre-reduction of pellets of titanrferous magnetite, ferrochrome and ferromanganese ores for the subséquent production of titania slag, chrome and manganèse, alloys respectively.

Référencé has been made to heating the air-dried pellets in a fixed bed reactor. This is exemplary only and non-limiting. A moving bed and a rotary kiln may be employed in place of the fixed bed reactor, in a pre-reduction stage. It is important that abrasion of the pellets is minimised and it should be possible to separate pre-reduced fines from other material, for example through the use of magnetic or équivalent techniques.

BRIEF DESCRIPTION OF THE DRAWING

The invention is further described by way of example with référencé to the accompanying figures wherein;

Figure 1 illustrâtes in flow chart form the pre-reduction of carbon-based, ilmenite micro-pellets and the subséquent smelting thereof;

Figure 2 is a diagram depicting an impact of the résidence time on pre-reduction and métallisation degrees at 1000°C and 0.5I CO / min; and

Figure 3 is a diagram depicting an impact of the CO flowrate on the pre-reduction and métallisation degrees at 1000°C and 1h résidence time.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention is hereinafter described with reference to the pre-reduction of carbonbased, ilmenite, micro-pellets. Although this is a preferred application of the principles of the invention it is possible to adapt the principles described herein for the pre-reduction of titaniferous magnetite, ferrochrome and ferromanganese ores.

Raw ilmenite ore 10 of a suitable size is fed to a blender 12. The blender also receives coal fines 14 of -106 micron in size and an organic binder 16 formed from a single binder or from a mixed binder composition.

The ratio of the input coal to the ilmenite is determined taking into account practical considérations. For instance a stoichiometric ratio which achieves a full réduction of iron in the ilmenite ore can be used. Further, the input of organic binder or mixes of organic binders, in the range of up to 1%, is dictated by the physical properties of the resulting pellets, particularly the green and air-dried strengths of the pellets. The resulting pellets should also hâve an acceptable behaviour (subsequently) in a hot reactor environment to avoid decrepitation due to excessive swelling.

The blender 12 produces carbon-based, ilmenite, micro-pellets of 2mm to 5mm in size. These pellets are then air-dried (step 20).

The air-dried, indurated pellets are then subjected to a thermal pre-reduction step 22, or to a hybrid, solid-state pre-reduction step 24. In each instance the air-dried indurated pellets are heated in a fixed bed reactor 26 for an optimal résidence time, generally from 0.5 to 4 hours.

If use is made of the thermal pre-reduction process the pellets are heated in the reactor 26 to a température in the range of 1100 to 1200°C. If use is made of the hybrid approach then the pellets are heated in the reactor 26 to a température of 900 to 1000°C in a controlled atmosphère of a reducing gas 30 which comprises one or more of CO, syngas, natural gas and hydrogen. The reducing gas is filtered through the hot burden of the pellets in the reactor 26. The reducing gas flowrate is regulated to achieve an adéquate pre-reduction yield. The flowrate should also be regulated to optimise the reactor operation, principally the thermal efficiency and the production cost.

Process parameters of importance, in respect of the of pre-reduction technique used, include: the ilmenite grain size distribution, the composition of the pellets, the sizes of the pellets, the operating température, the résidence time and the reducing gas flowrate.

Taken under consistent operating conditions each method is able to produce a consistent pre-reduction yield. The hybrid method, despite operating at a lower température then the thermal réduction method, appears to offer a higher prereduction yield than the thermal method.

The fully or partially pre-reduced ilmenite pellets 32, emerging from the reactor 26, can be fed, cold or hot, to a conventional ilmenite smelting process 34.

Without being bound by the following explanation it is believed that the organic binder provides a more intimate contact between the ilmenite and the coal fines. The small pellet size feature, in a highly reducing atmosphère, assists the transfer of heat and mass in the diffusion of gaseous reductants, such as CO and H₂, to the reaction sites. The organic binder 16 burns off at the process température, a feature which induces localised réduction and promûtes the formation of cracks and pores in the ilmenite ore grains contained in the pellets 32. The spécifie surface areas of the ilmenite pellets are therefore increased and the diffusion rate of the gas reductant to the reaction sites is enhanced. This in turn impacts on the pre-reduction yield. The réduction process can be smoothly and efficiently operated despite the minor sintering of the pellets that may occur at elevated températures.

The fully or partially pre-reduced, carbon-based ilmenite pellets which are fed, either hot or cold, into a DC open arc fumace 34 decrease the consumption of electricity in the furnace, help to address slag foaming and resuit in an improved grade of TiO₂slag 36 output by the furnace 34.

Through tests it has been established that iron oxide in the pellets was nearly completely reduced through the use of the hybrid pre-reduction process carried out at a température of 1000°C and for a résidence time of 2 hours. The pre-reduction yield was increased as température, résidence time and reducing gas flowrate were increased.

The use of the thermal pre-reduction process at a température of 1100 to 1200°C produced a pre-reduction yield of about 85% - a value which is adversely affected with an increase in ilmenite ore grain size and with an increase in the size of the coal fines.

About 4 tons of cold pre-reduced ilmenite pellets were smelted in a DC open arc furnace. The energy consumption of the furnace lay in the range of 0.6 to 0.7 kWh / kg of pre-reduced ilmenite pellets — a figure which représente an electrical energy saving of 30 to 40% compared to a conventional ilmenite smelting process. The smelting process was stable with no visible sign of foaming. The product 36 contained about 95% TiO₂ and about 3% FeO.

A higher grade TiO₂ slag (above 90%) can thus be achieved, using conventional ilmenite feedstock in smelting operations, with no foaming occurring. Using the method of the current invention, a lower grade ilmenite could be used as feedstock to produce TiO₂ slag of at least 85% TiO₂ content.

The invention has been described with référencé to the use of a gaseous reductant. It is possible though to make use of a solid reductant such as anthracite or coal, instead of the reducing gas 30. Also the reactor 26 which, typically, is a fixed bed reactor can be replaced by a moving bed or by a rotary kiln configuration provided abrasion effects between the pellets are minimised. It should be possible though to separate the pre-reduced pellets, for example using magnetic techniques, from the other material emerging from the reactor.

Additional carbonaceous soiid reductant can be used in excess to reduce residual iron in the slag to below 6% without inducing slag foaming.

DESCRIPTION OF A PILOT TEST OF THE PROCESS OF THE INVENTION

A 200 kW DC arc furnace facility was used for demonstrating the smelting of prereduced ilmenite pellets. The furnace had a 1 m outer diameter, water-spray cooled Steel shell lined with a single layer and three rows of magnesite-chrome bricks and a hearth lined with rammable magnesia. The refractory lining resulted in the furnace crucible internai diameter (ID) of 0.656 m. The furnace was equipped with an alumina lined conical roof and a shell bolted on a domed base. A single taphole was used to tap a stream of both molten slag and métal. The furnace was equipped with a single and centrally-located graphite electrode of 40 mm diameter operating as a cathode while the anode comprised steel pins buried in the hearth. The feed System comprised individual hoppers used to feed anthracite and pre-reduced ilmenite pellets through a furnace feed pipe. The furnace was equipped with an off-gas System for the cleaning of produced process gas prior to release thereof into the atmosphère.

Carbon-based ilmenite pellets containing the as received ilmenite, stoichiometric amount of anthracite, were prepared using a proprietary organic binder at a required dosage. The as-received ilmenite had a particle size distribution of D_wo in the 38 pm to 150 pm size range. The anthracite was milled to a D₈₅ passing 106 pm to facilitate its incorporation into an ilmenite pellet recipe. Pellets were prepared in a pilot-scale pelletizing unit comprising an inclined rotating pan of 985 mm diameter and 170 mm depth. The mechanical strengths of the pellets were measured and found to vary with the type and dosage of binder used, within a range of 0.01 - 0.03 MPa for green pellets and 0.81 -1.50 MPa for indurated pellets at ambient conditions.

Batches of 250 kg each of indurated pellets were reduced in an electrically heated muffle furnace operated at a controlled température of 1100 °C. During a three hour fi ring time, in total 5 kg of CO was blown intermittently through the reactor burden at intervals of 10 minutes. The pellets were loaded in a single tray of 1700 mm x 900 mm, having a loading area of a mesh screen acting as a distribution plénum for the reducing gas.

Both the raw and pre-reduced ilmenite materials at various conditions were chemically analysed; specifically an analysis of the iron oxidation States (Fe³⁺, Fe²⁺, and Fe°) was used to calculate the pre-reduction and métallisation yields for the pellets. Negligible réduction of titanium oxides was assumed throughout and the prereduction yield was therefore calculated based on the mass balance of oxygen associated with each gram of iron before and after pre-reduction. Equations [1] and [2] were used for the calculation of the prereduction and metallization yields, respectively.

Pre-réduction yield = lOOx_______^^{gen rem0Ved by the} P™ ~ Action process) (Oxygen associated -with each gram of iron in the ilmenite samplë)

Fe^ü

Métallisation yield = lOOx...... .............................................................[21

The Chemical analyses of the ore and anthracite are summarised in Table 1 and Table 2, respectively.

Table 1: Bulk Chemical composition ofthe raw ilmenite (mass %)

MgO	ai₂o₃	SIO₂	CaO	TiO₂	V₂Og	Cr₂O₃	MnO	FeO	Fe²⁺	Fe^ü	Fe/Ti
0.28	0.44	0.57	0.05	46.6	0.25	0.09	1.08	47.87	13.50	<0.05	1.33

<0.05%: the analyte concentration could not be accurately quantifiée! as it is below the limit of détection (LOD)

Total Fe in the sample is expressed as % FeO

Table 2: Summary of the bulk Chemical composition ofthe anthracite (mass %)

Moisture	Ash	Volatile	Fixed carbon	Total carbon	Total sulphur
3.57	4.74	6.19	89.1	90.7	0.56

In total, about 3.6 tons of pre-reduced pellets were produced. The pellets were bagged in 1 m³ bags from which five composite samples were collected. The Chemical analyses of the 5 composite samples are given in Table 3.

Table 3: Chemical compositions of the pre-reduced pellets

	MgO	AI₂O₃	SiO₂	CaO	TiO₂	V₂O₅	Cr₂O₃	MnO	Total Fe	Fe°	Fe²⁺	C	Ti³⁺	Fe/Ti ratio
TP Bag 1	0.53	0.33	0.31	0.10	44.4	0.36	0.07	1.05	34.92	25.55	9.37	7.23	6.55	1.32
TP Bag 2	0.55	0.30	0.26	0.07	44.5	0.36	0.08	1.06	35.23	25.44	9.79	7.72	6.76	1.32
TP Bag 3	0.50	0.30	0.24	0.15	43.5	0.33	0.07	1.03	33.60	21.90	11.7	7.97	5.45	1.29
TP Bag 4	0.48	0.32	0.39	0.11	42.4	0.33	0.07	1.05	34.46	22.66	11.8	8.01	4.78	1.35
TP Bag 5	0.36	0.45	0.64	0.18	41.9	0.33	0.07	1.08	36.01	24.21	11.8	7.20	4.66	1.43
Average	0.48	0.34	0.37	0.12	43.3	0.34	0.07	1.05	34.84	23.95	10.89	7.63	5.64	1.32
St dev	0.07	0.06	0.16	0.04	1.17	0.02	0.004	0.02	0.90	1.64	1.21	0.39	0.98	0.02

The calculated degrees of prereduction and metallization for the five composite samples are presented in Table 4

Table 4: Pre-reduction and métallisation degrees ofilmenite pellets

Composite sample no.	Pre-reduction degree, %	Métallisation degree, %
1	79.7	73.2
2	78.9	72.2
3	73.6	65.2
4	74.1	65.8
5	75.1	67.2
Average	76.3	68.7
St dev	2.8	3.7

Tables 3 and 4 show that pellets prereduced to a consistent extent were produced as a resuit of the uniform furnace operating conditions.

Results from laboratory tests in a tube reactor of 80 mm diameter showed a very important feature of this process that is presented in Figures 2 and 3. Tests conducted at a température of 1000°C, showed that pre-reduction and métallisation degrees are both related to the résidence time and CO flowrate. Increasing the CO flowrate appears to positively affect the yields, suggesting that CO diffusion would 10 play a significant rôle in this process.

Continuous smelting of partially reduced ilmenite pellets (approx.. 70% prereduction yield) was carried out to demonstrate stable furnace operation as well as production of a consistent slag quality, in particular, a slag TiO₂ grade above 85%. The test work also had the objective of confirming the process spécifie energy requirement. The 5 slag results are presented in Table 5.

Table 5: Analysis of slag from the stable smelting operation, in mass %

Tap	*TiO₂	Cr₂O₃	MnO	FeO	V₂O₅	SiO₂	CaO	ai₂0₃	MgO
A	84.74	1,34	1,37	2,06	0,30	1,31	0,34	1,96	6,58
B	87.84	0,60	1,18	2,59	0,21	1,16	0,28	2,49	3,65
C	83.15	0,31	1,68	2,73	0,29	1,20	0,38	2,76	7,50
D	88.86	0,45	0,68	4,89	0,17	0,49	0,31	1,45	2,70
b	88.19	0,45	0,99	4,37	0,16	0,43	0,35	1,53	3,53
F	91.27	0,31	1,47	1,58	0,15	0,28	0,32	1,47	3,15
G	94.28	0,08	1,32	1,25	0,10	0,21	0,17	0,98	1,61
H	93.32	0,09	1,21	1,48	0,10	0,16	0,14	1,10	2,40

*by différence

Table 6: Evolution of composition ofpig iron from the stable smelting operation, in mass %

Tap	Fe	Ti	V	Si	Cr	Mn	C	P	S
A	95.54	0.67	0.24	0.34	1.04	0.48	1.66	0.02	0.01
B	95.92	0.43	0.23	0.36	1.08	0.44	1.53	0.00	0.01
C	95.15	0.55	0.16	0.38	1.47	0.33	1.92	0.02	0.02
D	97.03	0.16	0.09	0.18	0.52	0.21	1.76	0.02	0.03
E	94.59	0.29	0.20	0.62	1.14	0.51	2.57	0.03	0.05
F	91.81	3.64	0.23	0.64	1.17	0.86	1.62	0.01	0.02
G	95.83	0.57	0.20	0.41	0.71	0.60	1.64	0.02	0.02
H	93.66	1.34	0.32	1.28	0.89	0.88	1.56	0.02	0.05

Slag FeO contents as low as 1.3 % were achieved without visible signs of slag foaming. This condition was maintained for a longer period during which stable furnace operation was demonstrated and slags of consistent FeO content were produced. Results for this particular test work suggest that smelting of partially reduced ilmenite and operating the furnace with lower FeO content in the slag are technically possible.

The 200 kW DC open-arc furnace was operated at a power level in the range of 115 - 140 kW and at a corresponding voltage of 100 - 115 V. Consistent furnace heat losses in the range of 60 — 90 kW were measured. Average tapping températures measured using an optical pyrometer were scattered within a range between 1670 and 1780°C. The spécifie energy requirement (SER) for the smelting of prereduced carbon-based pellets was measured between 0.6 and 0.7 kWh / kg prereduced ilmenite. A 30-40 % réduction in furnace electricity required relative to a conventional smelting process can be achieved assuming that a prereduction yield of at least 70 % can be achieved. Arc resistivities were measured for various conditions investigated in order to predict the furnace arc stability. Arc resistivity was found to be in the range of 0.0168 and 0.0240 Q.cm which range is close to 0.0175 O.cm, a typical value for arc resistivity in smelting processes with CO-rich atmosphères (in the absence of foaming).

Claims

1. A method of preparing a pre-reduced ilmenite ore for smelting wherein métal oxides, contained in the ore are selectively reduced in solid-state reactions over titanium oxide, the method including the step of pre-reducing carbonbased pellets of the ore, wherein the pellets are less than 6mm in size and are prepared from a blend of the ore, coal fines and an organic binder.

2. A method according to claim 1 wherein the métal oxides, other than titanium oxides, in the pellets are pre-reduced to a maximum extent.

3 A method according to claim 1 wherein the pellets are between 2mm and 5mm in size.

4 A method according to claim 1 wherein the coal fines hâve a size of -106 microns.

5 A method according to any one of daims 1 to 4 wherein the ratio of the coal to the metallic oxide content is determined using a stoichiometric ratio for the full réduction of Iran in the ore.

6 A method according to any one of daims 1 to 5 wherein the organic binder content is in the range of 0 to 1 %.

7. A method according to any one of daims 1 to 6 wherein a single organic binder or a mixture of organic binders is used.

8 A method according to any one of daims 1 to 7 wherein the pellets are air indurated for at least 4 days.

9 A method according to claim 8 wherein a mechanical strength of the pellets is of the order of 600N.

10 A method according to any one of daims 1 to 9 wherein iron oxide in the ore is présent in a quantity of less than 10%.

11 A method according to any one of daims 1 to 10 wherein the pellets are subjected to a thermal réduction process or to a hybrid, solid-state, réduction process.

12 A method according to claim 11 wherein the pellets are heated in a fixed bed reactor for a time period of between 0.5 to 4 hours.

13 A method according to daim 12 wherein a thermal pre-reduction step is adopted and the pellets are heated to a température in the range of 1100 to 1200°C.

14 A method according to claim 11 or 12 wherein a hybrid, solid-state, prereduction step is adopted and the pellets are heated to a température in the range of 900 toi 000’C.

15 A method according to claim 14 wherein a reducing gas is filtered through a hot burden in the fixed bed reactor.

16 A method according to claim 15 wherein the reducing gas comprises one or more of the following: carbon monoxide; syngas (CO+H2); natural gas; and hydrogen.

17. A method according to any one of daims 1 to 16 wherein the smelting is conducted in a DC open arc furnace at a température of between 1650 and 1750°C.