Reduction of ferrotitaniferous materials

A reduction process for the metallization of ferrotitaniferous materials comprising a partial metallization at a low temperature with hydrogen followed by a high temperature reduction with a carbonaceous reductant.

TECHNICAL FIELD 
The present invention relates to the metallization of the iron content of 
ferrotitaniferous materials, particularly naturally occurring ilmenites. 
It is desirable to remove from ferrotitaniferous materials their iron 
content in order to make them more suitable for conversion to titanium 
containing products, chiefly titanium dioxide pigments. 
BACKGROUND ART 
There are several known methods of removing the iron content from 
ferrotitaniferous material. Most of these involve a reduction step to 
convert the iron content to a state whereby it may be conveniently 
separated from the relict titanium oxide material. For example in 
Australian Patent Specification No. 247 110 there is disclosed a multistep 
process for the upgrading of ilmenite wherein the ilmenite is reduced so 
that its iron content is metallized; the thus formed metallic iron is 
leached away from the titanium oxide material by a `rusting` process 
using an aqueous reagent. In another process described in U.S. Pat. No. 
3,252,787 ilmenite is metallized and the iron content then removed by 
treatment with ferric chloride solution. In yet another process described 
in Australian Patent Specification No. 515 061 metallized ilmenite 
particles are subjected to a segregation treatment whereby small iron 
metal particles dispersed throughout each particle of metallized ilmenite 
are translocated and agglomerated to form large particles outside the 
relict particles. 
For such processes which require removal of iron by means of reactions with 
liquids or gases it is desirable to have the metallic iron particles in a 
very fine state, homogeneously dispersed and accessible to the reactants 
through pores from the outside of the particles of metallized 
ferrotitaniferous material. 
Studies on the reduction of ilmenite reported by G. Ostberg (Jernkont Ann 
1960 144) (1) p. 46-76) showed that iron precipitated in grains of 
ilmenite by reduction occurred as discrete particles, the number of which 
per given volume was greater when hydrogen was used as the reductant than 
when carbon monoxide or solid carbon was used. The size of the particles 
was inversely proportional to the number. The number of particles could be 
increased by lowering the temperature used. These observations would 
indicate that to obtain a homogeneous dispersion of fine particles of 
metallic iron, hydrogen reduction should be used at the lowest practicable 
temperature. Unfortunately because of unfavourable reaction equilibria 
hydrogen reduction is considerably more costly to carry out than reduction 
with solid carbon. 
DISCLOSURE OF INVENTION 
However we have now discovered a process for the metallization of the iron 
content of ferrotitaniferous materials by which the metallic iron is 
formed as fine particles homogeneously dispersed within the grains of the 
material. This process involves partial metallization of the 
ferrotitaniferous material with hydrogen at a low temperature, followed by 
completion of the metallization process using a solid carbon reductant at 
a higher temperature. 
Accordingly the present invention provides a process for the metallization 
of the iron content of ferrotitaniferous material, which process comprises 
in sequence, a first reduction step of heating the said ferrotitaniferous 
material to a temperature in the range of 600.degree. C. to 850.degree. C. 
in a reducing atmosphere comprising hydrogen gas so that more than 5% and 
not more than 50% of the iron content of the ferrotitaniferous material is 
converted to the metallic state, and a second reduction step of heating 
the product from the first reduction step containing residual oxidized 
iron to a temperature of at least 950.degree. C. in the presence of a 
solid carbonaceous material so that at least 90% of the iron content of 
the product from the second reduction step is in the metallic state. 
It is essential in order to achieve the desired result from the process of 
the invention, namely the formation of fine particles of metallic iron, 
that at least 90%, and preferably at least 95%, of the iron content of the 
ferrotitaniferous material be in the trivalent state when it is submitted 
to the first reduction step. Some ferrotitaniferous materials will meet 
this requirement but in many cases it will be necessary to submit the 
ferrotitaniferous material to an oxidation step prior to the first 
reduction step. 
Accordingly, in a further embodiment of the process of our invention we 
provide a process for the metallization of the iron content of 
ferrotitaniferous material, which process comprises in sequence an 
oxidation step of heating the said ferrotitaniferous material in an 
oxidizing environment to ensure that at least 90% of the iron content of 
the said material is in a trivalent state, a first reduction step of 
heating the product from the oxidation step to a temperature in the range 
of 600.degree. C. to 850.degree. C. in a reducing atmosphere comprising 
hydrogen gas so that more than 5% and not more than 50% of the iron 
content of the product from the oxidation step is converted to the 
metallic state, and a second reduction step of heating the product from 
the first reduction step to a temperature of at least 950.degree. C. in 
the presence of a solid carbonaceous material so that at least 90% of the 
iron content of the product from the second reduction step is in the 
metallic state. 
BEST MODE OF CARRYING OUT THE INVENTION 
For convenience, it is preferable to mix the product from the oxidation 
step with the carbonaceous material destined to be the reductant in the 
second reduction step before it is subjected to the first reducing step. 
It is essential, in this instance, that the temperature in the first 
reduction step does not exceed 850.degree. C. otherwise significant 
amounts of carbon reduction can occur which, in turn, will cause the 
metallic iron to be formed as relatively large particles. Below 
600.degree. C. no significant reduction with hydrogen will occur. 
Preferably the first reduction step is operated at between 700.degree. C. 
and 800.degree. C. 
The time during which the treatment of the first reduction step is applied 
depends on the total iron content and its rate of reduction. The 
conditions of this first reduction step must prevail for long enough for 
at least 5% of the iron content to be converted to the metallic state. To 
proceed beyond 50% metallization would be wasteful as regards utilisation 
of hydrogen. Preferably the degree of metallization achieved in this step 
is between 10% and 20%. 
In order to achieve optimum utilization of the hydrogen in the first 
reduction step the solids are preferably contacted with the hydrogen in a 
counter-current manner. If fluidized bed reactors are used this is 
conveniently achieved by using at least two reactors. In the first reactor 
the solid product from the oxidation step is contacted with, and partly 
reduced by partially oxidized gas from the second reactor. In the second 
reactor the reduced product from the first reactor is partially metallized 
by hydrogen. 
If the carbonaceous material has not been added to the ferrotitaniferous 
material prior to the first reduction step, the product from this 
reduction step is mixed with carbonaceous material required as the 
reductant in the second reduction step and the mixture is then subjected 
to the second reduction step. 
The proportion of carbonaceous material to ferrotitaniferous material in 
the second reduction step is not critical but there must be sufficient 
carbon present to reduce all the un-metallized iron content in the product 
from the first reduction step. Preferably the ratio of solid carbonaceous 
material to ferrotitanferous material on a w/w basis is in the range of 
0.1:1 to 0.3:1. 
Solid carbonaceous materials which are suitable for use in the process of 
this invention are coal, coke and char. Although no hydrogen gas has to be 
added in the second reduction step it is likely that residual amounts from 
the first reduction step will be present and some could be generated by 
reaction of water with the carbonaceous material. 
Provided the temperature is above 950.degree. C. in step 3 the remaining 
oxidized iron can be metallized by the carbonaceous reductant, however 
temperatures in excess of 1300.degree. C. are undesirable because the 
titanium oxide phase will start to sinter. Preferably the temperature used 
in the second reduction step is between 1000.degree. C. and 1200.degree. 
C. 
INDUSTRIAL APPLICABILITY 
The process of the invention is useful for the metallization of many 
titanium-bearing materials. Such ferrotitaniferous materials include, for 
example, those derived from titanium-bearing ores generally called 
"ilmenite" which term includes the mineral ilmenite, FeTiO.sub.3, and 
other minerals having the ilmenite structure such as (Fe,Mn,Mg)TiO.sub.3 
as well as oxidized forms of ores containing iron in the ferric state and 
weathered forms of such ores. Other names given to these materials include 
ulvospinel, arizonite, pseudobrookite, titanomagnetite and kalkowskyn for 
example. Other suitable titaniferous materials include any of the above 
which also contain iron oxide inclusions, as well as materials described 
as iron sands. The titanium-bearing ores usually occur in beach sand or as 
rock deposits. 
Generally, if the ferrotitaniferous material to be metallized is derived 
from beach sand, there is no need to reduce its particle size prior to 
carrying out the process of the invention on it, but if it is of rock 
origin then it is necessary to grind it to reduce its particle size to 
that suitable for use in the process. Preferably the ferrotitaniferous 
material to be treated by the process of this invention should consist of 
particles in the size range of 50 microns to 400 microns. 
The process of the invention has no substantial effect on the particle size 
of the ferrotitaniferous material. This is an advantage because, if the 
metallic iron content is removed by one of the known processes which also 
have no effect on particle size, the relict titanium oxide particle will 
be of convenient size for the processes commonly used to convert the 
titanium oxide to a pigment product. 
It is characteristic of the process of the invention that it produces 
particles of titanium oxide material in which the metallized iron content 
is dispersed as fine particles which are less than 3 microns in size. 
These particles are homogeneously distributed throughout the titanium 
oxide particles. 
It is this homogeneous distribution of fine metallic iron particles which 
provides the major advantage of the process of the invention. The same 
type of product can be made by reduction with hydrogen alone. However the 
process of the invention has a significant economic advantage in that a 
major part of the reduction is achieved by use of a solid carbon 
reductant. Efficient utilization of hydrogen requires complex and costly 
recycle processes. 
Metallized ilmenite having its metallic iron content in the form of fine 
particles, evenly distributed and accessible, such as produced by the 
process of this invention, has been shown to be a suitable starting 
material for the segregation process disclosed in Australian Patent 
Specification No. 515 061.

The process of the invention is now illustrated by, but not limited to, the 
following example. In this example all parts and percentages are by weight 
unless otherwise stated. 
EXAMPLE 1 
A sample of beach sand ilmenite from west coast area of Australia 
(containing 54.7% TiO.sub.2, 25.3% FeO, and 16.6% Fe.sub.2 O.sub.3 ; size 
range -52, +200 BS Sieve mesh) was heated in a shallow tray to 
1000.degree. C. for five hours in an oxidizing combustion gas atmosphere. 
The oxidized product obtained contained 30.1% iron, 98.5% of which was in 
the ferric state. A 50 g sample of this oxidized product was mixed with 
12.7 g of brown coal char (particle size range -52+200 BSS). 
The mixture was heated to 760.degree. C. in a vibrating bed reactor for 
approximately 5 minutes during which time 9.83 mole of hydrogen was passed 
through the bed. A parallel experiment using similar conditions showed 
that about 15% metallization of the iron would have taken place. 
The vibration of the bed was stopped and the temperature raised to 
1100.degree. C. over approximately 15 minutes. The static bed was 
maintained at a temperature between 1050.degree. C. and 1100.degree. C. 
for 2 hours. 
At least 95% of the iron content of the product ferrotitaniferous particles 
had been metallized. The product particles were in the same size range as 
the original ilmenite. Microscopic examination of the product particles 
showed that the particle size and distribution of the metallic iron 
particles in them were similar to those obtained when only hydrogen at 
about 750.degree. C. was used to metallize a similar sample. A portion of 
the product, which was magnetic, was leached with dilute hydrochloric 
acid, washed and dried. The leached product was non-magnetic and 
microscopic examination of it revealed that the metallic iron particles 
had been dissolved away. 
The average size of the metallic iron particles in the product particles 
was about 2 microns. 
By way of comparison another sample of similar beach sand ilmenite on 
reduction with carbon at a high temperature (1150.degree.-1200.degree. C.) 
gave a product consisting of titanium oxide particles with large metallic 
iron agglomerates varied greatly in shape and size but commonly appeared 
as particles having a largest dimension of 20 microns, some having a 
largest dimension of 100 microns.