WO2001042520A1

WO2001042520A1 - Method of high-temperature digesting titanium containing material with sulfuric acid

Info

Publication number: WO2001042520A1
Application number: PCT/ZA2000/000246
Authority: WO
Inventors: Gerard Pretorius
Original assignee: PACMIN INVESTMENTS Ltd
Current assignee: PACMIN INVESTMENTS Ltd
Priority date: 1999-12-13
Filing date: 2000-12-12
Publication date: 2001-06-14
Anticipated expiration: 2002-06-13
Also published as: AU2979101A; WO2001042518A1; AU2979201A; WO2001042520A8

Abstract

According to the present invention there is provided a method of digesting titanium containing material comprising the steps of providing the titanium containing material; and reacting the titanium containing material with sulphuric acid (H2SO4) at a furnace temperature above 340 °C to form titanium containing sulphate. The reaction product of said method may also be leached with water to separate titanium values from at least some other material.

Description

METHOD OF DIGESTING TITANIUM CONTAINING MATERIAL

AND PRODUCTS THEREOF

Technical Field

This invention relates to a method of digesting titanium containing material

to form titanium containing sulphate. The invention also relates to a method

of separating titanium values from at least some other material in titanium

containing material.

Background Art

Many different processes are known for extracting metal values from metal

containing material such as ore or slag. One such process is the sulphate

process which is widely used to beneficiate titaniferous material. The sulphate

process is a batch process and the first step is to digest the Ti0₂ bearing

material in excess hot H₂S0₄ . The excess H₂S0₄ is calculated not only to

ensure that sufficient H₂S0₄ is present to sulphate all sulphatable species in the

titaniferous material but also to ensure that sufficient H₂S0₄ is present in order

that for each mole of Ti0₂ present in the titaniferous material, at least two

moles of H₂S0₄ is present. This excess ensures that during the H₂S0₄ digestion process, Ti0₂ in the

titaniferous material reacts with the excess H₂S0₄ to form H₂TiO(S0₄)₂ also

known as acid titanium sulphate which is a water soluble compound.

H₂TiO(S0₄)₂ can also be represented as TiOS0₄.H₂S0₄.

The H₂TiO(S0₄)₂ is extracted with water from the reaction product (in the

form of a filter cake) and is then precipitated as TiO(OH)₂ whereafter it is

separated, washed and calcined to form Ti0₂.

The H₂S0₄ used to digest part of the titaniferous material may be concentrated

or may be diluted, and the H₂S0₄ digestion step takes place at a furnace (kiln)

temperature of about 200°C. In the case of the digestion of ilminite slag the

temperature is typically about 220°C. The reason for this temperature is that

H₂S0₄ has a high vapour pressure and a boiling point of 290°C and

furthermore, H₂S0₄ decomposes into S0₃ and H₂0 at 340°C and 1 atmosphere

pressure. The furnace temperature is accordingly retained below said

temperatures and H₂S0₄ digestion is not used at furnace temperatures above

340°C thereby to minimise decomposition of the H₂S0₄. Furthermore due to the high vapour pressure of H₂S0₄ , most H₂S0₄ digestion

processes operating at above 160°C require the use of pressure vessels, such as

lead vessels. This causes the digestion process to be carried out at pressures

substantially above atmospheric pressure. This equipment is expensive and

cumbersome.

Disclosure of the Invention

The inventor has now found that if H₂S0₄ digestion of titanium containing

material is carried out without the utilisation of pressure vessels, and at a

furnace temperature above 340°C (and typically at 600°C), that is where

H₂S0₄ decomposes to form S0₃ and H₂0, very good digestion of metal values

take place. It is totally unexpected that H₂S0₄ can be used at furnace

temperatures so far above its boiling point without the use of expensive

autoclave high-pressure technology. It would be expected that at such

temperatures (typically at 600°C), all the acid would rapidly evaporate and/or

decompose and would therefore not remain in contact with the material for

long enough to effectively sulphate sulphatable species.

It is accordingly an object of the present invention to provide an alternative

method of digesting titanium containing material to form titanium containing sulphates. It is also an object of the invention to provide a method of treating

titanium containing material to separate titanium values from at least some

other material in the titanium containing material.

According to a first aspect of the present invention there is provided a method

of digesting titanium containing material comprising the steps of

providing the titanium containing material; and

reacting the titanium containing material with sulphuric acid (H₂S0₄) at

a furnace temperature above 340°C to form titanium containing sulphate.

The titanium containing sulphate which forms may be H₂TiO(S0₄)₂ also

known as acid titanium sulphate, which is water soluble. Alternatively, and

preferably it may be TiOS0₄ which is also known as basic titanium sulphate.

The titanium containing material may comprise titaniferous material, which

may comprise titaniferous ore such as ilmenite, rutile, anatase, leucoxene and

titaniferous magnetite. Alternatively the titaniferous material may comprise

titaniferous slag which may include pseudo brookite and/or perovskite.

The titanium containing material may be sized prior to the digestion step. The particle size will depend on the nature of the titanium containing material to

be digested. The more difficult the material is to digest, the finer the particle

size will be. The particle size may be smaller than 75μm, preferably smaller

than 45μm, and it may even be smaller than 15μm.

The titanium containing material may be sized by milling. The milling may

comprise dry milling or wet milling. Wet milling may take place in the

presence of sulphuric acid (preferably concentrated sulphuric acid) thereby

forming a wet mixture of the titanium containing material and the H₂S0₄.

The H₂S0₄ may be concentrated or diluted. Preferably it is provided in a

concentrated form from 1 mol.dm^"3, to 40 mol.dm^"3 , preferably from 12

mol.dm^"3 to 20 mol.dm^"3 , and preferably at a concentration of about 18

mol.dm^"3.

With furnace temperatures above 340°C which is above the decomposition

temperature (340°C), of H₂S0₄ it is believed that the H₂S0₄ decomposes to

form S0₃ and H₂ O. It is believed that these decomposition products may react

with the titanium in the titanium containing material to form titanium

containing sulphate. The H₂S0₄ is preferably provided in a stoichiometric excess, where 100%

stoichiometric equivalent is calculated by determining the molar amount of

H₂S0₄ to fully sulphate all the sulphatable species of the titanium containing

compound. In determining the stoichiometric equivalent the molar ratio of

H S0₄ : Ti values may be calculated as 2:1, that is to form H₂TiO(S0₄)₂.

Alternatively and preferably, in determining the stoichiometric equivalent the

molar ratio of H₂S0₄ : Ti values is calculated as 1:1, that is to form TiOS0₄.

It will be appreciated that the molar ratio of the H₂S0₄ to titanium containing

material will depend on the composition of the titanium containing material

and more particularly it will depend on the nature and ratio of sulphatable

species in the titanium containing material. For example, titaniferous material

containing (a) Ti0₂, (b) Fe₂0₃, (c) Al₂0₃, (d) MgO, (e) CaO and (f) SiO (a to

f indicating the molar amounts present the composition) the following molar

amount of H₂S0₄ will be the stoichiometric equivalent, namely (la +

3b + 3c + Id + le + Of). Usually the stoichiometric excess will not be more than

0,6 times the stoichiometric molar amount to be added where the

stoichiometric molar amount is calculated as above. It also has to be taken into

account that when FeO is present (such as in ilmenite) the H₂S0₄ will oxidise

the FeO to Fe₂0₃ which will then be sulphated. In calculating the H₂S0₄ to be

added it has to be taken into account that some H₂S0₄ will be used to oxidise

the FeO to Fe₂0₃. Preferably the reaction may be carried out in a furnace.

Preferably the furnace temperature at which the H₂S0₄ reacts with the

titanium containing material (hereinafter referred to as the digestion stage) is

above 400°C, preferably above 450°C, more preferably from 450°C to 750°C

and most preferably from 550°C to 650°C. The use of higher temperatures is

also foreseen, especially up to 1000°C. During this stage the product

temperature of the H₂S0₄ is preferably above 240°C, preferably from 270°C

to 340°C. Most preferably from 270°C to 320°C. This temperature is

preferably maintained until the temperature of the reaction product starts to

rise. At this stage most of the H₂S0₄ has been consumed, either in the

oxidation reaction, sulphating reaction, or as lost to the system due to

evaporation and/or dissociation. In use, the particle size of the titanium

containing material and the amount of H₂S0₄ may be adjusted to ensure that

at the end of the digestion stage, substantially all the sulphatable species in the

titanium containing material are sulphated.

The reaction product of the digestion stage may be subjected to heat treatment

(hereinafter referred to as the dead burn stage) to ensure that substantially all unreacted sulphate is lost from the product and preferably that only TiOS0₄

is present as the titanium containing sulphate. This step is considered to be

most preferred, and is preferably carried out subsequent to the digestion stage.

The product temperature during the dead burn stage is usually higher than

320°C to cause the sulphate in the form of H₂S0₄ to dissociate into H₂0 and

S0₃ (S0₃ can also further dissociation into S0₂ and 0₂) which can then be

recycled with the vapour products produced during digestion. The product

temperature may be from 400 to 550°C. The temperature during this stage is

determined by the type of titanium containing material which is used and will

be less than the decomposition temperature of commercially important

sulphates. However, for certain ores it may be beneficial or desirable to

thermally decompose specific metal containing sulphates, typically to form

metal oxides. In such cases the temperature will be chosen to be above the

decomposition temperature of the metal containing sulphate in question but

below the temperature of the commercially important metal containing

sulphate (s) or vice versa.

The digestion stage and dead burn stage are preferably carried out as two

separate stages, but it is foreseen that they can be combined into a single stage.

Prior to the digestion stage the product may go through a warming stage wherein it is warmed to the temperature required for the digestion stage.

Preferably the warming occurs as fast as possible.

Subsequent to the digestion stage, but preferably only subsequent to the dead

burn stage the product is allowed to cool (hereinafter referred to as the cooling

stage). The cooling may be natural, but it may also be forced to speed up this

stage.

Preferably the titanium containing material and H₂S0₄ are reacted with each

other by mixing them together to form a wet mixture and then subjecting it

to at least the digestion and dead burn stages. Preferably the mixture of the

titanium containing material and H₂S0₄ goes through all four the above stages.

Different furnaces may be used during the different stages. For example, a

first furnace such as tunnel kiln may be used for the warming and digestion

stages and a second furnace such as a rotary kiln may be used for the other

stages.

The reaction product of the digestion stage may also be milled prior to

subjecting it to the dead burn stage.

The digestion process is preferably carried out at about atmospheric pressure, preferably in a range of 5% above or below atmospheric pressure, preferably

in a range of 2% above or below atmospheric pressure.

The reaction conditions are preferably such to prevent S0₃ from decomposing.

At temperatures above about 1000°C, S0₃ decomposes to sulphur dioxide and

oxygen which is not desirable.

The process may also include adding an additional source of sulphate or S0₃

to the titanium containing material to be digested with the H₂S0₄ , which

additional source is not H₂S0₄. The additional source may comprise a source

of sulphate. The additional source may comprise ammonium sulphate

((NH₄)₂S0₄), or ammonium bisulphate (NH₄HS0₄). Preferably it comprises

ammonium sulphate. Ammonium sulphate converts to ammonium bisulphate

in the presence of sulphuric acid, or decomposes to ammonium bisulphate with

the release of ammonia when heated to above 100°C, typically between 160

and 240°C.

If the additional source comprises ammonium sulphate, an amount thereof

equal or less than the H₂S0₄ used in the process is added, which amounts are

calculated on a molar basis. During the reaction (especially the dead burn

stage, if applicable) ammonium bisulphate decomposes to produce off gases including NH₃, S0₃ (or S0₂ and 0₂) and H₂0. These gases may be recycled.

If such an additional source of sulphate or S0₃ is used it may be taken into

account when calculating the amount of H₂S0₄ to be used. That is, the

sulphate and/or S0₃ of the additional source can be subtracted from the H₂S0₄

required.

The process may also include the step of removing sulphate, especially H₂S0₄

from the reaction product of the titanium containing material with sulphuric

acid in order that substantially only TiOS0₄ is present as the titanium

containing sulphate. The sulphate, especially H₂S0₄ may be removed by

means of heat treatment, preferably in the form of the dead burn stage as set

out above. It will be appreciated that when an additional source of sulphate or

S0₃ is present, the treatment should preferably also remove the sulphate and

S0₃ originating from such a source.

The digestion method may also include the step of separating the titanium

containing sulphate from other material. Where water soluble and water

insoluble species are present in the reaction product, the reaction product may

be treated with water to allow water-soluble species to dissolve in the water to

form a leachate, which leachate may then be separated from undissolved solids. The titanium containing sulphate may be in the form water soluble of

H₂TiO(S0₄)₂ which may soluble in water. Alternatively it may be in the form

of TiOS0₄ which is only soluble in water in the presence of relatively high

concentrations of S0₄ ²\ Whether H₂TiO(S0₄)₂ or TiOS0₄ forms will depend

on the amount of H₂S0₄ or additional source of sulphate or S0₃, as well as the

heat treatment to remove H₂S0 , sulphate or S0₃.

Where the titanium containing sulphate is soluble in water the leachate may

be treated to separate various sulphates if more than one water-soluble metal

containing sulphate is contained in the leachate.

Standard processes may be used for this purpose and may include pH control,

neutralisation, hydrolysis, electrolysis etc.

The invention also relates to a titanium containing sulphate formed by the

digestion process including water-soluble titanium containing sulphate. The

water-soluble titanium containing sulphate which forms is effectively

anhydrous. This is often not the case for many conventional low temperature

sulphate leach processes where the sulphates are hydrated or dissolved in water.

According to another aspect of the present invention a method of separating titanium values from at least some other material in a titanium containing

material comprises the steps of

providing the titanium containing material;

reacting the titanium containing material with H₂S0₄ at a furnace

temperature above 340° to form a solid reaction product including

titanium containing sulphate; and

leaching the solid reaction product including the titanium containing

sulphate with water to form a leachate and separating the leachate from

water insoluble solid.

The titanium containing sulphate may be contained in the leachate or in the

insolubles s.olid depending on the conditions.

The invention also relates to a product formed from the process.

The invention will now be further described by means of the following non-

limiting examples. Example 1

One current process for treating ilmenite involves smelting iron ore in high

temperature furnaces to produce pig iron plus a high-grade slag (i.e slag with

a high titanium value, typically 85%). Such slags can be used in either the

conventional sulphate or conventional chlorination process to produce TiO_a

pigments. These slags have a very high melting point and are quite corrosive

to refractories. The furnaces used in such reactors are expensive to build and

maintain and they are also troublesome.

The present invention allows ilmenite or similar materials i.e. titaniferous

magnetite to be handled in a less expensive and more benign manner. Such

feedstocks can be combined with fluxes like Si0₂, CaO, dolomite (CaO: MgO)

etc. These fluxes will lower the slag melting point and hence enable cheaper

and more conventional furnaces to be used to smelt and remove the molten

iron. The resulting slag would not normally be treatable by conventional

processes to recover the titanium values because of the presence of too high

levels of impurities from the flux and hence the presence of CaTi0₃ if CaO is

used CaTi0₃ is extremely stable and is not digested by the conventional

sulphate processes. However these slags would be treatable by the present

invention. In the present example titanium slag from Highveld Steel and Vanadium at

Witbank was used. This slag has similarities to ilmenite slags that were

combined with fluxes. The major morphologies (phases) of this slag are as

follows:

Augite, aluminium - Ca(Mg, Fe + 3, Al)(Si, A1)₂0₆

Perovskite, syn - CaTi0₃

Armalcolite, ferrian, syn - (MgFe)(Ti₃Fe)O₁₀

Pseudo brookite, syn - Fe₂TiO_s

A chemical analysis was conducted on the slag and the composition is indicated

in Table 1.

Table 1: Chemical composition of Highveld Steel and Vanadium slag and

H₂S0₄ required to digest the slag.

2 Kmol S0₄ ^2" amounts to 200kg H₂S0₄ at 98% purity.

The slag in an amount of 100kg was dry milled to a particle size of smaller than

25μm and was then mixed with 200kg H₂S0₄ (98% purity). The amount of

H₂S0₄ to be added was calculated on the basis as set out in Table 1. The

H₂S0₄ provided a 41,7% excess H₂S0₄ calculated on the basis of a TiO, : S0₄ ^2"

molar ratio of 1: 1. An excess of H₂S0₄ is used to ensure complete oxidation of all reduced species in the slag and complete sulphation thereof.

The resultant wet slurry mixture of 200kg H₂S0₄ and 100kg slag was poured

into open aluminium trays in order that the slurry did not exceed a depth of

200mm. The containers were introduced into a top hat furnace, and the

furnace was ramped up to 600°C to provide a product temperature between

240°C and 320°C. This period is known as the digestion stage during which

oxidation and sulphation of the slag occurs.

After about six hours at this temperature, the product temperature started to

rise above 320°C. At this stage most of the oxidation and sulphation had

occurred. This treatment was continued until the crust temperature of the

cake (which formed due to the reaction) reached 450°C at which time the

furnace was switched off in order that decomposition of the formed metal

sulphates did not occur. This period of increased product temperature is

known as the dead burn stage and during this period all remaining H₂S0₄ is

lost from the product in order that only TiOS0₄ is present as the sulphated

titanium value and that no excess H₂S0₄ is present in the cake to form

H₂TiO(S0₄)₂. This stage took about 4 hours.

The S0₃, S0₂ and 0₂ which form during the digestion and dead burn stages may be recycled. The said gasses may be fed through a ceramic lined tube from

the furnace to a mild steel scrubber containing oleum to recycle it as H₂S0₄.

The product was then allowed to cool for another 4 hours and dead burning

also occurred during the first period of cooling.

The cakes were then tipped out of the trays and crushed. The reaction yielded

approximately 200kg cake and had a composition as set out in Table 2. The

results indicate that the sulphation efficiency was not less the 85%. The

crushed cake was then leached with 3751 water at 60°C, thereby leaching the

soluble sulphates from the cake. After 2 hours of leaching the leachate was

filtered from the solids. The concentration of the metal sulphates in the

leachate are also indicated in Table 2. The insolubles are safe to be dumped.

Table 2 !: Cake composition, solubility of sulphated species and

concentration of sulphated species present in leachate.

The leachate was then treated to reduce Fe³⁺ to Fe²⁺. This was done by adding

iron filings to the leachate and reduction was enhanced by heating the solution

from 40 to 60°C. During reduction the solution changed colour from pale

green to light purple. The remaining iron metal was then removed from the

solution by filtration, but magnetic separation is also possible. The solution

1 0 was then heated to 90°C to hydrolyse the solution, thereby converting TiOS0₄ to TiO(OH)₂. The hydrolyses was completed after about 2 hours.

The hydrolysed product, TiO(OH)₂, is also known as crude anatase pulp

(CAP) and is insoluble in water. The CAP was filtered from the leachate. The

CAP had a high purity and also has the advantage that it is in activated form.

Accordingly it is a lucrative feedstock for the sulphate process to produce

titanium pigments.

Example 2

In this example ilmenite slag was treated. The slag had the chemical

composition as set out in Table 3.

Table 3: Chemical composition of slag and H₂S0₄ required to digest the

slag.

2 Kmol S0₄ ^2" amounts to 200kg H₂S0₄ at 98% purity. The slag in an amount of 100kg was milled with 200kg H₂S0₄ (98% purity) to

a particle size of smaller than 25 μm. An alumina lined pin mill, using 600kg,

6mm alumina beads was used for this purpose at 500rpm for 1 hour. The

amount of H₂S0₄ to be added was calculated on the basis as set out in Table 3.

The H₂S0₄ provided a 59.6% excess H₂S0₄ calculated on the basis of a Ti0₂

: S0₄ ^2" molar ratio of 1:1. An excess of H₂S0₄ is used to ensure complete

oxidation of all reduced species in the slag and also complete sulphation

thereof.

The resultant wet mixture of 200kg H₂S0₄ and 100kg slag was in the form of

a slurry at 60 to 80°C. The slurry was pumped into open aluminium trays in

order that the slurry did not exceed a depth of 200mm. After 1 to 2 hours the

slurry solidified and it is believed that this hard polymerised cake is the first

step at digestion involving co-ordination of the H₂S0₄ with slag via hydrogen-

oxygen-change bonding. The containers were introduced into a top hat

furnace, and the furnace was ramped up to 600°C to provide a product

temperature between 240°C and 320°C. This period is known as the digestion

stage during which oxidation and sulphation of the slag occurs.

After about six hours at this temperature, the product temperature started to rise above 320°C. At this stage most of the oxidation and sulphation had

occurred. This treatment was continued until the crust temperature of the

cake (which formed due to the reaction) reached 450°C at which time the

furnace was switched off in order that decomposition of the formed metal

sulphates did not occur. This period of increased product temperature is

known as the dead burn stage and during this period all remaining H₂S0₄ is

lost from the product in order that only TiOS0₄ is present as the sulphated

titanium value and no excess H₂S0₄ is present in the cake to form

H₂TiO(S0₄)₂. This stage took about 4 hours.

The S0₃, S0₂ and 0₂ which form during the digestion and dead burn stages

may be recycled as H₂S0₄ as described in example 2.

The product was then allowed to cool for another 4 hours and dead burning

also occurred during the first period of cooling.

The cakes were then tipped out of the trays and crushed. The reaction yielded

approximately 188 kg cake and had a composition as set out in Table 4. Table 4: Cake composition and solubility of sulphated species.

The results indicate that the sulphation efficiency was not less than 85%.

Insolubles and solubles may be separated by leaching the reaction product with

water. Insoluble TiOS0₄ is a lucrative feedstock to produce synthetic rutile

for the chloride process.

It will be appreciated that the process according to the present invention uses

H₂S0₄ at furnace temperatures significantly above the decomposition

temperature of H₂S0₄. In the examples no pressure vessels (autoclaves) were

used and it would be expected that the H₂S0₄ would gassiiy before it had time

to react with the metal containing material. Unexpectedly and surprisingly this did not happen. The H₂S0₄ and S0₃ + H₂0 remained in contact with the

metal containing material for long enough to react completely with the

sulphatable portion thereof.

It is known that some metal sulphates are not stable at 600°C, e.g. ferric

sulphate decomposes at 480°C. However these sulphates do not decompose

during the digestion step at 600°C as long as H₂0 and S0₃ are still present.

It will be appreciated that many variations in detail are possible without

thereby departing from the scope and spirit of the invention.

Claims

1. A method of digesting titanium containing material comprising the steps

of

providing the titanium containing material; and

reacting the titanium containing material with sulphuric acid (H₂S0₄)

at a furnace temperature above 340°C to form titanium containing

sulphate.

2. The method of claim 1 wherein the titanium containing material is sized

prior to reacting it with the sulphuric acid.

3. The method of claim 2 wherein the titanium containing material is sized

to a particle size smaller than 75μm.

4. The method of claim 3 wherein the titanium containing material is sized

by wet milling in the presence of sulphuric acid thereby forming a wet

mixture of the titanium containing material and the sulphuric acid.

5. The method of claim 1 wherein the H₂S0₄ is provided in a stoichiometric excess.

6. The method of claim 1 wherein the titanium containing material is reacted

with the H₂S0₄ (hereinafter referred to the digestion stage) at a furnace

temperature of above 400°C.

7. The method of claim 6 wherein the furnace temperature is from 550°C to

650°C.

8. The method of claim 6 wherein the reaction product of the digestion stage

is subjected to heat treatment (hereinafter referred to as the dead burn

stage) to ensure that substantially all unreacted sulphate is lost from the

product and that only TiOS0₄ is present as the titanium containing

sulphate.

9. The method of claim 8 wherein the titanium containing material and

H₂S0₄ are reacted with each other by mixing them together to form a

wet mixture and then subjecting it to at least the digestion and dead burn

stages.

10. The method of claim 1 which includes the step of adding an additional source of sulphate or S0₃ to the titanium containing material to be

digested with the H₂S0₄, which additional source is not H₂S0₄.

11. The method of claim 10 wherein the additional source comprises

ammonium sulphate ((NH₄)₂S0₄), or ammonium bisulphate (NH₄HS0₄).

12. The method of claim 1 which includes the step of separating the titanium

containing sulphate from other material.

13. The method of claim 12 wherein water soluble and water insoluble species

form in the reaction product of the titanium containing material and

sulphuric acid and wherein said reaction product is treated with water to

allow water-soluble species to dissolve in the water to form a leachate,

which leachate may then be separated from undissolved solids.

14. Titanium containing sulphate prepared by the method of any one of the

preceding claims.

15. A method of separating titanium values from at least some other material

in a titanium containing material comprises the steps of

providing the titanium containing material; reacting the titanium containing material with H₂S0₄ at a furnace

temperature above 340° to form a solid reaction product including

titanium containing sulphate; and

leaching the solid reaction product including the titanium containing

sulphate with water to form a leachate and separating the leachate

from water insoluble solid.

16. Titanium containing sulphate prepared by the method of claim 15.