Technical Field

This invention concerns the treatment of minerals containing titanium dioxide and iron, which are generally called titaniferous minerals. The most common of these minerals is ilmenite. More particularly, this invention concerns cold milling of particulate titaniferous minerals to form nanostructured products from which the iron content (or a significant amount of the iron content) of the mineral can be removed by a leaching process.

Background to the Invention

Ilmenite, FeTi0 ₃ , and titaniferous minerals generally, are relatively inert minerals that have long been a major source of titanium dioxide, Ti0 ₂ , which is used as a white pigment in paints, to produce welding rods, and in the cosmetic and ceramics industries. Extraction of the iron from the ilmenite and other titaniferous minerals has been, and still is, an expensive undertaking.

Most of the currently used metallurgical processing of ilmenite includes hot acid leaching of the particulate mineral, with associated environmental problems. For example, the "sulphate route" for the extraction of titanium dioxide from ilmenite involves the steps of

(a) mixing dry milled ilmenite with 85 to 92 per cent sulphuric acid;

(b) heating the mixture of ground ilmenite and sulphuric acid to a temperature of about 160°C, at which temperature an exothermic reaction is initiated, the

product of this reaction being a mixture of ferrous, ferric and titanium sulphates;

(c) forming an aqueous (or weakly acid) solution of the mixture of sulphates, then reducing the ferric sulphate to ferrous sulphate with scrap iron;

(d) clarifying the solution by sedimentation;

(e) removing the iron in the solution by crystallisation (as FeS0 ₄ .7H ₂ 0); and

(f) extracting the titanium dioxide using hydrolysis.

Another technique that is used to process ilmenite is the so-called "chloride route", which involves the chlorination of impure rutile using gaseous chlorine at a temperature in the range of from 650°C to 1,150°C to form titanium tetrachloride (TiCl ₄ ). The TiCl ₄ is then oxidised to produce pure titanium dioxide.

The "Becher" process is also used to upgrade ilmenite - to remove iron and provide a feedstock for the "chloride route". In the Becher process, ilmenite is reacted with coal and sulphur in an iron reduction kiln at 1100°C. This reaction reduces the iron in the ilmenite to the metallic form. After cooling the products of the reaction, the iron is rusted out in slurry form with ammonium chloride acting as a catalyst for the rusting. The remaining iron compounds are removed by leaching with sulphuric acid.

One of the more promising alternative ilmenite processing methods, developed jointly by Commonwealth Scientific and Industrial Research Organisation and Murphyores Pty Limited, known as the "Murso process", is described in the

specification of Australian patent No 416,143. The Murso process uses an oxidation of the ilmenite to convert substantially all the iron to the ferric state, then reduction of the oxidised material to convert substantially all the ferric iron to the ferrous state. These steps, of oxidation of the ilmenite followed by its reduction, produce a material which has an enhanced reactivity, from which the iron can be leached with dilute hydrochloric acid. The cost of using hydrogen as the reducing agent and the cost of regeneration of the hydrochloric acid used for leaching have made the adoption of the Murso process commercially unattractive. Several attempts have been made to modify the Murso process to improve its economics, but at the time of writing this specification, it has not been adopted commercially.

The search for other improved methods of producing rutile from ilmenite has continued.

Disclosure of the Present Invention

The prime object of the present invention is to provide a method of treating ilmenite and other titaniferous minerals to convert such minerals into a form from which the iron content can be removed by a simple leaching process.

This objective is achieved by a cold milling process.

Ball milling of ores, with and without additives to facilitate the comminution process (the reduction of particle size) is not new. The early potential of ball milling for the reduction and extraction of ores, however,

has generally not been fulfilled, and interest in such ore processing technology has waned. The development of a new form of high energy ball mill at The Australian National University, and the success that has been achieved in mechanical alloying work with that ball mill (see, for example, the specifications of International Patent Applications Nos PCT/AU91/00248, PCT/AU92/00073 and PCT/AU94/00057), have stimulated new interest in the cold milling of ores. That new ball mill, which is described in the specification of International patent application No PCT/AU90/00471 (WIPO Publication No WO 91/04810), enables controlled-energy milling of a charge to be effected. The present inventors have now discovered that under certain milling conditions, ilmenite can be reduced while being converted into a nanostructural form, and that iron can be removed from this product (for example, using hydrochloric acid at a temperature of about 100 ^C C).

The basic requirements of the cold milling process are: (i) that high energy milling is carried out at room temperature for a sufficient time period (up to 300 hours) to produce a powder having a nanostructural form, and (ii) that the milling is effected in the presence of suitable additives to the ball mill charge.

Thus, according to the present invention, there is provided a method of treatment of a titaniferous ore to facilitate the removal of iron from the ore to produce rutile, the method comprising high energy milling of the ore in particulate form in the presence of a suitable additive for a period sufficient to form a nanostructural titaniferous product.

The additives include both solid and liquid reducing agents. Amorphous boron has been found to be a useful additive, as have a range of surfactants and organic materials (particularly long chained hydrocarbons). Among the surfactants which may constitute the additive of the present invention are:

(a) dihexadecyl dimethyl ammonium acetate (DHDAA);

(b) didodecyl dimethyl ammonium bromide (DDAB);

(c) didodecyl dimethyl ammonium acetate (DDAA); (d) didodecyl dimethyl ammonium hydroxide (DDAOH); and

(e) sodium didodecyl sulphate (SDDS) - an anionic double chained surfactant.

Dodecane is a preferred long chained hydrocarbon which can be used as the additive of this invention.

To extend this method to include the production of rutile, the milled ore must subsequently be leached to extract the iron from it (for example, using hydrochloric acid at a temperature of about 100°C).

Preferably the milling is carried out in a ball mill of the type described and claimed in the specification of International patent application No PCT/AU90/00471.

A better understanding of the present invention will be obtained from the following description and discussion of experimental work in connection with the cold milling of ilmenite, which has been undertaken by the present inventors.

Description of Cold Milling Work

The present inventors hypothesised that subjecting a particulate titaniferous material to prolonged milling should change the mineral into a reactive nanostructural form, from which it should be possible to remove α-Fe by acid leaching or by magnetic separation. When this hypothesis was tested, simple milling of ilmenite did not produce a reactive nanostructural form of the mineral. However, milling ilmenite with an additive to the ball mill charge, surprisingly, did yield a reactive nanostructural product.

A series of experiments were then performed with ilmenite (in its mineral sand form) to investigate this phenomenon further. In each experiment, the particulate ilmenite was subjected to prolonged high-energy milling in a ball mill of the type described in the specification of International patent application No PCT/AU90/00471. (It will be appreciated that milling in other types of high energy ball mill for an appropriate period of time should have the same effect. ) The milling was effected for a period sufficient to convert the particulate ilmenite charge of the ball mill into a nanostructural material. This required high energy ball milling for a period of time which depended upon the milling conditions and the additive(s) to the ball mill charge.

The milled samples were then leached with no further treatment.

At different stages of the experiments, the composition of each sample was assessed using one or more of the following techniques: x-ray diffraction; Mossbauer spectroscopy; transmission and scanning electron microscopy; atomic absorption spectrometry; Rutherford backscattering spectrometry. In particular, structural development of the as-milled samples was monitored by x-ray diffraction of cobalt Kα radiation using a Phillips diffractometer, and Rutherford backscattering spectrometry was used to analyse the presence of iron in chemically leached samples.

A range of additives were used with the initial charge of the ball mill. In each experiment, the additive remained in the charge of the ball mill for the duration of the milling. Although there was some variation of the concentration of some of the additives, no attempt was made to optimise their concentrations or their reducing properties. One control experiment was carried out using ilmenite alone (that is, with no additive to the charge of the ball mill).

Particular additives used in the experiments were as follows:

1. DDAA ( idodecyl dimethyl ammonium acetate);

2. DDAB (didodecyl dimethyl ammonium bromide);

3. DHDAA (dihexadecyl dimethyl ammonium acetate); 4. DDAOH (didodecyl dimethyl ammonium hydroxide);

5. SDDS (sodium didodecyl sulphate);

6. Dodecane; and

7. Amorphous boron.

During the milling - which lasted for up to 300 hours - the charge of the ball mill was held at room temperature (about 25°C). For experiments using the additives 1 to 5 above, the milling was performed in aqueous solution. Considering the point of zero charge of the ilmenite particle, it was necessary to adjust the pH of the aqueous solution of the additives DDAA, DDAB and DHDAA so that it had a value of 10, which corresponds to negatively charged ilmenite particles, in order to get adsorption of the cationic species. For the experiments with the additive SDDS, the pH was adjusted to a value of 5, which corresponds to positively charged ilmenite particles, in order to get adsorption of the anionic surfactant SDDS. Potassium hydroxide and potassium chloride were used for pH adjustment. No adjustment of the pH of the aqueous solution was necessary for experiments with the additive DDAOH.

The nanostructural ilmenite products obtained by milling the ilmenite with the additives (i) DDAOH, (ii) SDDS and (iii) amorphous boron, were leached with 4M hydrochloric acid at temperatures ranging from 80°C to 100°C. The Rutherford backscattering spectroscopy spectra of the leached materials showed

(a) that less than 5 atomic per cent of the iron in the sample milled with DDAOH remained after leaching; and

(b) there had been a significant reduction of the iron content of the ilmenite samples that had been milled with SDDS and with amorphous boron.

The x-ray diffraction pattern of the material milled with DDAOH and then leached with hydrochloric acid showed only peaks corresponding to rutile.

Thus the experiments showed that prolonged milling of ilmenite with an appropriate additive produces a nanostructural powder that is amenable to leaching with hot hydrochloric acid to remove the iron component of the ilmenite and leave rutile (Ti0 ₂ ).