Method of Preparing Magnetite Having a Controlled Particle Size, Starting From Ferrous Sulphate Solutions

The present invention relates to a method of controlling the particle size of equiaxial magnetite in a process for obtaining same, starting from ferrous sulphate solutions.

Processes that permit one easily to control the particle size of the magnetite obtained from ferrous sulphate solutions are not known so far. On the other hand, for certain applications--especially in the field of red pigments obtained by calcining magnetite--it is useful strictly to control the magnetite particle size: in fact, the mass tone of the red pigment is closely related to the particle dimensions, and the tinting strength is the higher, the lesser (within certain limits) the particle size dispersion.

In the event a magnetite having a controlled particle size is not available for the preparation of pigments, then it is essential after calcining to carry out burdensome operations such as grinding and, sometimes, classifying with a view to bringing the particle size back to the desired values.

Therefore the advantage of having available a magnetite whose particle size is already that of the desired red pigment is evident. In fact, if sintering phenomena during calcining are avoided, it is possible to eliminate all the succeeding grinding and classifying operations, or to render them much less burdensome.

Thus, it is an object of the present invention to provide a method that permits one to obtain equiaxial magnetite with controlled particle size, by precipitation from ferrous sulphate solutions of any origin (pickling liquid, effluents of the TiO.sub.2 production process via sulphate, etc.).

This and still other objects are achieved by the present invention relating to a method for controlling the equiaxial magnetite particle size in processes for preparing said magnetite starting from ferrous sulphate solutions, such method being characterized in that the reaction forming the magnetite is conducted in a suspension to which .gamma. FeO(OH) has been added in such amount that the Fe of .gamma. FeO(OH) is 0.1-10% of the Fe++ originally contained in the solution.

It has been discovered in accordance with the present invention that in the process for preparing magnetite starting from ferrous sulphate solutions it is possible to regulate the particle size of the magnetite obtained by admixing with the ferrous sulphate solution, before magnetite precipitation, variable percentages of .gamma.FeO(OH).

In particular, it has been found that the average diameter of the magnetite obtained decreases as the gamma oxide percentage increases. Furthermore, the magnetite product exhibits a narrower particle size distribution and a more regular morphology.

The percentages of added gamma oxide vary from 0.1 to 10% (as Fe) with respect to the Fe.sup.+.sup.+ initially present in the ferrous sulphate solution and depend on the process employed for preparing the magnetite, the desired final particle size being equal.

Magnetite can be obtained from a FeSO.sub.4 solution following various procedures.

For instance, according to a process described in the co-pending Italian patent appl. No. 48632A/74 filed on Feb. 25, 1974, it is carried out in two steps.

In a first step, at a temperature between 15.degree. and 40.degree. C, an alkali is admixed with the ferrous sulphate solution in a stoichiometric amount sufficient to precipitate 2/3 of the Fe.sup.+.sup.+ ion as ferrous hydroxide, and then, at the conclusion of the precipitation, air is blown thereinto thus oxidizing ferrous hyroxide to goethide .alpha.FeO(OH).

In a second step, an alkali is added to the slurry obtained in the first step, the remaining Fe.sup.+.sup.+ is precipitated in the form of ferrous hydroxide, and the slurry is heated to a temperature ranging from 70.degree. to 100.degree. C, thus causing the formation of magnetite which is then separated from the solution.

If the method according to the present invention is applied to the aforesaid process, i.e. if some .gamma.FeO(OH) is added to the suspension, before the beginning of the reaction forming the magnetite, it is possible to regulate on the basis of the added product amount--the size of the magnetite obtained.

According to another process it is carried out in one step only. At first, almost all the iron present in solution is precipitated with an alkali, in the form of Fe(OH).sub.2, at a temperature ranging from 15.degree. to 40.degree. C, then the mass is heated to a temperature ranging from 70.degree. to 100.degree. C and the ferrous hydroxide is oxidized with air to magnetite; an increase of the solution redox potential and a corresponding decrease of the pH value indicate that the reaction has ended. The method of the present invention proves to be efficient also in this process; in fact the addition of a small amount of .gamma.FeO(OH) to the reaction mass, after heating and before oxidation, serves to regulate the dimensions of the magnetite obtained. The oxide .gamma.FeO(OH) may be prepared according to any method. However, it has been found that it is of particular advantage to prepare the gamma oxide according to the following procedures.

To a FeSO.sub.4 solution of any concentration (preferably between 15 and 70 g/l as Fe.sup.+.sup.+), containing a small amount of Mg.sup.+.sup.+ ions (about 0.5 g/l), an alkali is admixed, at room temperature and with stirring, until the pH value is 9. Air is then blown thereinto and, always keeping the pH at a constant value by addition of the alkali, the precipitated hydroxides are oxidized until the solution redox potential has risen from about -800 mV to at least +10 mV. Under these conditions the precipitate exhibits a reddish-brown color and shows on X-ray analysis to be made up of .gamma. FeO(OH) in the form of very small crystals, as revealed also by examination under the electron microscope.

The following examples are given still better to better illustrate further advantages and characteristics of the method of the present invention.

From a ferrous sulphate solution having 65 g/l of Fe.sup.+.sup.+, and coming from the TiO.sub.2 production process via sulphate, 70% of Fe.sup.+.sup.+ was precipitated with NH.sub.4 OH at 201 g/l and then oxidized to goethite, .alpha. FeO(OH). A slurry having the following composition was thus obtained;

By using .alpha.FeO(OH) thus prepared, four equiaxial magnetite preparations were carried out under the following operating conditions:

These various preparations differed from one another only in that different amounts of .gamma. FeO(OH) were added before the starting of the reaction forming the magnetite, in such a way as not to alter either the Fe concentration or the Fe.sup.II /Fe.sup.III ratio. For example, in the test in which the gamma oxide percentage was 1.33, the following was used: 3200 cc of .alpha.FeO(OH) slurry, 168 cc of titanium-free FeSO.sub.4 solution having 84 g/l of Fe.sup.+.sup.+, 141 cc of slurry containing 18.6 g/l of Fe in the form of .gamma. FeO(OH), and 24 cc of water, thus obtaining a mass exhibiting the following composition: 18.3 g/l of Fe.sup.+.sup.+, 53.34 g/l of Fe.sub.total, Fe.sup.II /Fe.sup.III = 0.52.

The gamma oxide additions are reported in Table I as percentage of Fe present in the form of .gamma. FeO(OH) with respect to the total iron. The characteristics of the various magnetites obtained and the productivity (expressed in g of Fe per hour per liter) of the conversion to magnetite are reported in the same table.

.gamma. FeO(OH) was prepared following the procedures illustrated above. In particular, from a FeSO.sub.4 solution coming from the TiO.sub.2 production process via sulphate at a concentration of 20 g/l of Fe.sup.+.sup.+, the whole amount of iron was precipitated with NH.sub.4 OH at 201 g/l and oxidized with air, at a constant pH = 9 and at room temperature. The final slurry concentration was 18.6 g/l of precipitated Fe.sup.III (as gamma oxide).

A 6 m.sup.3 reactor, provided with a heat exchange and a radial turbine stirrer, was fed with 3500 liters of a ferrous sulphate solution, coming from the TiO.sub.2 production process via sulphate, purified of titanium by precipitation of same at a pH = 3.5 and by subsequent filtration, and having a concentration of 60 g/l of Fe.sup.+.sup.+.

Ferrous hydroxide Fe(OH).sub.2 was precipitated with stirring by means of an NaOH solution at 650 g/l and the reaction mass was simultaneously heated to 90.degree. C. A slurry containing .gamma.FeO(OH), separately prepared at a concentration of 19.2 g/l of Fe.sup.III, and in such amount that the iron therein contained constituted a predetermined percentage of the total iron, and water in such an amount that the Fe.sup.III concentration of the reaction mass was 50.7 g/l, were admixed with said reaction mass. Air was blown thereinto, keeping the temperature constant, and after a time period varying according to the gamma oxide amount present, the pH value decreased while the redox potential increased: when the latter reached -450 mV, the reaction was considered as concluded.

The resulting magnetite was filtered, washed and dried, and thereupon characterized. The particle size resulted depended upon the Fe percentage existing in the form of .gamma.FeO(OH) at the beginning of oxidation, as shown in Table II, in which, besides the morphological, particle size, and chemical purity characteristics of the magnetite obtained, the reaction productivity, which appears to increase as the amount of .gamma. increases, are also reported.