CS239791B1 - Process for the preparation of iron (III) -oxide - Google Patents

Abstract

A method for producing alpha-iron oxide in the form of platelet crystals with a diameter greater than 20 microns, which is an effective pigment added to top anti-corrosion coatings for aggressive environments. The method according to the invention processes waste basic iron sulfates such as jarosites of the formula MeFe3/SO4/2/OH/g, where Me is an alkali metal, ammonium or hydronium cation, glockerite of the formula Fe4SO4/OH/io or mixtures thereof, into this final product by the action of sodium hydroxide solutions at temperatures of 150 to 350 °C. The essence of the invention consists in forming a homogeneous reaction mixture of the above basic iron(III) sulfates and a NaOH solution with a concentration of 5 to 20 mol NaOH per 1 liter of reaction mixture, after which this reaction mixture is heated at a rate of 1 to 10 °C/min to a selected temperature and the reaction time is 0.5 to 1 hour for the temperature interval 300 to 350 °C, 1 to 2.75 h for the interval 250 to 300 °C, 2.75 to 4 h for the interval 200 to 250 °C or 4 to 6 h for the interval 180 to 200 °C.

Description

BACKGROUND OF THE INVENTION The present invention relates to a process for producing alpha iron oxide in the form of platelet crystals whose diameter exceeds 20 microns by converting basic ferric sulphates.

Current sources of alpha-iron oxide (hematite), whose particles have a diameter of the order of tens of micrometers and a thickness of the order of tenths to units of micrometer (usually called Mio, mioaoeous iron oxide) were both sparse natural deposits and a synthetic procedure described 333 788.

This process utilizes the conversion of amorphous ferric hydroxide in a mixture of inorganic salts and sodium hydroxide at elevated temperature and pressure. Although this process makes it possible to convert some of the difficult waste products from titanium dioxide or pickling baths into valuable pigments, it has some drawbacks, in particular by using less suitable oxidizing agents to oxidize the feedstock, which then degrade the reaction medium.

These drawbacks are overcome by the invention according to U.S. Pat. aut. osv. No. 218 204, which utilizes the transformation of pure ferric oxide to produce platelet hematite, allowing the desired product to be produced by virtually waste-free technology with multiple use of the reaction medium.

With the development of hydrometallurgical processes used to extract mainly non-ferrous metals from less concentrated ores, the issue of iron removal, which is generally present in larger quantities in these ores, can be highlighted and can be eliminated during the process so as not to interfere with the separation of non-ferrous metals.

It is usually iron precipitates as first acidic liquors in the form of basic sulfates, often as sodium or ammonium jarosite formula MeFe ₃ / S0 _4/2 / 0H / _g wherein Me is sodium or ammonium. In this form, however, iron cannot be further utilized, as this is prevented by the high sulphate and water content. Thus, for conversion to metallurgical iron oxide, jarosites need to be further treated, usually by roasting, without obtaining a commercial product; the final product is only recoverable waste.

U.S. Patent No. 3,987,156 discloses that basic sulphates, namely jarosites, can be converted to platelet hematites, but that it is not possible to achieve the required particle size, i.e., at least 20 microns in diameter, to serve as an anticorrosive pigment . The process of said patent corresponds exactly to that of the aforementioned British patent.

It has now been found that the following procedure can convert basic ferric sulphates to platelet alpha-iron oxide, the particles of which have an average size greater than 20 microns, and which can be used as a pigment in topcoat corrosion coatings for aggressive environments.

Accordingly, the above-mentioned drawbacks have no process for the production of platelet alpha-oxide in the form of platelet crystals of more than 20 microns in diameter according to the invention by converting basic ferric sulphates such as jarosites of the formula , an ammonium or hydronium cation, a glockerite of the formula Fe 2 SO 4 / OH / 4 O or mixtures thereof, by treatment with sodium hydroxide solutions at a temperature of 150 to 350 ° C.

The process according to the invention is characterized in that a homogeneous reaction mixture of said basic ferric sulphates and a solution of NaOH in a concentration of 5 to 20 moles of NaOH / l is formed, and the mixture is heated in a temperature regime consisting of heating at 1 to 10 ° C / min. to a selected temperature and a hold time of 0.5 to 1 hour at a selected temperature in the range of 300 to 350 ° C, to 2.75 h in the range of 250 to 300 ° C, 2.75 to 4 h in the range of 200 to 250 ° C 6 h in the range of 180 to 200 ° C.

The advantage of this process is that it allows the waste material to be converted into a valuable product in a simple manner, and that it allows to obtain high product yields from the volume of the pressure chamber and thus also to achieve a high concentration of a single by-product, sodium sulfate, in the reaction mixture and facilitating its recovery crystallization. The content of the admixture of, for example, aluminum or magnesium, which is present in the ferric precipitates, is of no importance for the process result, since it does not affect the quality of the resulting platelet hematite.

The basic ferric sulphates are treated with a concentrated sodium hydroxide solution of 5-20 moles of NaOH per liter of reaction mixture at a pressure corresponding to the saturated vapor pressure above the reaction mixture with stirring.

Before gradually increasing the temperature, the reaction mixture is homogenized so that the ferric material particles are uniformly distributed throughout the volume of the mixture. In order to obtain a narrow particle size distribution of the resulting platelet hematite and to achieve a particle diameter higher than the lower serviceability limit for anticorrosive pigments, i.e. above 20 microns, the reaction temperature regime is important.

The relationship between the reaction temperature and the reaction time was found, as was the relationship between the heating rate and the length of the reaction. These relationships are understandable considering that the conversion of basic sulfates to hematite occurs by dissolving one component and crystallizing the other. The hematite itself is also soluble under the reaction conditions in the reaction medium, and therefore it is also likely to recrystallize, i.e., to change the particle size distribution during the reaction.

According to the invention, the reaction mixture is heated so that its temperature rises at a rate of 1 to 10 ° C per minute and the reaction time for a temperature range of 300 to 350 ° C is 0.5 to 1 hour, for a range of 250 to 300 ° C 1 to 2 75 hours for an interval of 200 to 250 ° C 2.75 to 4 hours and for an interval of 180 to 200 ° C 4 to 6 hours.

The following examples show several variations of the process according to the invention.

Example 1

720 g of ammonium jarosite were placed in an open container and 500 ml of an aqueous solution of 500 g of sodium hydroxide were added. The mixture was heated to boiling and ammonia gas was expelled. The mixture was then homogenized for 5 minutes using a high speed stirrer and poured into a 1 liter autoclave. The system was heated at a rate of 3 ° C / min to a temperature of 250-260 ° C and maintained at this temperature for 2 hours and 45 minutes. The autoclave was then allowed to cool to room temperature with rotation. The reaction product, platelet hematite (295 g), was isolated by filtration, washed and dried.

Example 2

760 g of glockerite was introduced into a 1 liter stirring autoclave and 600 ml of an aqueous solution containing 500 g of NaOH were added. After heating at 1 ° C / min, a temperature of 180-200 ° C was maintained in the autoclave for 6 hours with stirring. As in Example 1, 430 g of platelet hematite having an average particle size of 24-27 microns was then isolated.

Example 3

A 1 liter stirrer autoclave was charged with 480 g of ammonium jarosil and stirred with a submerged high speed stirrer in 750 ml of an aqueous solution of 360 g of NaOH in 15 minutes.

The stirrer was then removed and the autoclave was closed and heated to 160-190 ° C with a conventional stirrer at a rate of 1 ° C / min. The temperature in this range was maintained for 4 hours. After cooling, platelet hematite (232 g) having a particle size of 15-22 microns was isolated.

Example 4

600 g of a mixture of basic sulphates originating from an acid leach of a polymetallic ore, neutralized with ZnO and then with NaOH, containing 47.6% Fe ₂ O, were charged into an open vessel. This mixture was horogenized for 10 minutes in a solution of 600 g NaOH in 750 ml water using a high speed stirrer while heating to about 90 ° C. Then the reaction mixture was transferred to a rotary autoclave and heated at 10 ° C / min to 320-350 ° C. This temperature was maintained for 1 hour. After cooling, 274 g of platelet hematite having a particle size of 25 to 28 microns was obtained.

Example5 »

Into the open vessel was charged 970 g of sodium jarosite, obtained by neutralizing an acid leach of polymetallic ore, containing 15.7% Al2O4. Then 800 ml of an aqueous solution containing 512 g of NaOH was added and the mixture was homogenized with a high speed stirrer for 10 minutes. The mixture was then transferred to a 1.5 L rotary autoclave and an additional 512 g of sodium hydroxide added. After closing, the reaction mixture was heated at a rate of 3-4 ° C / min to 200-230 ° C with constant rotation. This temperature was maintained for 3.5 hours.

After completion of the reaction, the autoclave was cooled and 371 g of platelet hematite having a particle diameter of 25 to 30 microns was isolated.

Claims (1)

SUBJECT OF THE INVENTION A process for producing alpha-iron oxide in the form of platelet crystals over 20 microns in diameter by converting basic ferric sulphates such as jarosites of the formula MeFeF / SO ^ ^ / OH / g, where Me is an alkali metal, ammonium or hydronium cation, Fe? SO? / OH / ₁₀ or mixtures thereof by treatment with sodium hydroxide solution at a temperature of 150 to 350 ° C, characterized in that a homogeneous reaction mixture of said basic ferric sulphates and a solution of 5 to 20 moles of NaOH / l is formed, whereupon the reaction mixture is heated in a temperature mode consisting of heating at a rate of 1 to 10 ° C / min to a selected temperature and a holding time of 0.5 to 1 hour at a selected temperature of 300 to 350 ° C, 1 to 2.75 h in the 250 to 300 ° C, 2.75-4 h in the range of 200-250 ° C or 4-6 h in the range of 180-200 ° C.