INVESTIGATIONS ON THE PURIFICATION OF MAGNETITE IRON ORES FROM IMPURITIES BY THE AUTOCLAVE–ALKALINE METHOD

N. P. Kurin, N. F. Stas, V. F. Dabizha, E. V. Cherepanov

It is known that physicochemical methods of ore beneficiation, flotation, and also combined methods developed on their basis make it possible to obtain iron-ore concentrates containing, in the best case, 0.5% silicon dioxide and a total of 1.5% impurities /1/. Such a concentrate does not meet the requirements either of powder metallurgy, where it is processed by reduction methods into powdered and sponge iron, or of electrical engineering, where it is used to make plates for high-capacity alkaline storage batteries. The required degree of purity of the concentrate can be determined from the requirements imposed on the product of its reduction—powdered iron—since in reduction processes neither burnout of mineral impurities nor additional contamination occurs. Accordingly, the concentrate should contain: iron not less than 71.8%, \(SiO_2\) not more than 0.15%, and all other impurities \((CaO, MgO, TiO_2, MnO)\) not more than 0.35%. Obtaining a concentrate of such high purity is possible only by chemical means.

At the Tomsk Polytechnic Institute an autoclave–alkaline method has been developed for purifying iron ores from impurities; in experiments with Krivoi Rog ore it made it possible to obtain a concentrate with a residual content of silicon and aluminum oxides of 0.01% /2/. At the same time, an equation was derived theoretically and confirmed experimentally which describes the leaching process under a linear temperature increase in the autoclave:

\[ \lg \frac{2.3 \cdot 10^{6}\lg \frac{1}{1-\alpha}}{T^{2}} = A-\frac{E}{4.575T} \tag{1} \]

where \(\alpha\) is the degree of leaching at temperature \(T\) \((^\circ K)\);
\(E\) is the activation energy of the process;
\(A\) is a constant depending on the rate of temperature increase /2/.

Investigations of the autoclave–alkaline method for ore purification were continued at the Krasnoyarsk plant “Sibelektrostal.” The work used ore from the Olenegorsk deposit with an iron content of 64.5% and \(SiO_2\) of 8%. The choice of Olenegorsk ore is explained by its low content of titanium and manganese impurities—

which are undesirable in some powder-metallurgy operations, by the desire to test the possibilities of the autoclave-alkaline method on ores differing in composition and structure, and also by the need to fulfill a planned assignment to obtain a large batch of iron-ore superconcentrate for its further testing. In contrast to the Krivoi Rog ore, where the principal mineral is hematite with a small admixture of goethite, the Olenegorsk ore consists mainly of magnetite, \(\mathrm{Fe}_3\mathrm{O}_4\), with a slight admixture of hematite and pyrite. The non-ore impurities are represented chiefly by quartz, which occurs both in free form and in the form of intergrowths and chemical compounds with magnetite and other non-ore impurities; amphibole, chlorite, feldspar, garnet, mica, and carbonates are also present.

Taking into account the novelty of the object of study, laboratory experiments were first carried out to refine the technological parameters of the leaching process. The experiments were conducted in a 2-liter stainless-steel autoclave, Kh18N10T, with a propeller stirrer (150 rpm) and external electric heating. In each experiment, 100–200 g of ore and 400–600 g of caustic-soda solution were used. After the ore and solution had been charged, the autoclave was sealed, stirring and heating were switched on, and, upon reaching the specified temperature, the starting time of the experiment was recorded. After holding the autoclave for a specified time at the given temperature, heating and stirring were switched off, the excess pressure was released, and the pulp from the autoclave was discharged through an opening in the bottom into a stainless-steel vessel, in which settling, separation of the solution, and washing of the concentrate with water by repeated decantation were carried out. The washed concentrate was filtered and dried at \(100^\circ\mathrm{C}\). During the experiments, the process parameters and the \(\mathrm{SiO}_2\) content before and after leaching were monitored.

In the first series of experiments, the so-called “dust” of Olenegorsk commercial concentrate was used; in the process of obtaining this concentrate, the “dust” is carried off by gases from the drying furnaces and is collected in cyclones. The chemical composition of the “dust” is characterized by a reduced content of impurities and is expressed by the following data (%): iron — 70.1; \(\mathrm{SiO}_2\) — 2.24; \(\mathrm{Al}_2\mathrm{O}_3\) — 0.19; \(\mathrm{CaO}\) —

— 0.10; \(Al_2O_3\) — 0.19; \(TiO_2\) — 0.25; \(MnO\) — 0.05; particle size mainly (96.5%) less than 0.053 mm. In the experiments, a caustic-soda solution according to GOST 2263–59 was used. The effect of the duration of the process on the completeness of leaching of silica impurities was studied. The data obtained are presented in Table I.

Table I

Effect of leaching time on the degree of purification of Olenegorsk iron ore from silica impurities at a temperature of \(170^\circ C\), \(NaOH\) concentration 50%, and L:S = 4

It is evident from the table that magnetite Olenegorsk ore, by means of the autoclave-alkaline method, is purified to a considerable extent from silica impurities. Noteworthy are the data characterizing the kinetics of the process: most of the silica (68%) is leached during the first 30 minutes; apparently, this is the portion of the impurities located in the surface layer of the particles, as well as free quartz inside the grains. With a further increase in the duration of the experiment, the increment in the degree of leaching decreases sharply, which can be explained by the entry into reaction of complex, chemically stable compounds of silica with iron oxides and other impurities, as well as by diffusion inhibition as the reaction proceeds into the interior of the ore particles.

The second series of experiments was carried out using the same initial product, but preliminarily beneficiated by wet magnetic separation (WMS), as a result of which the content of the \(SiO_2\) impurity decreased to 1.2%. In the experiments, the temperature was varied; the other parameters were kept constant. Obtaining

The results are given in Table 2.

Table 2

Effect of temperature on the process of leaching silicon impurities from Olenegorsk iron ore at a NaOH concentration of 50%, L:S = 4, and a time of 1 hour

Experiments showed that, as the temperature is increased, the degree of leaching and the purity of the product increase; however, this increase is not as substantial as in experiments with Krivoy Rog ore, where, when the temperature is raised by 10°C, the degree of leaching increases by 15–20% /2/.

Comparison of the result of the experiment at 170°C from this series with the preceding experiments at the same temperature (Table 1) indicates that reducing the $SiO_2$ content in the initial product from 2.24 to 1.2% decreases the reaction rate, but makes it possible to attain, in the same time, a higher purity of the concentrate obtained (0.2% instead of 0.5%).

Processing of the experimental data presented in Table 2 by equation (I) showed that this equation can be used to describe the process of leaching silicon dioxide from Olenegorsk ore. At the same time, with increasing temperature a change in the activation energy is observed: at temperatures below 160°C this value is equal to 1.45, and at temperatures above 160°C it is 14.8 kcal/mol. An analogous phenomenon is observed in the leaching of silicon impurities from Krivoy Rog ore; however, in this case the change in activation energy occurs not at 160°C, but at a temperature of 135°C /2/.

Thus, in accordance with the experimental data and the results of their processing by equation (I), which linearizes ...

stage of the process at temperatures below \(160^\circ\mathrm{C}\) is diffusion; in this case the degree of leaching, depending on the temperature in the autoclave, can be calculated from the equation

\[ \lg \frac{2.3 \cdot 10^6 \lg \frac{1}{1-\alpha}}{T^2} = 1.52 - 320/T \tag{2} \]

At higher temperatures the limiting stage becomes the rate of the chemical reaction, and in this case the dependence of the degree of leaching on temperature is expressed by the equation

\[ \lg \frac{2.3 \cdot 10^6 \lg \frac{1}{1-\alpha}}{T^2} = 8.29 - 3240/T \tag{3} \]

The equations obtained make it possible to calculate the degree of leaching of \(SiO_2\) at various temperatures and, conversely, the process temperature required to obtain a concentrate of a specified purity.

Further laboratory experiments were carried out on commercial concentrate from the Olenegorsk Mining and Beneficiation Plant, which was additionally ground to a size of 0.053 mm and enriched by the MMS method. In order to facilitate the diffusion processes, as well as the washing of the ore after leaching, the concentration of caustic soda in the solution was reduced to 33%. In the experiments, which were carried out at a constant temperature of \(170^\circ\mathrm{C}\), the time and the L:S ratio were varied. The results obtained are presented in Table 3.

Table 3

Effect of time and L:S ratio on the degree of purification of Olenegorsk iron ore from silicon impurities at a temperature of \(170^\circ\mathrm{C}\) and a \(NaOH\) concentration of 33%

It is evident from the table that the degree of leaching of silicon impurities increases with an increase in L:S and in the duration of the process; in all experiments, with the exception of the first, the residual content of $\mathrm{SiO_2}$ in the ore is lower than that required for the production of higher-grade iron powder. Additional experiments carried out at a temperature of $170^\circ\mathrm{C}$ using more dilute $\mathrm{NaOH}$ solutions (25 and 20%) showed that a decrease in concentration leads to an increase in the residual $\mathrm{SiO_2}$ content, respectively to 0.15 and 0.20%, and is irrational.

On the basis of the results of the investigations carried out, the following regimes may be recommended as optimal for leaching impurities from Olenegorsk ore: particle size of the material not more than 0.053 mm, temperature not less than $170^\circ\mathrm{C}$, $\mathrm{NaOH}$ concentration not less than 30%, weight ratio of alkali solution to ore L:S 4:1, process duration not less than one hour.

Because the plant lacked a special industrial autoclave, the production of a large batch of concentrate was carried out in a horizontal autoclave intended for boiling liquid glass, having the following characteristics: capacity $3\ \mathrm{m^3}$, operating temperature up to $150^\circ\mathrm{C}$, operating pressure up to 4 atm, heating method—“live” steam, mixing method—rotation of the shell at a speed of 9.5 rpm, material—steel 3. In view of the low operating temperature, the time of ore treatment in the autoclave was increased to 3 hours, and the concentration of caustic soda—to 40%. As the starting material, the “slime” of the Olenegorsk concentrate was used, previously treated by wet magnetic separation.

A single charge of ore and alkali into the autoclave amounted to 500 and 2000 kg; heating and pressure build-up lasted 30–40 minutes. During heating and the leaching process, 400–450 kg of “live” steam were consumed, as a result of which the alkali concentration by the end of the operation decreased to 33%, and the L:S ratio increased to 5:1. After leaching, the ore was washed free of alkali with hot water by repeated decantation until a neutral reaction of the wash liquid was obtained. After washing, the pulp was filtered on Nutsche filters; the concentrate with a moisture content of 10–15% was dried on an electrically heated tray to an air-dry state. Nine leaching operations were carried out, as a result of which about 5 t of concentrate were obtained with the following content (%): iron—71.8; $\mathrm{SiO_2}$—

—0.14; $Al_2O_3$—0.11; $TiO_2$—0.07; $MnO$—0.10; $CaO$ and $MgO$—0.12; the total impurity content is 0.54%.

The concentrate obtained is suitable for producing the highest grades of iron powder and for the manufacture of storage batteries. At the Sulin Metallurgical Plant the concentrate was reduced to iron powder, which in purity surpasses the analogous product of foreign firms. A batch of iron powder was sent to the Volga Automobile Plant for the manufacture of experimental automobile parts.

The results obtained by no means exhaust the possibilities of the autoclave–alkali method. Using equation (3) given above, it is not difficult to show that increasing the leaching temperature to $220^\circ C$ will make it possible to bring the residual $SiO_2$ content in the ore down to 0.012% and, correspondingly, to reduce the content of other impurities. Consequently, in the industrial use of the autoclave–alkali method for purifying magnetite ores, it should be recommended that the leaching process be carried out at a higher temperature.

References

1. N. A. Vozova, L. L. Kuzmin, Izvestiya Vuzov. “Khimiya i khim. tekhnologiya”, 8, 5, 623, 1965.

2. N. P. Kurin, N. F. Stas, ZhPKh, 44, 10, 2251, 1971; 44, 8, 1710, 1970.

INDUSTRIAL TESTS OF THE AUTOCLAVE–ALKALI METHOD FOR PURIFYING IRON ORES FROM IMPURITIES

N. P. Kurin, N. F. Stas

In previous works [1, 2] we showed that, when iron ores are treated with sodium hydroxide solutions, impurities of silicon and aluminum are leached into solution in the form of sodium silicate and sodium aluminate. A sharp increase in the rate of the reactions is observed when the process is carried out at elevated temperatures in an autoclave: for example, at $200^\circ C$ the leaching of silicon and aluminum oxides is completed in 15 minutes, at $140^\circ C$ in 2.5 hours, whereas at $100^\circ C$ the process continues for several days. The rate of the process at constant temperature is expressed by a first-...