High-Intensity Magnetic Separation of Limonite Iron Ores

Results of investigation into the development of HIMS technology 
for separation of weakly magnetic finely disseminated limonite iron ores are 
discussed.


INTRODUCTION
Among the world iron ore deposits, the llmonite ores are the third most abundant source of iron. The overwhelming part of these ores is represented by low-grade varieties which cannot be used in iron and steel industry without preliminary beneficiation.
Low contrast properties of metallic and non-metallic minerals, fine dissemination and their tendency to sliming represent basic difficulties in the development of technologies for beneficiation of these ores.
Successful application of high-intensity magnetic separation in beneficiation of hematite ores forms a basis for a wider use of this technological process for treatment of such difficult-to-treat ores as limonite.
This article presents the results of investigations performed at the Mekhanobrchermet Institute to develop a technology for beneficiation of the Kremikovsky (Bulgaria) and Kerch (Ukraine) limonite ores by high-intensity magnetic separation.

MAGNETIC SEPARATION OF LIMONITE ORES FROM THE KREMIKOVSKY DEPOSIT
Two samples of limonite ores representing 60 per cent of iron ore from the deposit were investigated. The ores are characterized by varying mineral composition, textural and structural peculiarities and complex mineral intergrowth.
The metallic minerals are represented by iron hydroxides, hematite, manganese oxides and hydroxides containing barium and lead, magnetite, plumbojarosite, clayey minerals, barite and quartz. Mineral composition of the ore is shown in Table I, while the chemical composition is given in Table II. Tables III and IV summarize macroscopic description and morphological characteristics, respectively. To evaluate the metallurgical properties of the ore, sedimentation, gravity separation (in heavy suspensions with specific gravities of 2800, 4000 and 4200 kg/m 3) and magnetic separation (magnetic flux density up to 1.2 T) were carried out and magnetic characteristics were measured.  The results of gravity separation indicate the presence of iron minerals in fractions of specific gravity of 2800 to 4200 kg/m ; fractions with specific gravity greater than 4200 kg/m are rich in barite.
The maximum values of specific magnetization and Sl.,cific magnetic susceptibility are characteristic for + 10 tm 45 m fraction, as can be seen in Figure 1.
The dependence of specific magnetic susceptibility and specific magnetization on size of the material, at magnetic field intensity of 80 kA/rn (1 kOe) (curves 1 and 3), and 800 kA/rn (10 kOe) (curves 2 and 4).
Magnetic characteristics of the products investigated depend both on magnetic properties of gangue minerals and on the presence of magnetite.
Laboratory tests were performed using a laboratory magnetic separator with horizontal magnetic field, height of the separation zone of 220 mm and the maximum magnetic flux density of 1.5 T in the separation zone.
A three-rotor magnetic separator with horizontal rotor was used in pilot-plant tests. The height of the separation zone in each rotor was 220 mm. The maximum magnetic flux density in the upper rotor was 0.4 T while 1.4 T was available in the other two rotors.
The non-magnetic fractions from the upper and middle rotor were cleaned in middle and lower rotors, respectively. Each rotor has two independent power supplies and, consequently, six operations can be performed simultaneously.
The 6-ERM 35/315 separator is a commercial prototype of which six units have been installed at the Central Mining and Beneficiation Complex in Krivoy Rog.
The following matrices were used in the laboratory and pilot-plant magnetic separators: Grooved plates with3.2 mm pitch and 2 mm gap Grooved plates with 3.2 mm pitch and 4 mm gap Combination of grooved plates and expanded metal sheets.
Size distribution in laboratory tests was maintained at 30 to 65 per cent -74 m, the feed density ranged between 35 and 40 per cent solids.
It was found that when the concentration of the final size fraction in the feed increases, the iron content in the product and the recovery of iron increase. Further size reduction results in a considerable reduction of the recovery, as a result of low efficiency of recovery of-10m fraction.
When the gap between the plates is reduced from 4 mm to 2 mm, a marked increase in the recovery of iron is observed, while the quality of the product decreases.
The use of the combination matrix which has a higher magnetic field gradient compared to grooved plates matrix results in an increase in iron recovery, as a consequence of the presence of fine grains of feebly magnetic minerals. Results of magnetic separation tests are summarized in Table V.
Chemical composition of the concentrate produced in pilot-plant tests is given in Table VI. The recovery of iron into the concentrate was 74.1 per cent.  Figure 2 is recommended. The flowsheet includes the following operations: ore grinding and classification up to 65 70 per cent 74 m the scalping of the material prior to separation high-intensity magnetic separation of the ore in two stages (double cleaning of the non-magnetic product in each stage); magnetic product from the first stage is used as the feed in the second stage the thickening and the filtration of the concentrate.
It is recommended to use the 6-ERM-35/135 rotor-type highintensity magnetic separator equipped with grooved plates in the upper and middle rotors, and the combination matrix in the lower rotor.

MAGNETIC SEPARATION OF LIMONITE ORES FROM THE KERCH DEPOSIT
The existing processing plant treats two types of the limonite ore: "brown" ore, i.e. coarsely disseminated oolite ore and "tobacco" ore which is finely disseminated with a cement binding. The beneficiation method used is jigging. With 97:3 ratio of brown to tobacco ores (1970) in the mix, the concentrate produced by this technique contained 45.8 per cent of Fe at 75.1 per cent recovery. With 50:50 ratio of brown to tobacco ores (1985) in the mix, the grade of the concentrate decreased down to 44.6 per cent and the recovery of iron dropped to 63.1 per cent.
With a further reduction in the concentration of the brown ore down to 20 per cent, a continuous decrease in the grade to 43.8 per cent is expected. At the same FIGURE 2 The flowsheet for the beneficiation of the Kremikovsky ore "r mass yield (%), a head grade (%) recovery (%), grade (%) time, the recovery is expected to drop to 59.4 per cent. Evaluation of these data indicates that the tobacco variety of the limonite ore is not beneficiated by jigging.
In order to find an effective technology for beneficiation of tobacco ores, laboratory and pilot-plant tests of two samples taken from the operating open pits were conducted.
The iron content in the samples was 39.9 per cent and 38.5 per cent, respectively. The metallic part of the ore is represented by hydrogeothite in fine flaky form. The specific gravity of the ore is 3360 kg/m 3 and the specific magnetic susceptibility is equal to: 5.3x10 6 m/kg at the magnetic field of 80 kA/m (1 kOe) 1.5xlq mZ/kg at the magnetic field of 800 kA/m (10 kOe).
Chemical composition of the ore is shown in Table VII.
Under the laboratory conditions, the gravity analysis was made' and gravity concentration of the feed ore by washing and jigging was carried out. Similarly, magnetic separation of the feed ore, of the underflow after washing and of the jigging tailings was performed. The gravity analysis showed that with 3 to 0 mm size it is possible to separate only a part of the high-grade concentrate. The main mass of 50 per cent Fe can be produced at the grinding size up to 0.5 0 mm.
Microscopic observation of products after the gravity analysis indicated that the liberated oolites were concentrated in 1 + 0.25 mm size fractions. High-grade and low-grade intergrowths are present in-3 + 1 mm size fraction, non-metallic cement prevails mainly in 0.25 mm size fraction.
Comparison of different flowsheets for beneficiation of tobacco ore using pilotplant circuits with capacities of 0.1 t/h and 2 t/h showed that the best process parameters were obtained with the flowsheet which includes crushing the feexl ore to 5-0 mm, washing the crushed ore in a classifier, screening the underflow after washing at 1 mm, separation of the final concentrate into undersize, regrinding the oversize to 96 per cent 5 mm, HIMS of the ground product combined with the overflow after washing, concentrate thickening and filtering. The flowsheet is depicted in Figure 3. Two types of HIMS separators were used in the tests: a rotor-type separator with horizontal magnetic field and with separation in a pulp flow (in which the pulp is fed directly into the air gaps between the matrix elements); and a carousel-type separator with vertical magnetic field and with separation in aqueous medium (in which case the matrix is pre-filled with water).
Magnetic flux density in the separation zone was 1.2 to 1.7 T. The grooved plates and expanded metal sheets were used as a matrix. The unit with separation in aqueous medium which offers the treatment of both grainy material and of the slimes after washing was the most efficient machine for a given application.
The chemical composition of the concentrate is shown in Table VIII. The recovery of iron into the concentrate was 55 per cent.

CONCLUSIONS
It was shown that high-intensity magnetic separation can be used to treat limonite iron ores from two deposits. The process equipment that makes the application of this technology to various deposits of limonite ore possible has been developed in the Ukraine. Dr. Maliy's main scientific interest lies in the beneficiation of weakly magnetic difficult-to-upgrade iron ores. He is a chief technologist of the project on oxidized ores, currently under construction at the Krivoy Rog Mining and Beneficiation Complex. He is an author of numerous papers, including contributions to the VIIIth, XVIth (Stockholm) and XVIIth (Dresden) International Mineral Processing Congresses.