DEVELOPMENT OF A COMBINED GRAVITY-MAGNETIC SEPAItATION PROCESS FOR MAGNESITE ORE USING HGMS

Most magnesite deposits in Yugoslavia contain serpentine and olivine as impurities. Heavy medium separation is used for the beneficiation of raw magnesite ore with particle size greater than 0.35 ram. The results of investigations reported in this work indicate a possibility of efficient removal of serpentine and olivine using HGMS and superconducting magnetic separators for fines smaller than 0.35 ram. Based on these results a new technological process for magnesite upgrading was established. The process combines the gravity and magnetic separation processes for treatment of raw material, as well as for magnesite after its decarbonisation. It is possible to upgrade the magnesite fines below 0.35 mm which are rejected from the conventional processes. The grade of the magnesite concentrate of 98% and above may be obtained using this process.


INTRODUCTION
The plant with gravity separation in magnetic suspensions built in the magnesite mine "Goles" contains two technological circuits: -gravity concentration of raw magnesite, preconcentration and -gravity concentration of the pre-concentrate from the first circuit, after decarbonisation and hydration.
The flowsheet of this plant is shown in Fig. 1 [1].The technology of the process consists of the following stages: ao The crushing of the ore to the desired size limit which depends on the required degree of liberation of impurities on the one hand, and on the capabilities of the concentrating equipment and imposition by the subsequent products (the production of the dead-burned magnesite and refractory bricks) on the other hand.
bo Heavy medium separation to discard light fractions containing low-density gangue.This stage is necessary only if the ore contains considerable amounts of light-density impurities (below 9.400 kg/ma).

Co
Calcination of the sinks to decarbonise carbonates.The calcination temperature is conditioned by the following factors: if the impurities are only SiO-bearing minerals, the calcination temperature is between 870 K to 1370 K, and depends only upon the sintering process to be subsequently applied on cleaned products when the magnesite ore contains dolomite beside SiO as gangue, the calcination temperature should be below 950 K so that only magnesite is decarbonised and dolomite remain unaffected.should the magnesite ore contain, among other impurities calcium carbonate, the calcination temperature should go up to 1370 K in order to bring up full decarbonisation.Calcined products are then immersed in a water solution of a surface-active reagent.Under such conditions, beside other reactions, free CaO is hydrated to Ca(OH)2.This calcium hydroxide forms the "milk of lime" with the liquid phase and is easily washed away with water.
do Decarbonised products are then subjected to hydration which takes place in a water solution of a surface-active reagent of the type: Cfi-12n+ -----SO3Na and CnH2n+ )-O (CH:CH:O)m H The hydration process represents the beginning of the second concentration stage. e.
Separation in heavy medium of decarbonised and hydrated products.In this stage the magnesium oxide and magnesium hydroxide float while silicate-bearing impurities such as undissociated carbonates (dolomite) report in the sink product.It should be noted that densities of decarbonised and hydrated products are reduced to about 2000 kgm3.
The conditioning of products with surface-active reagents in the hydration stage and in the concentration process itself is applied in order to prevent flocculation, to increase the wetting of minerals and to disperse fine particles.The surface-active reagent plays a very important role in separation of decarbonised and hydrated magnesite products, taking into account the fact that heavy medium separation is carried out at a high pH value (10)(11).By decreasing the surface tension of water to about 0.032 N]m, on top of the effect already mentioned, the surface-active reagents speed up the rate of hydration due to better wetting and keep up dispersion of the fines which is favourable to calcium hydrate washing.It is to be noted that under such conditions, the medium is kept dispersed during hydration and is prevented from blocking this reaction.This has been proved by a large number of tests.By this technological process can only be treated material with grain size +0.35 mm.The material with grain size -0.35 mm, which is contained in total mass of 31.7%, cannot be treated by this process.Table 1 shows the particle size distribution of class -0.35 mm.As a result of significant concentration of these particles, the third concentration phase was introduced after the second concentration stage.The third phase represents magnetic concentration by high-gradient magnetic separation (HGMS) at the magnetic field of 2 Tesla [2].The flowsheet of this stage is shown in Fig. 2.
The major part of Ca(OH), as a harmful compound, leaves the system with the slurry, while Mg(OH)2 and serpentine with olivine as harmful components proceed into further concentration, because they are the main carriers of SiO.The laboratory experimental results and those from the industrial-scale tests on magnesite ore from Goles were reported in detail in [1,3,4,5,6].Final results of gravity concentration in two stages are summarised in  The reagents were used in the hydration process and in the second stage of gravity separation because of very high pH (10-11) of the suspension.In order to overcome flocculation in the process of magnetic concentration, the same reagents were used.
For the wetting angle 8 0, the minimum particle diameter d which can be held by the matrix, is calculated using expression ( 1 where k 0.65, taken from earlier experimental work [7]. After desliming (removing classes smaller than --0.023 mm) which cannot be treated at the magnetic field of 2 T the sample is once again treated by the water solution of the surface-active reagents, as a result of the fact that hydration of magnesium has not been completely finished.Subsequently, the tests were carried out.The results obtained in these tests are summarised in Table . 3. In can be seen from Table 3 that the size fraction ---0.023mm which represents more than 14% of the total mass could not be treated by the magnetic separator owing to insufficient magnetic field (maximum 2 T).As a result of the experimental data which confirmed the theoretical determination of the minimum particle size as a function of the magnetic field strength it can be concluded that the magnetic field up to 5 T is necessary for this type of magnesite.It means that additional 12% of magnesite could be upgraded.It follows from eq.( 1) that the minimum particle diameter of 0.008 mm could be separated at 5 T, compared to 0.023 mm from previous HGMS experiments.According to literature, superconducting magnetic separators with magnetic field in excess of 5 T are available and further investigations will performed in this direction.By introducing such separators to the beneficiation of the magnesite ore it will be possible to recover material rejected from the hydration process of the caustic magnesite and from the gravity concentration process.
(Paris), October-November 1980, p. 437   R. Ignjatovic, R. Stanojlovic, M. Ignjatovic, D. Milanovic: Influence de la tension superficielle de l'eau prealablment conditionnee avec un agent tensio actif appropfie sur la separation manetique haute intensive par voie humide.Congress de l'Industr.Miner.Ales, Ales (1989), 128 R. Ignjatovic, S. Markovic, F. Gerxhaliu: Performances de l'installation de Belgrade, in 1975.Afte graduation, he worked fo ten years in two industrial mineral processing plants for beneficiation of industrial minerals and coal In 1985 he joined the Technical Faculty Bor, University of Belgrade.In 1989 he obtained his M.Sc.degree.His main fields of interest are froth flotation and gravity and magnetic separation.
Mr.Markovic published more than 20 papers in national and international journals, and presented over 15 papers at various symposiums and congresses.He is a member of the Serbian Mining Society and of Serbian Chemical Society.Since 1993 he has been the secretary of the editorial board of the national journal Mining and Metallurgical Journal.

Table 2 .
The final result of two-stage gravity separation

Table 3 .
The results of magnetic separation tests Anomalies de la concentration gravimetrique, en suspension dense, des magnesites finnes et poreuses.D.Sc.Thesis, Faculte Polytechnique de Mons, Belgique, 1971 Z.S. Markovic: Mogucnost gravitacijske koncentracije magnezita sa rudnika Goles u teskim suspenzijama pri predhodnoj dekarbonazaciji (in Serbian), Thesis, University of Belgrade, Technical Faculty Bor, 1975 I. Budic: The influence of the hydration degree previously decarbonised magnesite ore on the removing efficiency of CaO and SiO in the concentration procoesses.(inSerbian).D.Sc.Thesis, University of Belgrade, Technical FacultyBor, 1987M.R. Ignjatovic: Uticaj povr.sinskognaponafluidanakoncentraciju minerala u HGM separatoru (in Serbian).M.Sc.Thesis, University of Belgrade, Technical FacultyBor, 1991Mizoslav R. Ignjatovic was born in 1959.He graduated from the Faculty of Mining and Geology, University of Belgrade.He obtained his M.Sc.degree from the University of Belgrade, Technical Faculty in Bor, in 1991.After graduation he joined the Institute for Technology of Nuclea and Other Mineral Raw Materials in Belgrade and he has worked on projects and studies in gravity and magnetic concentration of mineral raw materials.Mr.Ignjatovic is an author of two monographs, 10 papers in scientific and professional journals.He also participated, with 15 reports, in national and international symposiums and congresses.(ali6 was born in 1945.She graduated from the University of Belgrade, Faculty of Mining and Geology.She obtained her M.Sc.andD.Sc.degrees in 1976 and 1979, respectively, from the above mentioned Faculty.After graduation, she joined the Department of Mineral Processing, Faculty of Mining and Geology, where she is presently the professor.Dr.Calic is an author of a University textbook, of two monographs and of more than 70 papers in national and international scientific journals.Her main field of interest is magnetic concentration and flotation.Zoan $.Markovid was born in 1949 and graduated from the Technical Faculty Bor,University of Nadela