Introduction

An intentional search for chromium ore on an industrial scale in the Karelia-Kola region has only started recently. The location of chromite ores of Karelia (Burakov Aganozer) Kola peninsula (Greater Varak, Sopchezer deposit) are comparable with respect to structure and metallogeny to the well-known ore-bearing massifs of Kemi (Finland), Bushveld (RSA), and Steelworcester (USA) [1]. Chrome-spinelids are the main ore-bearing mineral of these deposits and are classified as an iron variety containing 45 – 52% Cr2O3.

Prediction of the resources of these ores in the Karelia-Kola region is estimated in several hundreds of millions of tons. Chromite ores are used for preparing chrome-magnesite and magnesite-chromite refractories, the main ones of which periclase and chrome spinelids. These refractories are used extensively in view of replacement of dinas in arches and other elements of a lining of steel-smelting furnace. The arches of electric melting furnaces, made from magnesite-chromite, compared with dinas permit an increase in temperature within the melting space by approximately 100°C, which facilitates melting of alloy steels, and improving a furnace campaign.

State of the Question

The name chromite may concern both the mineral composition (Mg,Fe)O·(Cr,Al)2O3, and also rock containing more than 80% this mineral. This formula also corresponds to minerals of chrome-picotite and chrome-spinelid. Chrome-spinelid is encountered almost exclusively in magmatic ultra-basic rocks in the form of disseminations and complex accumulations. Harmful impurities reducing chromite refractoriness concern serpentine (magnesium hydrosilicate), olivine, and chromium-containing chlorites. Chlorite or magnesium hydroaluminosilicate impurities are present in rock together with serpentine.

Ores with a Cr2O3 content not less than 33% are suitable in order to produce refractories; ores with a lower Cr2O3 content are unsuitable for refractory production [2]. The most valuable deposits of Russia are the Don in the western flank of the southern part of Urals and Saranov in the west Urals. The chrome-spinelid part of the ore of these deposits is represented by chrome-picotite and the non-ore part is represented by a mixture of serpentine and chlorite. Other contaminating admixtures of the Ural chromite deposits are chalcedony, hematite, carbonates, and their ores are different with respect to physical properties and structure in a natural con dition. The Saranov ore has a massive stone structure, and ore of the Don deposit contains powder-like and loose ores; before use they are enriched with respect to 48% Cr2O3 content. Magnesia-spinelid materials used in smelting steels and other alloys have been studied by domestic scientists [1,2,3,4]. Chromite ores of the Burakov-Aganozer deposit of Karelia and other deposits of the Karelia-Kola region are classified as lean with respect to Cr2O3 content requiring enrichment.

Production tests of ores have been conducted on a sample weighing 600 kg in the Kola Scientific Center of the Russian Academy of Sciences. A gravitation scheme has been proposed for enrichment with recleaning of concentrate by electromagnetic separation. Ore samples on which production studies were conducted contained, wt.%: Cr2O3 26.5, Al2O3 7.0, SiO2 25.3, MnO 0.02, MgO 6.96, CaO 4.97, K2O 0.39, Na2O 0.20, Fetot. As a result of enrichment chromite ore chromite concentrate containing 47.5 wt.% Cr2O3 was obtained and tested during production of forsterite-periclase refractories. It has been established that chromite concentrate may be used in order to prepare fuzed

chromite-magnesite refractories and spinels. Synthesized magnesia-alumina and chromium-aluminum-magnesium spinels may be used during smelting steels in a vacuum induction furnace and for crucibles of these furnaces there are chemically pure oxides of magnesium, calcium, aluminum, zirconium, and some of their compositions.

Pure zircon (see Fig. 1) exhibits low linear expansion, and therefore deserves attention, although in the majority of countries for lining induction crucible furnaces during cast iron melting there is mainly use of quartzite with an increased volume within the limits of 5% after 3 h exposure at 1400°C. However, it is considered that only alumina and magnesia mixtures will be used in future since the rest of the liming mixtures have some disadvantages limiting their further application.

Fig. 1.
figure 1

Linear expansion of different materials.

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Recently in order to prepare high quality refractories there has been use if chemically synthesize compounds, synthesis methods have been developed, and there is use of magnesium chromate and bichromate for preparing refractory objects. In addition, there is an intensive search for technology for producing objects based on chrome-alumina-spinelid [6, 7]. Production tests for chromium oxide refractories with different density have confirmed their good corrosion resistance towards alkali-free aluminoborosilicate glass. The use of chromium oxide refractories makes possible by a factor of three to increase the furnace service life and improve glass quality [8].

The optimum composition has been established for thermally stable ceramic based on magnesium chromite with an overall ratio of molar fractions of MgO and Cr2O3 in a charge of 0.6, and Cr2O3 coarse fraction grain size of 0.25 – 0.60 mm. The material obtained based on magnesium chromite exhibits good heat resistance and other engineering indices [9].

A number of periclase and periclase chromium-containing refractories have been studied in an MLB (melting in a liquid bath, copper metallurgy) bath. The maximum life may be provided by melted and cast chromite-periclase refractories with addition of Al2O3 [10]. It has been established that the best stability within the lining of an MLB furnace is demonstrated by periclase-chromite refractory prepared from fuzed materials. Technology has been developed for preparing fuzed chromite-periclase used for obtaining fuzed periclase-chromite refractories according to TU 14-8-368–81 [11].

Chromite refractories with a high Cr2O3 content are used in the form of ramming mixes for refractory object pieces. Magnesite refractories based on magnesium oxide with addition of silica, alumina, iron, calcium, or chromium oxides are distinguished by good resistance to the action of basic slags. The raw material for preparing these refractories is magnesite or purified magnesium oxide.

From experience of the Ural research school (P. S. Mamykin, K. K. Strelov, and others) it is well known that in order to prepare magnesite-spinelid refractories with maximum heat resistance the optimum ratio of chromite and sintering magnesite is 30:70, and refractory slag resistance is improved with a reduction in chromite content to 20 wt.%. During service in steel melting furnaces magnesite-chromite objects with 20 – 30% chromite are more stable than with a greater content, and include 60% periclase, 12% magnesioferrite, 17% chrome-spinelid, 6% forsterite, and 5% monticellite. As far as charge grain size composition is concerned, then there are no fractions of chromite finer than 0.5 mm and magnesite is added in the form of fractions finer than 1.0 mm. Thermally stable objects of periclase-chromite composition contain a 50% fraction of coarse magnesite and 50% fraction of finely milled mixture of chromite and magnesite.

Research Procedure

Fired raw material was milled and a charge was prepared with addition of plastifier (sulfite-yeast mash concentrate).

Specimens were compacted from a charge under a pressure of 150 MPa 36 mm in diameter and 50 mm thick that were fired at 1700°C. Charge composition: 75 wt.% fuzed periclase-chromite fraction 4 – 1 mm and 25 wt.% binder. For comparison as a binder a mixture of 75% chromite and 25% sintered magnesite was used [2]. Specimens were compacted under a pressure of 170 MPa, and has been fired at 1770°C for 4 h.

The possibility of using chromite concentrate (10 wt.%) obtained from ore of the Greater Baraka Kola peninsula deposit was checked during production of forsterite-chromite refractory using within a charge composition 20 wt.% of sintered magnesite with addition of 70 Kovdor deposit olivinite. The temporary sintering additive used was polyvinyl alcohol in an amount of 8 wt.% (above 100%). [1].

Test Results

Fired specimen properties: ultimate strength in compression 52 MPa, open porosity 12.%; temperature for the start of deformation under load of 0.2 MPa 1680°C, thermal stability 7 thermal cycles (1300°C – water), LTEC 12.7 × 10–6 K–1, gas permeability 13 nPm. Specimen chemical composition, wt.%: SiO2 1.75, Fe2O3 5.7, Al2O3 2.4, CaO 1.45, MgO 77, Cr2O3 12.5. Mineral composition, wt.%: periclase with inclusions of secondary spinelid of complex composition 89, residual spinelid 6, silicates in the form of monticellite, forsterite, and β-2CaO·SiO2 4. Object resistance for melting 0.7 – 0.75 mm.

Physicochemical Processes During Periclase-Spinelid Object Service

With one-sided heating of refractory objects there is impregnation with melt components rich in iron oxides. Objects in this case acquire a zonal structure and consist of three zones: least changed, transition (with a changed structure), and working (with a changed chemical composition and structure).

The main changes proceed primarily within the working zone, especially in a region that was in contact with molten metal and slag of the furnace workspace. As a rule this region has a dense structure, and its fracture has a metallic luster and consists of spinelids of complex composition; individual particles (grains) of chromite and periclase are not differentiated. The central region of the working zone is composed of periclase and spinelids, but grains of chromite are absent and pores are observed in their place. The upper region of a working zone (coldest) is composed of chromite, periclase, forsterite, and monticellite silicates. This region of a working zone has the same density as the region in contact with molten metal and slag. The iron oxide content in the contact region with molten metal and slag comprises 38 wt.%, the content of MgO and Cr2O3 in total is 8.5%, and CaO and SiO2 in total are 6.5%.

The change in refractory chemical composition proceeds as a result of melt migration under the effect of a temperature gradient and other factors. In this case silicate melts migrate in a direction from a high temperature to the lowest, and iron-manganese compounds migrate towards a high temperature. In the central part of transition zone refractory porosity and pore size increase. Temperature variations in the furnace increase stress within a refractory lining, causing development of cracks and spalling of part of an object. Magnesite-chromite objects wear predominantly as a result of this spalling, and not as a result of melting, as for dinas refractories, although under action of a high temperature and reagents of the melting space they also melt.

Refractory wear by melting depends on slag basicity. If the slag is acid, then there is transfer of refractory material into slag (melting). Under action of basic slags the refractory – slag boundary should remain unchanged and only at high temperature (1700°C) does the refractory surface melt.

Conclusion

The mineral and chemical composition of chromium ore of the Burakov-Aganozer deposit of Karelia compared with the mineral and chemical compositions of chromite ore of the Kola peninsula (Greater Baraka) and chromite concentrates obtained during ore enrichment of these deposits have been studied.

A two-stage gravitation-magnetic scheme of ore enrichment has been developed by which chromite concentrate is obtained containing 47% Cr2O3 (Naihuo Cailiao extraction 67.38%). Technological developments of antiburning materials for casting iron and steel have demonstrated the possibility of using chromite concentrate and chromite ore of the Karelo-Kola region in preparing refractories and molding materials, and also the promising nature of the behavior of subsequent exploration of the Karelian chromite ore deposits.

From the point of view of obtaining the maximum thermal stability of magnesite-chromite refractory chromite is the most expedient component within a mixture with magnesite. Neither sintered magnesite nor chromite separately are thermally stable, although a combination of them at high temperature creates in magnesite-chromite refractory a microcracked structure that also gives rise to their good thermal stability.

Periclase-chromite objects from normal magnesite powders and enriched chromite exhibit increased temperature for the start of deformation under load (1630°C) and heat resistance of ten thermal cycles. With a 90:10 ratio of finely milled enriched magnesite and chromite object thermal stability increases to 14 thermal cycles. Periclase-spinelid refractories from enriched chromite contain 7 – 8% silicates instead of 11% for traditional (standard) objects.

Chromium oxide forms with magnesite a very stable spinel (chrome-magnesite) that due to high bond energy in a reducing atmosphere and a vacuum is more resistant than pure periclase [15]. In periclase-spinelid objects made from enriched materials there is 4 – 5% silicates instead of 8 – 12% in refractories prepared from normal powders and also within them (from enriched materials) there is formation of a high quality structure containing more than 50% of direct intergranular bonds between highly refractory minerals. Within structural elements of these refractories intergranular bonds of the periclase-secondary spinel-periclase type predominate.

Sintering of mixtures up to 1750°C proceeds due to removal of open pores. Addition of chromite to magnesite only facilitates sintering with addition of small amounts, decreasing with an increase in temperature. This reduction is determined by the amount of chrome-spinelid (~ 2.5% of the limiting possible concentration) dissolved in periclase under these conditions. However, at 1750°C and addition of 20% chromite material porosity is quite low and comprises 2.5 – 3.0%.