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14OBTAINING CARBON-CONTAINING COMPOSITES BASED ON ILMENITE AND CHROME CONCENTRATE BY SHS
S. Tolendiuly*", S. M. Fomenko", A. Akishev", N. Rakhym", and D. Kashkynbai"
aThe Institute of Combustion Problems, Almaty, 050012 Kazakhstan
*e-mail: sanat_tolendiuly@mail.ru
DOI: 10.24411/9999-0014A-2019-10206
The creation of new materials and technologies for their production is one of the most important scientific and applied problems of physical materials science. The particular interest presents the new class of materials MAX-phase. These are ternary compounds that correspond to the general formula Mn + 1AXn, where M is a transition metal; A is an element IIIA or IVA of the periodic group of elements, X is carbon or nitrogen. The structural features of their crystal lattices determine the unique combination of the properties of metal and ceramics in such materials. To obtain materials based on MAX phases, various methods are used. The main method of obtaining materials based on MAX phases is sintering, which requires a lot of energy and time. An alternative to sintering is self-propagating high temperature synthesis (SHS) [1]. Such compounds can be present in the carbon-containing refractories and give them the unique properties [2, 3].
Carbonaceous refractory materials have high thermal conductivity, good chemical resistance in contact with metal melts, slag and lining applied for the substructure domain electro thermal furnaces, smelting units for nonferrous metals. The general approach to the synthesis of carbon-containing refractory materials is to conduct aluminothermic solid-phase combustion of metal oxides in the mode of SHS in the presence of carbon. SHS products are a composite material of refractory compounds: aluminum oxide, metal carbide, carbon [4-8].
In this work, the phase composition, microstructure and some properties of SHS materials based on ilmenite and chromite concentrates were studied.
The following reagents taken to get carbonaceous refractory materials: aluminum powder PA-4 (99% purity), ilmenite concentrate, chromite concentrate, carbon in the form of electrode graphite (purity 95%), and silica powder (purity 98, 5%). Silica sol and 15% of MgSO4 aqueous solution were used as a cohesive. The silica sol prepared by hydrolyzing ethyl silicate brand ES-40 with a weak solution of sulfuric acid.
A number of laboratory experiments was carried out to determine the optimal ratio of initial components for the production of carbon-containing composite materials with desired properties. The temperature during the SHS process recorded using a high-precision pyrometer Raytek 3I.
A mixture of the green components thoroughly mixed in an agate mortar to obtain a homogeneous structure. Then, the samples were pressed in a tablet form using a hydraulic press with a 30 kN force to obtain dense samples, which then left on a special table for natural drying at room temperature of 18-22°C for 24 h. Next, the samples placed in a muffle furnace preheated to 950°C to initiate SH-synthesis. The temperature during the SHS process was about 1200°C.
The phase composition of the obtained materials was determined using X-ray phase analysis on a Dron-4M diffractometer using cobalt Ka-radiation. The completeness of the reaction was determined by the phase composition of the products of synthesis. X-ray phase analysis of synthesized carbon-containing samples from the ilmenite concentrate showed that the final product consists mainly of aluminum oxide and contains a small amount of useful silicon carbide phase. It found that when an aqueous solution of magnesium sulfate used as a cohesive during SH-synthesis, an undesirable intermetallic phase of FexSiyTi formed. The maximum amount of silicon carbide phase formed (the FexSiyTi phase is not formed) when carbon content was 40-45%, the ilmenite concentrate was 30-32% by weight in sample.
X-ray phase analysis of synthesized carbon-containing chromite concentrate-based composites showed that the final combustion product consists mainly of spinel, pentachrome trisilicide and contains a small amount of useful silicon carbide, forsterite and aluminum oxide. When an aqueous solution of magnesium sulfate used as a cohesive during SH-synthesis, double CrxSiyCz carbide formed, which can be identified as the MAX phase. MAX phase is very important for imparting useful properties to the composite. The optimum content of components was to yield the maximum output CrxSiyCz phase: carbon 33%, chromium 35% by weight. It established that a silica sol does not form such phase during SH-synthesis when used it as a cohesive.
The morphology (Fig. 1) of the synthesized samples based on ilmenite concentrate analyzed using a Quanta 200i 3D scanning electron microscope. Figure 1a shows a sample based on ilmenite concentrate with silica sol, where it can be seen that the sample has a dense structure (with small pores) with particle sizes ranging from 0.8 to 3 p,m and have a fibrous form. According to Fig. 1b, the sample based on the ilmenite concentrate with magnesium sulfate was loose with numerous micropores whose particle size is in the range of from 3 to 10 p,m.
(a) (b)
Fig. 1. Images of samples based on ilmenite concentrate with: (a) silica sol; (b) magnesium sulfate.
Figure 2a shows a sample based on chromite concentrate with magnesium sulphate, where it can be seen that the sample has a porous structure (with small pores) with particle sizes ranging from 1 to 3 p,m and have a fibrous form. According to Fig. 2b, a sample based on a chromite concentrate with a silica sol has a loose structure with numerous micropores whose particle sizes are in the range of 4 to 10 p,m.
Mechanical properties of the samples were determined by using a testing machine YES 2000 Type. The strength characteristics of the samples vary depending on the carbon content in the initial mixture. It is established that samples containing carbon about 40-45% by weight showed the best result of 5-8 MPa. Further studies have shown that the mechanical compressive strength decreases when trying to increase the carbon content in the mixture.
Carbon containing composite materials based on ilmenite and chromite concentrates were obtained by self-propagating high-temperature synthesis. The optimal conditions for the SH-synthesis of carbon-containing composites based on ilmenite and chromite concentrates with various cohesive were selected experimentally, providing the maximum content of titanium, silicon, chromium carbides and MAX-phase analogs in the material.
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(a) (b)
Fig. 2. Images of samples based on a chromite concentrate with: (a) magnesium sulfate; (b) silica sol.
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