METHOD FOR MIXING FERROCHROMIUM SLAG IN ORDER TO PRODUCE FIRE-RESISTANT AND CHEMICALLY RESISTANT FIBER

Abstract: The invention relates to a method for preparing ferrochromium slag, by adding SiO2 and Al2O3 bearing mixing agents, into slag melt which can be defibrated into fibers resistant to high temperatures and alkaline conditions. Mechanically strong fibers are obtained when the SiO2/Al2O3 ratio of the added mixing agents is within the range of 0.8-1.85 and the MgO content of the mixed slag, as reduced to the MgO-Al2O3-SiO2 system, is within the range of 10-20% by weight.

Description: METHOD FOR MIXING FERROCHROMIUM SLAG IN ORDER TO PRODUCE FIRE-RESISTANT AND CHEMICALLY RESISTANT FIBER The present invention relates to a method for preparing ferrochromium slag, by adding SiO^ and Al^O^ bearing mixing agents, into a slag melt which can be defibrated into fibers resistant to high temperatures and highly alkaline conditions mainly for industrial use. Ferrochromium slag is chromium-bearing magnesium-aluminium- silicate, which is a by-product from the process of metallic ferrochromium. It is typical of this slag that it is created in extremely reductive conditions in electric furnace smelting. Therefore it contains hardly any metal oxides that are reduced more easily than chromium oxide, nor does it contain easily evaporable components. Ferrochromium slag is suited to be used as a raw material for special fibers resistant to high temperatures and alkaline conditions for the following reasons, among others: - the slag is homogeneous, once smelted and in the solid state partly glazed material, - generally over 50% of the slag is covered by SiO~ and Al^O- which are needed in the manufacture of the slag melt, - the K o, Na_0 and CaO contents of the slag are very low, which is of great importance with respect to both the fire resistance and the alkaline resistance of the fiber to be produced, - the slag is an economically advantageous raw materia1. The use of f rrochromium slag in the production of fibers is known from the Finnish patent application 845115. The said application discloses two methods for mixing ferrochromium slag, with silicon oxide and aluminium oxide. The purpose of mixing the ferrochromium slag is to change the physical properties of the slag melt so that the melt can be processed into fibers by means of generally known defibration methods. Moreover, the purpose of the mixing is to make sure that the desired properties for the end-product are achieved. Therefore the defibration capacity of the slag melt cannot be improved for instance at the cost of the heat-resistance of the end-product fibers. In order to make the ferrochromium slag melt so as to be wel defibrated, the temperature dependence of its viscosity must be weakened, the surface tension lowered and susceptibility to crystallization reduced. All of these desired aims are achieved by mixing the slag so that it becomes acidic. In addition to this, it is important to reduce the activity of MgO in the slag melt. In practical defibration tests, where ferrochromium slag was mixed according to the FI patent application 845115 - so that the composition of the mixed slag melt was set, in the phase diagram describing the MgO-Al 0 3 -SiO z system, within the crystallization range of cordierite - it was observed that the slag melt was well defibrated. But the mechanical strength of the fiber was small and its processibility weak. In closer examinations the reason for this was found to be the crystalline components contained in the fiber. The present invention introduces an improved method for mixing ferrochromium slag, whereby it is possible to prepare such slag melt that the fibers produced thereof are resistant to high temperatures and alkaline conditions. In addition to this, the fibers are mechanically strong and easily processible, which is of crucial importance with respect to their usability. The mixing method of the present invention differs essentially from the one described in the FI patent application 845115 as for the mixing ratios and amounts of the employed agents. The chemical composition of tne slag melt to be defibrated, and naturally that of the end-product fibers as well, is different and its properties better. The essential novel features of the invention are apparent from the appended patent claims. According to the invention, to ferrochromium slag there are mixed SiO., and Al^O^ bearing mixing agents (such as quartz, anorthosite, kyanite, andalusite, bauxite etc. ) so that the total SiO /Al j ^O- j , ratio of the mixing agents is 0.8-1.85, and the MgO content of the mixed slag, reduced to the system MgO- Al 2: -Si0 2 , is 10-20% by weight. By reducing we here mean a calculation method whereby the MgO, Al O, and Si0 2 contents of the real multicomponent slag are accumulated up to 100% by maintaining the mutual relations of the contents. In the mixing method of the present invention, the reduced MgO content of the slag melt to be defibrated is limited to below 20% by weight in order to ensure the defibrating capacity of the slag melt and above 10% by weight in order to prevent excessive increases in the defibration temperature of the slag melt. As for the SiO /Al 2> 0 3 ratio between the mixing agents, it is determined in order to secure the mechanical strength of the fiber. The invention is also illustrated with reference to the appended drawing, which is a phase diagram of the system MgO -Al g O- j -SiO j ,. The drawing describes a composition of the FeCr slag, and the corresponding content range which falls within the scale of the present invention is shaded. If the composition of the FeCr slag is changed, the shaded area also changes, but the ratio between the mixing agents is maintained the same. In the mixing method of the present invention, the amount of mixing agents added to the ferrochromium slag, depending on the slag composition and the desired MgO content, is 33-60% of the amount of the final slag. The mixing takes place in a smelting furnace where the temperature and outflow of the melt are easily adjusted. A suitable smelting furnace is for instance an electric furnace. The ferrochromium slag mixed according to the method of the present invention can, by applying generally known defibration methods, be produced into special fiber with a better fire-resistance than with the known mineral or slag fibers. As for strength and processibility, this fiber is comparable to them or better. Its alkaline resistance is better as well. The slag melt itself has a strong inclination for glazing. When cooled in air from the molten state, the slag forms a completely glass-like structure. The created glass has a deep blue colour, and the fiber made thereof is light blue. The invention is further illustrated by means of the following example. Example The chemical composition of a typical ferrochromium slag is: Fe 4.9%, Cr 9.0%, SiO j , 29.6%, CaO 3.1%, MgO 25.9% and Al 0 22.7% (percentages by weight). When reduced to the system MgO-Al g O j -SiO^, the composition is: MgO 33.1%, Al^0 3 29.0% and SiO^ 37.9%. When the slag is mixed according to the method of the present invention, the obtained composition range for the mixed slag is the shaded area of figure 1. The initial composition of the ferrochromium slag is also indicated in the drawing.

Claims: PATENT CLAIMS 1. A method for utilizing aluminium oxide and silicon oxide based ferrochromium slag in the production of fire-resistant and alkaline-resistant fiber materials, where the ferrochromium slag is mixed with SiO^ and Al^O^ bearing mixing agents, c h a r a c t e r i z e d in that in order to improve mechanical strength and durability, the total SiO«/Al_0_ ratio of the mixing agents to be added in the slag melt i s adjusted to be within the range of 0.8-1.85, and simultaneously the MgO content of the mixed slag, as reduced to the MgO-Al-O g -SiO system, is adjusted to be within the range of 10-20% by weight. 2. The method of claim 1, c h a r a c t e r i z e d in that the amount of the added mixing agents is 33-60% by weight of the slag to be defibrated. 3. The method of claim 1, c h a r a c t e r i z e d in that the smelting and mixing of the raw materials of the molten slag is carried out in an electric furnace.

Inventors: SALERVO AARNO TANELI, TUOVINEN FRANS HEIKKI, SALERVO, AARNO, TANELI, TUOVINEN, FRANS, HEIKKI
Applicants: OUTOKUMPU OY
Application year: 1988
Main classification: C03C13
IPC codes: C03C1, C03C13