PROCESS FOR PRODUCING SILICA BRICK

Abstract: A process for producing silica brick by firing silica stone mainly comprising silica and containing 0.2 to 5 wt % of solidified molten soda lime silicate added thereto, whereby silica brick with a lowered content of residual quartz can be produced readily and efficiently even by using a silica stone which difficultly causes the transformation of quartz. The process comprises crushing silica stone, adding thereto solidified molten soda lime silicate and a binder, molding the mixture, and firing the molding at 1350 to 1500 °C to cause a reaction between the silicate and the silica in the silica stone to thereby cause rapid transformation of the silica into cristobalite and tridymite.

Description: The present invention relates to a method of producing silica brick and, more particularly, to a method of producing a silica brick which can suitably be used in building up or repairing various industrial furnaces such as a coke oven, hot blast stove and glass kiln. Silica brick is a very important material which is used as a refractory for building up and repairing industrial furnaces, mainly coke ovens, hot blast stoves and glass kilns. It is well known that silica, which is the main component of siliceous stone as the principal material of silica brick, is changed into cristobalite and tridymite due to crystalline transformation in the course of firing in the production of the silica brick. Any silica brick having a large non-transformed silica content is undesirable because it exhibits extraordinary expansion which impairs the stability of industrial furnaces which employ such brick as the refractory. Therefore, the extent of transformation of silica is a very important factor which has to be considered in designing an industrial furnace with regard to the selection of material and evaluation of the adequateness of the use of the selected material. As is well known, a conventional process for producing silica brick employs the steps of pulverizing a silica material such as red-white silica or blue-white silica into particles of a suitable particle size and mixing such particles with lime (CaO) which is an additive effective also as a binder. A silica brick for use as a material for a glass kiln is also known which contains,for the purpose of preventing generation of frost-like matter on the brick surface of the upper structure of the glass melting cell, 0 to 1.5 % of CaO, 0.3 to 1.5 % of K₂O, the sum of the CaO and K₂O contents being not more than 1.5 %. (Japanese Patent Publication NO. 49-16087). It has also been attempted to further add elements such as Fe, Fe₂O₃, Cu₂O, TiO₂, Si and so forth. GB-A-463 949 relates to the manufacture of silica bricks and mentions a mixture containing in addition to siliceous material, lime and water soluble alkali compounds, iron oxide or MgO which converts the quartz wholly or substantially into tridymite. The degree of ease of transformation of silica in silica material into cristobalite and tridymite caused by heating tends to be affected by factors such as the crystal grain size of the silica and impurities in the siliceous stone material, and also varies according to the place where the siliceous stone material is yielded. In general, a greater crystal grain size of silica, as well as a smaller impurity content, tends to hamper the transformation of silica. Hitherto, there has been no proposal in regard to any additive which promotes transformation of silica in silica material in which transformation of silica is not easy, nor any method of producing silica brick using such an additive. The present invention has been accomplished in view of the above-described problems of the prior art, and is aimed at providing a method which enables the production of silica brick with a small residual silica content using a process similar to the conventional method with same degrees of ease and efficiency as those of the conventional process, even from a siliceous stone material in which transformation of silica is not easy. The invention thus provides a method of producing a silica brick, comprising the steps of pulverizing a siliceous stone material containing silica as the main component, forming the pulverized material and firing the formed material, characterised in that 0.2 to 5 wt% of Na₂O-CaO-SiO₂ fused and solidified material is added to said siliceous stone material so as to cause said Na₂O-CaO-SiO₂ fused and solidified material to react with the silica of said siliceous stone material thereby promoting transformation of said silica into cristobalite and tridymite. Preferably, the siliceous stone material contains 97.5 wt% of SiO₂. Substances which may be present in the siliceous stone, besides SiO₂, are Al₂O₃, Fe₂O₃, CaO, MgO, Na₂O and K₂O. Na₂O-CaO-SiO₂ fused and solidified material promptly reacts with silica which is the main component of siliceous stone and promotes transformation of silica into cristobalite and tridymite. With the method of the present invention, it is possible to reduce the content of residual silica in siliceous stone to 1 wt% or less. The Na₂O-CaO-SiO₂ fused and solidified material used in the present invention is a fused and solidified material which contain Na₂O, CaO and SiO₂, preferably as well as trace amounts of MgO and Al₂O₃, and preferably has the following chemical composition: SiO₂: 70 to 75 wt% CaO: 5 to 13 wt% Na₂O: 10 to 40 wt% Al₂O₃: 0.5 to 2 wt% MgO: 0.5 to 4 wt% The Na₂O-CaO-SiO₂ fused and solidified material used in the present invention may be obtained by mixing siliceous sand, sodium ash, lime carbonate, magnesium carbonate and sodium feldspar, melting the mixture and then cooling the mixture to solidify it. Preferably, the solidified material exhibits an amorphous vitreous state, but it may contain crystals derived from the original material. Preferably the Na₂O-CaO-SiO₂ fused and solidified material used in the present invention has the form of beads or powder,and the grain size is preferably 0.3 mm or less. A small grain size increases the specific surface area, which contributes to the prevention of the generation of pin-holes in the silica brick as the fired product and also to the promotion of the reaction with silica as the main component of the siliceous stone material, thus accelerating transformation. Examples of chemical compositions of Na₂O-CaO-SiO₂ fused and solidified-material are shown in the following Table 1. Too large a content of the Na₂O-CaO-SiO₂ fused and solidified material impairs the hot characteristics of the silica brick product, particularly the load softening temperature and anti-creep characteristics, and increases the apparent porosity which makes the brick unsuitable for use as a material for industrial furnaces. Furthermore, the addition of an excessive amount of the Na₂O-CaO-SiO₂ fused and solidified material makes no appreciable contribution to the reduction of the residual quartz. Conversely, too small a content of the Na₂O-CaO-SiO₂ fused and solidified material cannot reduce the amount of residual silica to the level of 3 wt% or less. Silica brick containing residual silica in excess of 3 wt% is not suited as a structural material because such brick exhibits extraordinary expansion during use at high temperatures above 1450°C. For these reasons, the content of the Na₂O-CaO-SiO₂ fused and solidified material is 0.2 to 5 wt%. preferably 1.0 to 2.0 wt%, with respect to the siliceous stone material. The production of siliceous brick in accordance with the method of the present invention may be conducted by, for example, adding the Na₂O-CaO-SiO₂ fused and solidified material to siliceous stone and crushing the mixture or by adding a predetermined amount of Na₂O-CaO-SiO₂ fused and solidified material to crushed siliceous stone and blending the mixture. A plastic binder such as carboxymethyl cellulose, lignosulfonate or molasses is added to the siliceous stone material during crushing or to the crushed siliceous stone material during addition of the fused and solidified material. The mixture is then formed, dried and then fired. The firing may be conducted in a single kiln or in a tunnel kiln, preferably at a firing temperature ranging between 1350 and 1500°C. Firing at temperatures below 1350°C undesirably allows a large amount (more than 3 wt%) of residual silica to remain without being transformed, whereas firing at temperatures exceeding 1500°C is not preferred because in such a case the tridymite generated by transformation is unstable and is liable to be changed into cristobalite. The present invention is particularly useful when the siliceous stone material used is of such a type that the transformation of silica does not easily occur. In the method of the present invention it is possible to add one or both of pre-fired siliceous stone material and brick dust in a total amount which is not more than 40 wt%. In the method of the present invention, it is also possible to add micro-fine silica powder containing infinite-form silica containing not less than 90 wt% of SiO₂. In such a case, the content of the micro-fine silica powder is less than 5 wt% with respect to the siliceous stone material. Addition of such micro-fine silica powder provides increased density of structure of the siliceous brick. It is also possible to add a CaO-containing material such as lime stone, slaked lime and burnt lime to the siliceous stone material. The CaO component effectively promotes transformation of silica into tridymite and enhances bonding strength. Preferably, the CaO content ranges between 1.5 and 3.0 wt% with respect to the siliceous material. When the amount of CaO added is less than 1.5 wt%, the corners of the silica brick product become crumbly and spots are generated on the brick surface which blacken it, resulting in a deterioration in the appearance of the product brick. Conversely, the presence of CaO in excess of 3.0 wt% reduces the strength of the silica brick product at normal temperature which makes the brick fragile. The presence of CaO in excess of 3 wt% also causes detrimental effects such as an increase in the porosity and degradation of the hot-characteristics. In particular the load softening point is lowered and the anti-creep characteristic is impaired. The invention will be more fully understood from the following illustrative Examples. Example 1 Siliceous stone materials having compositions and crystal grain sizes as shown in Table 2 were prepared, and sheet glass powder (grain sizes 5 to 300 µm) was mixed with these materials at ratios as shown in Table 3. Silica bricks were then produced by a conventional process. In each case, lime slurry was used as CaO and was added in an amount of 2 wt% with respect to the siliceous stone material, and firing was conducted by using a single kiln at 1410°C. The characteristics and compositions of the thus-obtained silica bricks are shown in Table 3. The mineral compositions were determined by X-ray diffraction. From Table 3, it will be seen that the present invention can remarkably reduce the ratio of silica remaining in the siliceous stone material which is not transformed, thus enabling production of silica brick which is superior in hot characteristics. As has been fully described, the present invention provides a method of producing a silica brick in which transformation of silica into cristobalite and tridymite is effectively promoted, thus making it possible to produce a silica brick in which the amount of residual silica is remarkably reduced. This produces superior hot characteristics, even when the siliceous stone material is of the type in which transformation of silica does not easily occur. The silica brick of the present invention can very suitably be used as a high-temperature structural material for industrial furnaces or the like.

Claims: A method of producing a silica brick, comprising the steps of pulverizing a siliceous stone material containing silica as the main component, forming the pulverized material and firing the formed material, characterised in that 0.2 to 5 wt% of Na₂O-CaO-SiO₂ fused and solidified material is added to said siliceous stone material so as to cause said Na₂O-CaO-SiO₂ fused and solidified material to react with the silica of said siliceous stone material thereby promoting transformation of said silica into cristobalite and tridymite. A method as claimed in claim 1, wherein the content of residual silica in the formed material is reduced to 1 wt% or less in the firing step. A method as claimed in claim 1 or claim 2, wherein said Na₂O-CaO-SiO₂ fused and solidified material comprises: 70 to 75 wt% SiO₂; 5 to 13 wt% CaO; 10 to 40 wt% Na₂O; 0.5 to 2 wt% Al₂O₃; and 0.5 to 4 wt% MgO. A method as claimed in any one of the preceding claims, wherein said Na₂O-CaO-SiO₂ fused and solidified material is an amorphous material. A method as claimed in any one of claims 1 to 3, wherein said Na₂O-CaO-SiO₂ fused and solidified material is partly crystalline. A method as claimed in any one of the preceding claims, wherein said Na₂O-CaO-SiO₂ fused and solidified material has a grain size not greater than 0.3mm. A method as claimed in any one of the preceding claims, wherein said Na₂O-CaO-SiO₂ fused and solidified material includes glass beads. A method as claimed in any one of the preceding claims, wherein said Na₂O-CaO-SiO₂ fused and solidified material includes a powder of sheet glass. A method as claimed in any one of the preceding claims, wherein said Na₂O-CaO-SiO₂ fused and solidified material is added in an amount of 1 to 2 wt%. A method as claimed in claim 1, wherein lime material is added in an amount of 1.5 to 3 wt% in terms of CaO content to said siliceous stone material. A method as claimed in claim 10, wherein said lime material is one or more of calcium carbonate, slaked lime and burnt lime. A method as claimed in any one of the preceding claims, wherein the firing temperature is from 1350 to 1500°C.

Inventors: AKAHORI SHUSHI, HARAKO HISAYUKI, AKAHORI, SHUSHI, HARAKO, HISAYUKI
Applicants: SHINAGAWA REFRACTORIES CO, SHINAGAWA REFRACTORIES CO., LTD.
Application year: 1992
Main classification: C04B35
IPC codes: C04B35