DRYING PROCESS

Abstract: A process for the drying of objects or bodies containing water and made of a refractory material by heating to an increased temperature exceeding the boiling point of water. The drying takes place from the interior of the material, the heat necessary for the drying being supplied from the inside of the material through holes or channels provided in the material.

Description: Drying process The present invention relates to a process for the drying of objects or bodies containing water and made of a refractory material. By the expression "objects or bodies" there are also included such refractory constructions as are constituted by linings of tundishes, the bottom part of ladles, chute furnaces for casting, electric arc fur¬ naces, etc. The expression also, of course, covers sepa- rate objects or bodies such as bricks, larger constructio¬ nal elements of different types of furnaces, lances etc. The drying of constructions of refractory materials intended for use in applications requiring a high tempe¬ rature requires the water present in the material to be removed in an effective and safe manner. The techniques hitherto prevailing are drying while supplying heat from the outside or in some other way. When drying self-suppor¬ ting objects or bodies such drying takes place in furnaces of different types, and when drying linings of different types the heat is supplied also from the outside by the furnace being heated after drying temperature for the purpose of removing the water to a sufficient degree so that the lining can be taken into use. The prior art in¬ volves substantial draw-backs, such as high energy re- quirement, extended and thereby costly drying, difficul¬ ties in determining when the drying is finished, etc. Fur¬ thermore, there are risks involved in connection with the drying while supplying external heat residing in the risk for bursting. In the heating the water present in the ma- terial will be evaporated under heavy volume increase, and this results in obvious risks to the environment. This problem is the foundation of the techniques disclosed in Swedish patents 7315942-8 and 8206043-5, wherein a solu¬ tion, is presented residing in the advance inclusion in the material of channel-forming elements or fibres to facili¬ tate release of the water in connection with the heating. The present invention has for its object to provide new drying techniques, wherein the disadvantages of the prior art are avoided. Thus, one object of the invention is to provide a new drying process resulting in substan- tially reduced drying costs at the same time as the drying period will be substantially shortened. Another object of the invention is to provide a pro¬ cess which in its preferred form involves eliminated or at least substantially reduced risk for bursting. These and other objects that will be clear from the following description of the invention are provided by a process for the drying of objects or bodies comprising al¬ so fixed arrangements, such as linings of furnaces, con¬ taining water and made of a refractory material by heating to an increased temperature exceeding and preferably sub¬ stantially exceeding the boiling point of water. The pro¬ cess according to the invention is characterized in that the drying takes place from the interior of the material, the heat necessary for the drying being supplied from the inside of the material through holes or channels provided in the material. Even though the heat can be supplied through any hea¬ ting medium it is preferred that the heating takes place via electric resistor elements. These resistor elements can be arranged in holes or channels in the refractory ma¬ terial in different ways. Thus, the resistor elements can be removable and thus renewed useful or they may be fixed¬ ly casted to the material and thus left behind in the ma¬ terial after finished drying. It is particularly preferred in the process according to the invention to carry out the drying of objects or bo¬ dies made of a refractory material containing channel-for¬ ming elements, which are substantially evenly distributed in the material. These techniques known per se in connec- tion with drying through externally supplied heat are de¬ scribed in connection with such conventional techniques in for example Swedish patent 7315942-8. Such channel-forming elements suitably constitute about 0.05 to 0.5% by weight of the material, and a particularly preferred range is about 0.1 to 0.3% by weight. Preferred channel-forming elements are based on natu- ral materials originating from plants, such as for example straw or stalks from ordinary cereals, grass, rice roots, fibres from palm trees, needles from pine trees and simi¬ lar. For the purpose of facilitating release from the re¬ fractory material the diameter of said channel-forming elements can vary from some microns up to several milli¬ meters. In practice palm tree fibres are preferred, since such fibres have a small diameter, a high density and are relatively rigid. This type of fibre is often sold under the tradename "Bassine". Alternative materials for the channel-forming ele¬ ments are pre-fabricated fibres made of suitable plastic materials, such as polypropylen. Such materials or ele¬ ments may have a diameter of from some tens of microns up to a few millimeters, for example up to about 5 mm. In order to provide for suitable exit paths for the vaporized water in the refractory materials it is essen¬ tial that the channel-forming elements are elongate, i.e. have high ratio length/diameter. Said ratio is suitably above about 5 and the length of the elements may vary within broad limits. In order to facilitate admixing these elements into the refractory material it is preferred that the length has not too high a value. With regard to the composition of the refractory ma¬ terial any common refractory oxides can be included. Thus, refractory compositions containing essentially SiC or Zr0 2 may be used, and also neutral refractory compositions containing mainly Al^O, or Cr^O or a basic refractory mixture containing essentially MgO or CaO, can be used when applying the techniques according to the present in- vention. The refractory oxide constitutes the major part of the material and the weight ratio thereof suitably lies within the range about 75 to about 95% by weight of the material as a whole. It may also contain a minor amount of binder such as high grade aluminumoxide cement, phosphoric acid or clay. The refractory material used in the drying process according to the invention is prepared in a conventional manner by mixing of the constituents, such as in a mixing container containing a mixer, and the container can be heated if desirable, and when used the channel-forming elements are introduced into the material during some phase of its manufacture. The invention will in the following be further illus- trated by non-limiting examples. In said examples the per¬ centages relate to weight if not otherwise stated. Example 2 refers to the appended drawing which in two different views shows the design of a block for a foundry. EXAMPLE 1 The present example relates to drying in accordance with the present invention of a so called injection lance intended for the feeding of gas or other material into a molten metal. The lance consists of a central steel tube surrounded by a mantle made of a refractory material, in the present example Victor Korund WR, Hδganδs, Sweden. This refractory material has a composition of 96% 1 2 0 3 , 0.5% Si0 2 and 0.1% Fe-Og. The apparent density according to Din 51065 is 2.800 kg per m3. The quantity of refractory material in the lance is about 330 kg. The re¬ fractory material contains evenly distributed palm tree fibres of the type Bassine having a length varying from about 15 to about 20 mm. The fibres constitute about 0.2% of the refractory material. After completing the lance the water contained in the material is expelled while using the techniques according to the present invention. Drying takes place by introdu- cing an elongate electric resistor element in the center of the steel tube and energy is supplied at a power of about 3 KW. The drying time is about 24 hours, and no signs of bursting or other problems could be observed during drying. A normal drying time for such a lance in a furnace while supplying external heat is multifariously longer and normally requires three days or more. In addi¬ tion to the advantage of short drying time there is also, of course, obtained the advantage of substantially lower energy consumption, since the energy losses to the envi¬ ronment are substantially reduced. EXAMPLE 2 The present invention relates to the manufacture and drying of a block for a foundry, the block having the de¬ sign as is clear from the drawing. The block is made of a refractory material, Ultracast CD 4 SR, Bohlin & LOfgren, Sweden, and its weight is about 1.350 kg. The refractory material contains channel-forming fibres in the same man- ner as stated in Example 1. In connection with casting 14 through-going holes having a diameter of about 20 mm even¬ ly distributed over the block are arranged. The drying takes place by insertion of electric re¬ sistor elements in tubular form, one into each hole, and an added power of 12 KW is supplied for the drying. The drying time is about 12 hours for the complete removal of contained water. Compared to conventional drying in a gas-heated fur¬ nace the energy required can be reduced by 8 to 9 times. EXAMPLE 3 The present example relates to the drying of a so called heart body for an LB-furnace. The heart body is cast starting from Victor Korund ES, Hoganas, Sweden, as a refractory material and the quantity of the material is 750 kg. The chemical analysis for the refractory material is 94% A1 2 0 3 , 0.5% Si0 2 and 0.1% Fe 2 C* 3 . Its apparent den- 3 sity according to Din 51065 is 2.750 kg per m . The ma¬ terial is supplied with channel-forming fibres in the sam way as in Example 1. In connection with the casting there are arranged 12 through-going holes in the heart body in- tended for electric resistor elements. After finished casting an electric resistor element of tubular type having an effect of about 1 KW is intro¬ duced into each of the 12 holes. The drying time while using a total power of thus about 12 KW is 33 hours and the drying takes place without problems. It should be observed that the present invention is not limited to the embodiments illustrated above. Thus, the process according to the invention can be applied to the drying of refractory masses irrespective of shape and device. Even if the examples relate to the drying of self- supported bodies or objects the invention may equally well be applied to permanent refractory constructions, such as furnace linings and the like. The drying can take place using ' reusable electric resistor elements or using resis- tor elements permanently cast into the material.

Claims: CLAIMS 1. A process for the drying of objects or bodies con¬ taining water and made of a refractory material by heating to an increased temperature exceeding the boiling point of water, characterized in that it is carried out on objects or bodies made of a refractory material containing chan¬ nel-forming elements which are essentially evenly distri¬ buted in the material, and that the drying takes place from the interior of the material, the heat necessary for the drying being supplied from the inside of the material through holes or channels provided in the material. 2. A process according to claim 1, characterized in that the heat is supplied via electric resistor elements. 3. A process according to claim 2, characterized in that said resistor elements are removed from the material after the drying. 4. A process according to any of claims 1 to 3, cha¬ racterized in that said elements constitute about 0.05 to 0.5% by weight of the material. 5. A process according to claim 4, characterized in that said elements are selected from the group consisting of straw or stalks from ordinary cereals, grass, rice roots, fibres from palm trees, needles from pine trees and plastic fibres. 6. A process according to any of claims 1 to 5, cha¬ racterized in that said elements have a high length/dia¬ meter ratio. 7. Objects or bodies made by the process according to any preceding claim.

Inventors: BLOM INGVAR, BLOM, INGVAR
Applicants: HOEGANAES ELDFAST AB
Application year: 1992
Main classification: F26B3
IPC codes: B28B11, C04B35, F26B3, F26B23