Continuous Casting of Particulate Silica Gels
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CONTINUOUS CASTING OF PARTICULATE SILICA GELS E. M. RABINOVICH, K. A. JACKSON* AND NONNA A. KOPYLOV
AT&T Bell Laboratories, Murray Hill, New Jersey 07974 ABSTRACT
A feasibility study of continuous casting of particulate silica sols has been conducted. The process consists in measured pumping of the sol and a gelling agent into a heated mold where the sol gels. The gel is continuously pushed out of the cylindrical mold by the pressure of the pumped sol. Gel rods of 2.5 cm diameter, up to 150 cm long, were prepared by this method. Temperature dependence of the gelation was studied, and activation energies of the process were determined. INTRODUCTION
Particulate silica gels [1-4] have been used to prepare glass articles significantly larger than those made by the alkoxide process, without using an autoclave. The shaping process is casting which is essentially a batch process, where the sol is cast, held in a mold until gelation, then pushed out and dried. The purpose of this work was to study a feasibility of transforming the batch casting process into a continuous process. PRINCIPLE AND DESIGN
Particulate silica sols stabilized by steric or electrostatic means are known to gel more rapidly at elevated temperature [5]. Different kinds of additives, e.g. fluoride-containing silica powder [6] or some chemicals can be added to accelerate gelation, so it can occur at room temperature in 5 to 10 min. The sol can be cast in a mold, e.g. a cylindrical glass tube stoppered from both ends. If a central glass rod is added to this mold, tubes can be cast. Upon gelation, the shaped gel body can be removed from the mold. This is a batch
process which is not very suitable for production. On our first attempt to make a continuous casting, the gel was pulled out of the mold. This was a complete failure, because the tensile strength of the wet gel is so low that it cannot tolerate even the smallest pulling effort. It was evident that discharge of the gel could only be accomplished by pushing, and that this could be done by pumping sol into the inlet end of the mold. A schematic diagram of the process is shown in Fig. 1. The sol is pumped with a large metering pump and, just before entering the pump, the gelling agent is delivered by a small metering pump and admixed. The sol arrives, still ungelled, in the mold where it gels during passage through the mold either under the effect of time alone or with heating. The rod or tube can then be dried and sintered. This study has dealt only with the casting process.
In the beginning it was suggested that intimate mixing could be achieved in the delivery pump, but some non-uniformity in gelation was observed. A small mixer was therefore added, consisting of a small conical flask with a magnetic bar resting on a magnetic plate. The sol entered near the bottom of the flask, and a liquid gelling agent was added near the top. Small amounts of the reagents were well mixed during the several seconds residence time. * Presently at Arizona Materials Laboratory, University of Arizona, Tucson, AZ
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