Patent GB-641415-A (GB)
Title: Improvements in or relating to magnesia insulation and method of manufacturing same
IPC: C04B40/02
Filed: 1946-11-15
Abstract: 641,415. Preparing and moulding fibrous magnesium carbonate blocks. STEVENS, A. H. (Johns-Manville Corporation). Nov. 15, 1946, No. 33939. [Classes 70 and 87 (ii)] Mouldable thermosetting magnesia insulating material is prepared by forming a slurry containing normal magnesium carbonate crystals, water and reinforcing fibres, and increasing the water-holding capacity of the slurry solids by subjecting the slurry to quiescent reaction at a temperature above 140‹ F. to develop therein voluminous crystalfibre flocks, this quiescent reaction being the preparatory conditioning of the slurry prior to moulding by permitting it to react in a quiescent state of slurry solid suspension for a few minutes while undergoing an initial stage of conversion to a more basic form of magnesium carbonate. The reinforcing fibres may be asbestos, raw cotton, spun glass, fine rock wool or steam-treated wood fibres and to impart high temperature resistance the mixture may include inert fillers such as diatomaceous silica and finely divided calcium carbonate. The slurry preferably has a solids content of 1-8.5 per cent by weight and is preferably formed by dilution with hot water of a more concentrated slurry having a temperature not exceeding 120‹ F. The quiescent reaction preferably takes place at 150-170‹ F. for 1 to 15 minutes. On completion of the reaction the slurry may be partially dewatered and moulded to block form by direct application of mechanical pressure, the moulded block being then heat cured, preferably in hot water, followed by a heat drying operation. The moulding may take place in a filter mould while the mould is at least partly immersed in a bath of hot water. In a preferred apparatus the mould comprises a flat perforated mould base-plate 20 faced with fine mesh screen 21, a cylinder 22 which is relatively movable with respect to the baseplate, and the walls of which form side walls of the mould, and a piston 23 which is reciprocally mounted within the cylinder and which is perforated and faced with fine mesh screen to form a filtering top for the mould. The moulding and preliminary heat curing operations take place beneath the surface of water bath 19. An endless multiple-flight conveyer mounted in the water-bath carries a plurality of uniformly spaced flights 20, each flight serving in one position as the base element of the mould, and in subsequent positions supporting a moulded block 10. Cylinder 22 may be moved through a short vertical path by means of a piston 27 mounted in a hydraulic pump 28, and piston 23 is reciprocated within cylinder 22 by piston 35 mounted within a hydraulic cylinder 34. In a modification cylinder 22 may be held stationary and the conveyer flight 20 which is in the moulding position may be reciprocated by means of member 40 actuated by piston 38 in hydraulic cylinder 37. In carrying out the process of the invention a concentrated slurry is placed in tank 90, preferably at a temperature of 80‹ F. A measured volume of hot water at 180-210‹ F. is introduced into one of a nest of tanks 92, and the slurry in tank 90 is run into the hot water in tank 92. The mixture of slurry and hot water is agitated and then allowed to stand quiescent for a controlled time period, after which the charge is transferred to the mould 18 through the pipe 95, piston 23 being then at the top of its stroke in cylinder 22 and the base of cylinder 22 being seated in tightly sealing relation against the top face of one of the flights 20. The temperature of the water in the bath 19 is preferably 180-210‹ F. As soon as the measured charge has entered the mould the piston 23 is forced down, thus placing the charge under mechanical pressure and partly dewatering the wet solids content of the charge by filtering water out of the mould through the perforate base 20 and the perforated.piston 23. The pressure may be as high as 100 lbs. per sq. inch. To release the block from the mould the cylinder 22 is raised off the mould base 20 and piston 23 then raised. The conveyer is then operated to bring an empty flight to the moulding position and the moulding operation then repeated. After a second movement of the conveyer, a perforate cover-plate 44 is applied to the wet solids block and kept thereon during further advance of the block through the hot water curing section of the apparatus, this curing process being usually completed in 5-10 minutes. At the end of its travel the block is lifted from the hot water bath by suction member 48 and finally placed in a drying oven operated at 220-350‹ F. Apparatus is also described (Fig. 3, not shown) for moulding semi-cylindrical pipe insulation shapes.