USE OF LIQUID NITROGEN IN SPRAY FREEZING

THE FEASIBILITY HAS BEEN SHOWN OF PRODUCING PURE SUBMICRON CERAMIC RAW MATERIALS USING A CRYOCHEMICAL TECHNIQUE. THE OXIDE PRECURSORS (SULPHATES, NITRATES, ETC.) ARE HELD IN AQUEOUS SOLUTION WHICH IS RAPIDLY FROZEN BY SPRAYING INTO STIRRED HEXANE, COOLED WITH A DRY ICE-ACETONE BATH. THE PRODUCT IS SUBSEQUENTLY FREEZE-DRIED AND CALCINED TO THE OXIDE. HEXANE IS USED AS A HEAT TRANSFER MEDIUM TO CONVEY HEAT FROM THE SPRAYED DROPLETS TO THE DRY ICE REFRIGERANT. HEXANE IS USED WHEN SPRAY FREEZING IS AT ROOM TEMPERATURE OR RANGES TO BELOW THE TEMPERATURE OF SUBLIMATION OF DRY ICE. LIQUID NITROGEN IS USED AT ITS BOILING POINT. THE FREEZING RATE IN NITROGEN IS KNOWN TO BE QUITE HIGH. NITROGEN ELIMINATES THE USE OF FLAMMABLE HEAT TRANSFER MEDIA AND THE SPRAY ENTERS THE REFRIGERANT DIRECTLY. THE GREATEST ADVANTAGE REALIZED THROUGH THE USE OF NITROGEN IS THE ABILITY TO RAPIDLY DRY THE FROZEN SPECIMEN BECAUSE THE NITROGEN QUICKLY BOILS OFF THE SPRAYED MATERIAL PRIOR TO PLACING IT IN THE FREEZE-DRYER. SAMPLES CANNOT USUALLY BE EXTRUDED FOR TRIAXIAL OR IN CONFINED COMPRESSION TESTING. THE DIRECT SHEAR TEST WAS WIDELY ADOPTED FOR DESERT SOIL CONDITIONS BECAUSE THE TESTING APPARATUS CAN BE DESIGNED TO RECEIVE THE LINEAR RINGS WITH SOIL SPECIMENS AS SECURED IN THE FIELD. SIDE SHEAR AND END BEARING COMPUTATIONS ARE PRESENTED. BOTH STRAIGHT PILES AND THOSE WITHIN LARGE BASES (BELLS) ARE DISCUSSED. PILES CONSTRUCTED WITHOUT THE USE OF CASING AND THOSE IN WHICH CASING IS EMPLOYED, IN ADVANCING EXCAVATIONS AND WITH-DRAWN AS CONCRETE IS PLACED, ARE DESCRIBED. THE ULTIMATE BEARING CAPACITY OF PILES BEARING ON SANDS, SATURATED CLAYS AND SOFT TO MODERATELY FIRM PARTIALLY SATURATED COHESIVE SOILS PROBABLY CAN BE PREDICTED WITHIN PLUS OR MINUS THIRTY PERCENT BY CAREFUL ANALYSIS USING THE PROCEDURES OUTLINED. ULTIMATE BEARING CAPACITY COMPUTATIONS IN CONJUNCTION WITH CONVENTIONAL SETTLEMENT ANALYSIS PROVIDE A SOUND BASIS FOR DESIGN. ADDITIONAL LOAD TESTS ARE NEEDED TO VERIFY THE CAPACITY OF PILING IN PARTIALLY SATURATED COHESIVE SOILS, PARTICULARLY STRONGLY CEMENTED SOILS WHERE SATISFACTORY SAMPLES ARE DIFFICULT OR IMPOSSIBLE TO OBTAIN.