per sq.in. The filter-presses are kept in sheds which are well insulated from outside conditions and chilled to the necessary temperature by radiators in which cooled brine is circulated.
When the filter-press is fully charged with wax, it is opened and the contents are discharged into a trough situated below the press and conveyed by a screw conveyor into the refining section. If a second cooling is necessary, the expressed oil, commonly called the "blue oil," is re-cooled to a lower temperature in similar plant and filter-pressed again.
A modern filter-press consists of a series of thin circular steel plates fitted with steel gauze on each side and provided with a circular opening in the centre to permit oil to be fed into each chamber. Heavy canvas is cut to fit the plates and is either sewn or firmly clamped round the hole in the centre. Spacing rings are placed between each pair of plates, thereby forming chambers in which the wax cakes form. The series of plates and rings are assembled on the press frame and the whole closed under a pressure of about 700 lb. per sq.in. by means of a hydraulic ram. This type is built to hold up to 400 or 500 plates 48 in. in diam. and to work up to Soo lb. pressure. The spacing rings used to form the chambers vary in thickness from in. to I in., enabling the thickness of the wax scale cakes to be altered according to the quality of the oil being pressed. For instance, should the oil contain an easily pressable crystalline wax scale a thick cake would be desired in order to work the press at its maximum capacity by reducing the number of plates in use, but should the oil contain a soft wax it might be advantageous to reduce the thickness of the cake.
The paraffin distillate from the stills, usually having a settling point of from 7o° F to 8o° F, and a recoverable wax content ranging between 5% and o%, is chilled to the temperature which will give a pressed oil having the desired set point and then fed into the presses. The oil is let into the presses very gently and when the pressure starts to rise the rate of increase is kept slow and constant. When the pressure has reached its maximum the press is allowed to remain under pressure until it is required to be emptied ready for a new charge and this permits the wax scale to become as free from oil as possible.
The process that follows achieves the removal of low melting waxes and adherent oil and is called the "sweating process," first developed by Price's Candle Company. There are innumerable modifications of the sweating process, which may be regarded as a fractional melting operation, and perhaps the simplest to understand is that devised by N. Henderson. In this there is a frame on which are set nine shallow trays one above the other. In each tray wire gauze is stretched 2 in. from the bottom. The frames can be set in rooms or stoves provided with heating and cooling arrangements. The trays are first set level and water is run in to the depth of the gauze. Then the melted wax is run on to the water surface and is allowed to solidify. After solidi fication the trays are tilted and the water run off. The tem perature inside the stove is gradually raised and fractional melt ing goes on until the material on the gauze is of the correct melting point. The "sweats" are run off and may be again put through the process. Alter drawing off the sweats the temperature is still further raised and the sweated wax is melted and run to the next stage of the process.