Bleaching Kettle.
—This can be of a predominant size dependent upon the volume of business. For a moderate business the most suitable dimensions are about six feet in diameter and four feet deep. A sectional view of such a kettle is shown in [Fig. 151] and a smaller kettle is illustrated in [Fig. 152]. It was the practice to use compressed air for agitation so as to mix the fullers earth with the lard. This has been discontinued in the best refineries, as it was found that agitation with air had a tendency to make the lard turn rancid more readily than if agitated by mechanical device.
The kettle is supplied with a vertical shaft, to the bottom of which is attached a propellor about fourteen inches in diameter. Around this is fitted a wire screen about thirty inches high made of galvanized wire, ³⁄₁₆-inch mesh, this screen clearing the blades of the fan about an inch. On top of this screen is fixed an iron pipe approaching to within about eighteen inches of the surface of the lard when the kettle is filled. A kettle of this size will hold about 5,000 pounds of lard for treatment. The agitating blade should revolve at a speed of from 125 to 175 revolutions per minute. This has the effect of producing a miniature whirlpool in the body of lard by driving the lard through the screen against the sides of the kettle, where it rises to the surface and goes down through the funnel-shaped tube to be again driven through the screen. In this way the fullers earth is thoroughly mixed in and if there is a tendency to “lump” the lumps are readily broken up by impact.
Bleaching Lard.
—If cold lard is used it should be melted and any sediment or moisture which settles should be drawn off. It is best not to draw off too close to the bottom so as to avoid any moisture.
The temperature at which lard should be bleached is an important matter to be determined. If the lard has been heavily cooked and has a strong steam flavor, a temperature of 180° F. is correct. If the lard has not been well cooked a somewhat lower temperature is necessary. As a rule the lower the temperature at which bleaching is done, the more fullers earth is required to get the proper bleach and at the same time as little earth flavor as possible is a point for careful and intelligent consideration. The best way to determine, where there are large quantities of lard, is to take a small sample and make tests in the laboratory. The point to be obtained is to get the bleach with as little of the flavor of the earth as possible. Generally speaking, from three-fourths to one and one-half per cent of fullers earth will get the desired result.
When the lard is brought to the proper temperature, fullers earth is put in the kettle, when it is agitated from eight to twelve minutes, then pumped through the filter press as rapidly as possible. It takes a certain length of time for fullers earth to do its work properly, and a strong agitation of from eight to twelve minutes appears to be the best method to adopt in order to obtain the full effect of the earth.
Refining Tallow Separately.
—Where tallow is used in lard, it is always advisable to bleach the tallow separately. This is done in the same manner as described for lard, with the exception that about three per cent of fullers earth is used, after which it is pumped through the filter press and into the receiving tanks.
