ASPII ALT REFINING AND WORKING varies with both the natural product and the uses to which it is to be put. Generally speaking, the object of the process is to remove water and volatile and other foreign matter from the crude ma terial; but for some purposes it is highly de sirable to leave the mineral matter mixed with the asphalt. This is particularly true of as phalt that is to be used for paving and similar purposes, since for such uses the asphalt really serves merely as a cementing and waterproofing medium, the bulk of the paving material con sisting of sand. or powdered limestone or sand stone. The rock asphalts, when used for pave ments direct, are not refined: but are simply crushed, reduced to powder by heat, and then compressed in place, as described under FAIT MENTS. The crushing is done by means of rolls, or by toothed cylinders, and the heating is effected in revolving cylinders, at a temperature of some 300° F.
One of the simplest methods yet attempted for abstracting asphalt from the rock products is by boiling the rock in water. This is possible in the case of the lighter asphalts; for, with a specific gravity less than water, the asphalt will rise to the top of the kettle, whence it may readily be removed. Either calcium or sodium chloride (common salt) may be substituted for the water in the boiling kettles. The heat may he applied directly or indirectly, steam being used in the latter case. The asphalt thus sepa rated must he refined by evaporation and sedi mentation. Carbon hisulphide may be used as a solvent to extract asphalt from the rock, with out the use of heat. This material, however, costs more than naphtha, the solvent most com monly applied, and a smaller proportion of it can be recovered. The same general process is followed in the case of the bisulphide as with naphtha, except for the absence of heat and the introduction of the solvent at the bottom in stead of the top of the vessel.
The Alcatraz sandstone rock asphalt already described is treated as follows: The rock is dumped from the cars coming from the mines into a hopper, from which it passes through a succession of fluted crushing-rolls, each nearer together than the previous ones, the last set be ing three inches apart. From this crusher the fragments go to a steam-heated drum melter, which reduces the material to a mushy state.
Next naphtha is added, dissolving the asphalt proper and leaving the sand below. The sand is treated a number of times, until the asphalt has been practically exhausted, after which the sand is wasted. The combined solution of naphtha and asphalt flows by gravity for 30 miles, through a 5-inch steel pipe-line, to Alca traz Landing. Here the naphtha is recovered by distillation and pumped back to Sisquac to be used again. The asphalt left after the dis tillation is then barreled and shipped.
The California maltha already mentioned is refined as follows: The bituminous sand is thrown into a steam-jacketed cylinder with re volving arms, where it is broken into frag ments. It is then placed in boiling water, whereupon the maltha floats to the top of the vat, the sand settles to the bottom, and is re moved mechanically. The crude maltha is sent to a storage tank, then to a refining kettle, where the volatile matter is evaporated, the temperature being increased from 100 to 240' F. during the 24 hours of final treatment.
Asphalt is also produced in California by distilling crude petroleum oil, the heat being applied through steam-coils. The kettles are open at first, then closed, the temperature being increased during the process from 220° F, to 700° F.
Trinidad Lake asphalt is refined, or has its water and volatile matter driven off, by heat ing it in tanks about 10 feet square and 15 feet high. The heat is applied by means of steam coils placed in the tanks. To keep the mineral matter from settling, jets of steam are passed into the bottom of each tank. The steam pressure employed is about 100 pounds. giving about 325° F. At this temperature the asphalt is a liquid. so that after the water has been driven off the refined product can be run into barrels for shipment. In the older method of refining, the heat was applied by means of a fire, which was directly in contact with the vessel containing the asphalt. Aside from the danger of overheating, this process was ob jectionable because it required 6 to 7 days, instead of the 12 hours taken by the new proc ess. In addition, the sedimentary matter set tled to the bottom of the old tanks, and was lost; whereas it is now retained as a valuable com ponent of the asphalt.