In the treatment of green oils for recovery of anthracene, the following method is usually followed on the large scale, though slight modifications may be made according to the plant employed, and the degree of purification required. The oils are completely cooled ; this facilitates the separation of the anthraeene, and reduces the solvent action of its associated oils, en that they present the appearance of s, soft buttery mesa, in which are scattered, more or less, grains of anthracene, which may even give the mass a granular aspect. They are pumped into a aeries of pipes a, b (Fig. 483), furnished with T pieces opening at the bottom, and to which are attached strong canvas or flax bags d, to receive and filter the oils. These bags are 8 ft. or 4 ft. long, and are stitched with packthreads, which can be drawn out, so as to open them lengthwise for the removal of their contents. The pipes are 4 in. diameter ; the T pieces may be 8 or 10 in. apart ; at the end of each, is a collar c, for attaching the bags. A shallow tank e receives the liquid filtering from the bags, and returns it to the stills or the store tanks. At first, a large quantity of liquid comes away ; after a short time, the bags aro well distended by the oils being forced into them. As the solids accumulate, the liquids cannot so freely escape ; they are then allowed to drain for some hours, after which they are removed to a powerful hydraulic press, which squeezes out a further quantity of the liquid oils. Since the adhering oils, when heated, readily dissolve the anthracene, cold pressure must be first used, to remove the liquids ; hot pressure may be etnployed to complete the operation.
The whole treatment of crude anthracene is open to vorrgreat improvement. Crude anthra,cene submitted to the same process of refining will have a higher percentage in Bummer than in winter, because of its diminished solubility in the adhering oils. It is further purified by being very finely powdered, and agitated with about 25 per cent. of high boiling naphtha, or "solvent," (petroleum is sometimes used instead), submitted to straining, pressing, &c., as before. The washing increases its percentage value from 20-25 to 30-35. The washing process is repeated two or three times, in closed iron cylinders fitted with mechanical agitators. The principal aim is t,o obtain the anthracene as finely divided as possible, and to bring every portion well into contact with the naphtha. For the final washing, the anthracene is sublimed, so s,s to obtain it in as fioe a powder as possible.
Authracene from Pitch.—It has been proposed by Fenner and Versmann (1871), and Lucas (1872, No. 747), to obtain it by distilling pitch to coking. Whether this can be made a profitable operation must depend upon the quantities obtained; some coals yield considerably greater quantitiea of anthracene than others. Kopp says that the soft pitch from the gas used in Turin contains as much as 4 or 6 per cent., which is three or four times the amoun t present in tar. It has been suggested t,o distil soft coal-tar pitch with superheated steam, for the production of anthracene, and this plan has been tried on a working scale with varying success ; hut the process does not appear to be carried out to any great extent, though it is one which commands attention. In distilling pitch to coking, certain disadvantages are said to arise. Anthracene is drawn over between 171° and 371° (840° and 700° F.), and the stills are emptied whilst the residue is still liquid. If the distil lation be earried to coking, the distillate will contain much chrysene and pyrene, from which it is best freed by a second distillation. It is well to maintain a partial vacuum in the still by exhausting the vs,pours. A current of superheated steam blown through the still does not appear to act so well as exhaustion, though probably it is more convenient in most cases. Large quantities of anthracene are now obtained thus ; the main precautions are to keep the stills supplied with melted pitch to one constant level, and, as the heavy vapours do not rise, to bring down the still head as near as possible to the surface of the pitch, and to surround the condensing pipes with boiling water ; the vapours are assisted over by suction or blowing. In most cases, the pitch is run into
the stills direct from the tar stills. The plan adopted on the Continent is to coke the pitch in ovens or retorts of clay ; on opening them, the coke which is formed is burnt off. The preliminary washings, &e., are the same as before ; but a little more care is necessary. The liquids which aro collected from the crude anthracene are frequently added to the creosote oils ; but as they are always saturated with antlaracene, some method should be adopted for its recovery.
The principles associated with crude anthracene are phenantrene, ; ehrysene, OuTI„; pyrcne, 0,611,0. No colouring matters are at present made from these substances, although they yield compounds which may eventually become sources of profit in this direction. When the oils recovered from the crude anthracene are distilled, there remains in the retort, after the anthracene has been drawn over, a soft tarry residue containing much ehrysene and pyrene ; this may be mixed with the creosote oils, or, if burnt in suitable chambers, :nay be a source of a black pigment for which the " last runnings " are frequently employed. These residues are frequently, and with greater advantage, added to the pitch, for making softer kinds.
It may be worth while to point out, that tars differ widely as regards their richness in anthra cene ; as a rule, those which yield much henzol will be the best, and probably this fact may be usefully applied by gas cbmpanies who are interested in working up their own refuse. There are many natural bitumens in which benzol and anthracene exist ; and it is not improbable that an examination of petroleum residues may lead to some important additions to the list of dyeing pro ducts. The amount of anthracene contained in a sample of the crude article is of great importance, since it is always sold by analysis. Its point of fusion will approximately indicat,e the amount of the oily impurities ; these may be absorbed by pressing between blotting-paper in a hot-press. The anthraeene may be washed with cold alcohol, which will further free it from naphthalene and other oily matters. Tbe melting-point of the product will give a very fair idea of its purity ; that of pure anthracene is about 210° (410° F.). The only method practically used for valuing samples of anthraeene is Luelr's anthraquinone test. A wide-mouthed flask of sufficient capacity has a wide glass tube 3 ft. long fitted into it with a cork ; the upper end of the tube is open, and carries a funnel furnished with a stop-cock, which regulates the flow of chromic acid soIutioo. The whole is supported on a stand over a Bunsen's burner, and protected with wire gauze. One gramme of anthracene is dissolved, by boiling with 45 c. e. glacial acetic acid, in the flask ; 10 grm. chromic acid, free from lead, in 5 e. c. glacial acetic acid and 5 c. c. water, is added gently by the funnel, so as not to disturb the steady boiling, which is continued until a distinct and permanent greenish-yellow colour appears, or =MI a drop of the liquid produces a reddish spot on a piece of silver foil. It is then cooled, and gradually diluted with 150 c. c. water ; the anthraquinone is precipitated, collected, and washed with water on a filter, then with hot dilute potash lye, again with water, dried at 100° (212° F.), and weighed on the filter, which is afterwards w.eighed and deducted. The operation lasts four to six hours. Tc the net weight is added •01 grm., as, by Luck's experiments, this quantity is taken up by the acetic acid and water used. The un oxidized impurities are thoroughly removed, by adding drop by drop, until the red colour -does not vanish, a solution, of permanganate potash to the residue on the filter; after washing it off into a beaker, oxalic aud sulphuric acids aie added, to remove the excess of permanganate; the whole is thrown into the same filter, washed to remove the acid, ezc., then with dilute boiling soda lye and water, finally dried at 100° (212° F.) as before. Sand and other fixed impurities are determined by incineration.