The process in question is the Scott solvent recovery invention to which I have referred, and which represents the greatest achievement yet recorded in the whole science of fat reclamation from organic waste. The process was perfected and patented shortly before the war, and although hostilities militated against its immediate and rapid development, thereby delaying the recognition of its overwhelming virtues, it is satisfactory to learn that many plants operating upon this principle have been laid down, not only in this country, but in other parts of the world. It is the process which at the moment is arousing the most intense interest, owing to the progressive stride which it represents in this field.

The process is delightfully simple, although apparently it involves an intricate plant and demands a higher level of skilled labour, but where the work of reclamation is conducted along ambitious lines it cannot be excelled. Briefly described, it turns upon the employment of benzine, or some other equally volatile solvent which, as we all know, will readily dissolve fat and absorb it. What can be done with this agent is familiar to every housewife who practises the removal of grease spots and other unsightly marks from clothing by the aid of benzine, while it is the medium whereby dry-cleaning is rendered practicable.

The raw material—condemned meat, offal and other organs of the animal recovered from the slaughter-house which possess no edible value—is charged into a steam-jacketed horizontal extractor fitted with stirring gear. When condemned carcasses are to be treated there is no need to carry out preliminary deboning; it is merely necessary to reduce the material to rough pieces for convenience of handling. It will be observed that the steam is not brought into contact with the mass, but is circulated through the jacket as in the dry vacuum process.

The solvent is introduced in the first instance in the form of vapour, being passed through boxes of special construction, to pass finally into the extractor. The contents of the latter being in a condition of constant agitation as a result of the manipulation of the stirring gear, the benzine vapour is able to permeate the mass. The heat radiated from the steam circulating through the jacket converts the moisture present in the material into vapour and with which the solvent comes into contact. Vaporization of the moisture causes the solvent itself to condense to a certain degree, and in the liquid form it dissolves out the grease. The process is continued until the bulk of the moisture has been eliminated, when the grease and solvent are withdrawn. When the grease has been fully extracted down to a limit which will result in a dry meat-meal, containing about 1 per cent. of grease, the benzine is steamed off in the usual manner. The benzine itself is recovered because it is only permitted to work in a closed circuit, and, after fulfilling its purpose, is passed to a still to be cleaned and purified, after which it is again passed to the extractor to repeat the cycle of operation.

The process, it will be observed, is continuous, while the benzine may be used over and over again. All that is required is to place a sufficient quantity of the solvent into the circuit to carry out the operation with the essential efficiency. Naturally, the quantity involved varies with the size of the plant and the work to be fulfilled, but it may run up to 5,000 or more gallons. The plant is generally laid out upon the unit principle, which is the most satisfactory, because it facilitates the adaptation of the installation to the volume of work in hand. One or more units can be shut down during the “off” period, allowing the remainder to be worked up to their full capacity, which, of course, is the most efficient and economical method. The losses of benzine are very low—not exceeding 1 per cent. of the weight of the raw material treated. In fact, there are many installations in operation where, over a period of one year, the benzine loss recorded is actually below 1 per cent. This factor is vitally influenced by the care and attention bestowed upon the plant. If it be carefully tended, all joints being kept in the tightly packed condition, and the condenser maintained in a high degree of efficiency, the benzine loss may be reduced to an infinitesimal degree, the value thereof representing but an insignificant fraction of the value of the increased yield of oil and fat.

The solvent acts upon the grease only. It does not affect in any way the gelatinous material, and, consequently, the nitrogenous or ammonia value of the ultimate meal is considerably enhanced as compared with the results achieved with the digesting plant. The meal is discharged from the extractor in a dry crisp condition ready for immediate grinding, and is admirably adapted for poultry and cattle feeding. No traces of the benzine remain.

The bones may be ground immediately, if desired, but if these should be forthcoming in sufficient quantity they should be passed on to the glue and gelatine plant. There is no necessity to submit them to a further degreasing process, because this has been completed in the one operation in the extractor. As a rule, however, with installations devoted to the treatment of condemned meat and other offal, the bones are not forthcoming in sufficient quantities to justify the attachment of a glue recovery plant although, of course, they can be sold to other works specializing in this work. It is merely a question as to whether it would pay to transport the degreased bones to the glue works. If not, they can be ground up to be utilized as fertilizer, for which, it is needless to say, a good price can be obtained.

The recovery of fat down to 1 per cent. of that contained in the crude material does not constitute the only outstanding advantage of the solvent extraction process. It enables the whole of the operations to be condensed into one task, completely dispensing with all auxiliary apparatus. The refuse is merely charged into the extractor and withdrawn in the form of powder, and, if condemned carcasses have been exploited, bone as well. What this means may readily be realized. Under the open steam digesting system—even with the wet and dry vacuum systems to a lesser degree—the refuse must first be cooked. The material upon withdrawal from the digester must be passed through the press, after which treatment it has to be disintegrated and dried. If the reclamation of the gelatinous or “stick” liquor, as it is called, be part of the process this also demands handling. Thus one may safely anticipate having to conform with five distinct and separate operations, involving intermediate handling and supplementary plant, while the loss of fat in passing from stage to stage is far heavier than may possibly be imagined. But, with the solvent extraction process, the numerous above-mentioned operations are resolved into one, and one only—the charging of the extractor with the refuse. The saving in labour by the elimination of all interhandling is obvious, which in these days of enhanced wage costs demands consideration, while there are no intermediate losses of oil. In so far as saving of time is concerned there is little, if any, difference. Under the solvent extraction method a period of eight to ten hours is required to deal completely with a charge of 4,500 to 9,000 lb.

The fruits accruing from this latest manifestation of ingenuity in connection with the reclamation of waste may be tersely emphasized. The reclamation of the fat down to 1 per cent. being accepted, it may also prove interesting to indicate how effectively the nitrogenous or ammonia value of the product is preserved. The following represents a typical analysis of a meat meal, which, it should be pointed out, contains no bone whatever. The figures are:—

Per cent.
Tribasic phosphate of lime (superphosphate)3·25
Nitrogen11·37
⤷ = ammonia13·81