The tardy recognition of this fact is responsible for a curious reversion in practice. The open-air sifting of house refuse for the recovery of substances possessed of commercial value was unequivocably condemned from health motives, as previously mentioned. Yet, in order to recover these articles, some system of selection and hand manipulation are inevitable, notwithstanding the high degree of intellectuality to which machinery has been advanced. But the old system of hand-picking was primitive in its simplicity. The circumstance that household refuse, both organic and inorganic, possesses virtues which the vogue of the destructor caused to be blindly ignored, has been responsible for a manifestation of marked ingenuity upon the part of the engineering profession. The necessity to recover every ounce of material possessing a market value was never so acute as it is to-day. Supplies are short and are likely to remain inadequate for some time to come, while the high level of prices is apt to compel more rigid economy. Yet the strains encountered in this direction may be very sensibly lessened by the practice of salvage along more intensive lines.
It would seem as if refuse recovery were destined to develop into a highly specialized branch of the engineering craft. Hitherto for the most part the engineer has confined his efforts towards garbage-disposal by destruction, but the new tendency is far more logical and deserving of every encouragement. Certainly it is a field in which abundant scope is offered for brilliancy and ingenuity of thought. This is demonstrated by the activity of certain firms, more particularly of one in the North of England, the guiding hand of the destinies of which has evolved a complete recovery plant, having many decidedly ingenious features, and which is already being installed by certain of our more progressive corporations and municipal authorities.
This plant is self-contained, and, so far as is feasible, is automatically operated. While hand-picking cannot be entirely eliminated it has been reduced to the minimum. The system adopted facilitates the task, and renders hand-picking as congenial as the peculiar conditions will permit. Furthermore it is an individual entity. While it can be established in an isolated centre it can also be coupled up to the existing dust-destructor, or power-generating station if preferred, thereby complying with the general desire to centralize municipally-controlled installations. This is certainly a powerful recommendation, because it avoids superfluous transport and handling.
Under this scheme the refuse-collecting vehicles discharge their loads into a receiving hopper from which the material falls by gravitation into a hexagonally-shaped revolving riddle. This screen or reel for two-thirds of its length is perforated to allow the fine ash associated with the waste to escape into another large hopper placed immediately beneath. The ash may then either be withdrawn directly from this hopper into wagons or carts for removal, or should arrangements be made for its combination with other ingredients to produce a fertilizing agent, it may be led by conveyor from the hopper to the compounding-room.
For the remaining third of its length the hexagonal revolving screen is perforated with a coarser mesh to permit the cinders to escape into a separate hopper, at the base of which is a worm conveyor which receives the cinders and bears them to a washer. The washing operation is introduced to allow the separation of the light or combustible fuel—cinders—from the heavier clinker, fragments of glass, pottery, and other incombustible substances. At the same time all fine dust clogging the interstices or pores of the cinders is removed, thereby facilitating the subsequent combustion of the cinder, while, of course, the heat produced from the cleansed fuel is greater than that derived from such material loaded with incombustible dust.
After being washed the cinders are picked up by a scraper elevator. If it be intended to utilize this fuel for raising steam in the adjacent power plant it can be carried by conveyor direct to the boiler-room, to be discharged into the bunkers or furnaces. Should it be decided to dispose of the cinders, either wholly or in part, to the general public, they may be taken by the transporter to any suitable point to be stored against sale in bulk or in bags.
A second scraper elevator gathers the heavier debris separated from the combustible fuel in the washer, and carries it to a pulverizer, to which it is delivered through a chute. If the fine dust associated with the raw refuse, and which fell through the receiving screen, be not delivered from its hopper into vehicles for immediate disposal, it may be led to this point to be stored in the pit receiving the material from the pulverizer with which it may be mixed. Of course, the dust is not passed through the grinding plant.
The elimination of the dust and coarser material from the crude garbage in the receiving screen leaves an appreciable quantity of organic and inorganic matter, comprising such divers substances as paper, fragments of wood, bottles, jars, bones, tins, and vegetable material to be handled. As these cannot pass through the perforations in the sifting screen they are delivered on to a broad endless conveyor-belt travelling between two platforms. This is the “picking belt,” from the fact that as the material is borne along between the two platforms the useful material is removed by the hands of pickers, to be cast into suitably disposed bins. In this manner the process of segregation is carried out with the minimum of effort, while the material is in movement, and under the most congenial conditions the character of the work will permit. It represents the only stage at which recourse to manual labour is required, so that it will be seen that hand-selection is reduced to the absolute minimum.
The waste-paper is not touched by hand. At a suitable point a specially designed hood, connected to an exhauster, is mounted over the picking belt. When this is set in motion the induced draught is sufficiently powerful to suck up the paper, and to bear it through a special conduit to be discharged into a convenient receptacle, whence it may be removed to the baling press.
This plant, known as the Hoyle refuse-recovery installation, after its inventor and designer, Mr. H. P. Hoyle, is extremely efficient. Simplicity is the outstanding feature, while its operation is economical and requires only the minimum of labour. So far as power is concerned a single 10 horse-power electric motor suffices for all operations. The capital cost has also been kept down, the price of the complete plant being from £1,500 to £2,000—$7,500 to $10,000. At this figure the installation of the system should prove distinctly profitable, more especially in conjunction with one or two auxiliary appliances which offer the means to enhance the market value of the recovered materials, although they are not essential. For instance, an appreciable proportion of the tins thrown into the dust-bin are in a bright condition and free from rust. Such tins can be made to yield so much crude tin plate for the production of further tins, instead of being subjected to the less economic process of crushing, baling, and detinning or transference to the furnaces in billet form to be melted down.