Coincident with the accumulation of sawdust are the fabrication of huge piles of wood-slats—the trimmings from the logs. These also represent sheer refuse, the only possible disposal being in the form of fire or kindling wood. At one lumber-camp in Scotland there was found a pile, a sprawling, ragged and jagged stack, house-high, covering 20 acres, and containing, at a modest estimate, from 300 to 500 tons of wood-waste. It was ideal for paper-making as investigations proved, but was then merely being allowed to rot.
The process of preparing sawdust for the paper-maker is very simple and inexpensive. It may be described as an application of the system for producing mechanical pulp, because, in the main, the resultant product is very similar to the latter in its essential characteristics. The waste, being the product of the buzz-saw, is coarse in texture. It is first passed over a riddle of wide mesh, which, while allowing the dust proper to fall through readily, collects the pieces of bark, chips, and other fragments of wood which may have become associated with the dust. This residue is thrown to one side for conversion by a different method. The sifted sawdust is dumped into a hopper to fall by gravity in a steady stream into the mill, which is somewhat reminiscent of the familiar mortar-mill, below. As it enters the latter it is caught up by the revolving grindstone and crushed against the stationary stone, the result being that it is disintegrated and pulverized. By virtue of the centrifugal action set up the dust, as it is whirled round, naturally works from the centre to the periphery of the wheels, the coarser particles or tailings being flung out, while the finely-divided dust, produced by the grinding action, falls into a separate receptacle.
The tailings are recovered to be re-passed through the mill, and, in time, for the most part are also ground to the desired degree of fineness. A certain proportion of residue defies reduction in this manner, but it is not discarded. It is retrieved to be used in the manufacture of coarse brown paper. Two methods of grinding, even in the vertical mill, are practised. The one known as the wet process involves the addition of water to the dust, which thus becomes hydrated, the resultant saw-pulp, as it is called, being somewhat similar to the familiar wet mechanical pulp. The alternative process is described as dry grinding, the sap in the wood constituting the only moist agent.
It may be mentioned that, in the very earliest attempts to emulate the wasps’ paper-making process, the experimenter ground the wood to dust by applying the log to the face of a grindstone which was revolving, water being the lubricant, the practice recalling the grinding of tools. The particles of wood fell, with the water, into the trough beneath. The surplus water was drawn off, leaving a mashy residue or pulp—hence the name.
In grinding the sawdust the coarse material is reduced to a fine powdery substance, soft and silky in texture when dry, but which retains the essential fibrous characteristic, though naturally the length of the individual fibre is extremely minute. But pulp so produced possesses one advantage for the paper-maker—it demands no preliminary boiling. It can be discharged direct into the beater, as the machine which prepares the raw material for the paper-making machine is called, with the waste-paper, sulphite or mechanical pulp, or a mixture of both, it only being necessary for the agitation of the contents of the beater to be conducted thoroughly to bring about the perfect blending of the ingredients.
I have emphasized the circumstance that this saw-pulp may only be considered as a diluent. This may be varied from 10 to 35 per cent. according to the quality of the paper desired. The issue of the Times, dated June 15, 1918, was printed on paper containing 20 per cent. of this saw-pulp, but I have seen other newspapers the paper for which was prepared from pulp diluted to the extent of 35 per cent. with the saw-pulp. With the accumulation of experience in the working up of this material marked improvements are to be recorded in regard to quality of the resultant paper which has enabled dilution to be carried to an enhanced degree without imperilling the factor of strength which the finished product must possess to enable it to be passed through the newspaper printing machine at a speed of 500 feet per minute without breaking. Under modern conditions it is difficult to determine whether or not saw-pulp has been introduced into the composition of the paper, which testifies conclusively to the perfection of production.
This economic utilization of one waste from the sawmill is of decisive financial significance. Cost of production is extremely low, because the power for driving the grinding mill may be obtained by firing the steam boilers either with sawdust itself, the consumption thereof being small, or with the refuse resulting from the preliminary sifting of the dust. Indeed, the process holds out such alluring possibilities that there is no reason why every sawmill should not include a grinding mill to treat the residue on the spot, shipping the saw-pulp direct to the mill, thus turning what is now an unmitigated nuisance and a source of danger into a distinct commercial asset. It is estimated that a grinding mill requiring 25 h.p. for its operation could turn out 1¹⁄₃ tons of saw-pulp in the course of the ordinary 8 hours’ working day or 7 tons a week. The cost of such a plant would be about £400—$2,000—and the price obtainable for the product should be sufficient to render the conversion of the waste to this useful purpose attractive after paying all outgoings. At the time the practice was brought into operation the cost of reducing the sawdust to saw-pulp of the desired character was from £5 to £6—$25 to $30—per ton. It is estimated that the saw-pulp maker would be equitably rewarded with £8—$40—per ton for the finished material ready for transport to the mill. On this basis a grinding mill, working to full capacity through the 44 hours’ working week, should be able to show a gross profit of £21—$105—which should leave an adequate margin of net profit to encourage such exploitation of the waste. The expansion of this young industry, however, depends entirely upon the conditions which will obtain upon the restoration of normal trading. It is a moot point whether the Scandinavian pulp-makers will ever be able to revert to pre-war quotations for their product, owing to the increasing costs of production, and this fact should render the outlook distinctly promising for the home producers, more especially if the sawmill trade be destined to undergo a decided revival. Every ton of saw-pulp produced from the waste will prove beneficial to the nation, for the simple reason that it will enable us to reduce our purchases from foreign sources of pulp by a corresponding amount.
While saw-pulp can only be regarded as a contribution to the paper-making problem, there happens to be another waste product suitable for this purpose, one which is available in much larger quantities, and the supply of which would seem to be increasing rather than decreasing. I refer to straw. Hitherto we have sadly neglected the many possibilities offered in this connection, having preferred to turn our by-product of the grain fields to other applications and to import vast quantities of strawboard for the manufacture of boxes, containers, and what not. Other countries have been more industrious and enterprising than we, but what they have achieved is equally feasible in these islands. To bring home the magnitude of this industry it is only necessary to relate that our annual pre-war imports of strawboard from Holland reached 250,000 tons.
There is no reason why such a lamentable state of affairs should continue. Straw is not only useful for the production of strawboard, but it constitutes an excellent material for the manufacture of paper. Its yield efficiency, while lower than that of esparto grass, being only 33·3 per cent., is sufficiently high to render its exploitation in this direction highly promising, especially as the material can be obtained in huge quantities.
At the present moment our supplies of straw for civilian needs may rule low and prices may be high. But this is due to the heavy military demands. Once the latter retire from the market and leave the article to take care of itself, a marked drop in price may be confidently anticipated, particularly if our new agricultural policy be maintained. So long as it pays the farmer to grow corn he will continue to do so, and the more acres he brings under this indispensable commodity the greater will be the quantity of the by-product thrown upon the market. It is anticipated that, when things settle down, from 2,000,000 to 3,000,000 tons of straw in excess of civilian needs will be available, and the only possible outlet then for this waste from our grain-fields will be the paper-mill. The utilization of the straw in this direction will be influenced by charges for fuel and labour, while, of course, the price of the imported pulp will affect any decision which may be contemplated in regard to the exploitation of our home resources. But assuming that the Scandinavian pulp will be dearer as a result of enhanced production charges, and assuming that dumping tactics just to hold the market will be frustrated, it is quite possible that we shall find it cheaper to depend upon our own exertions with domestic materials. If the quantity of straw which I have mentioned should become available and be absorbed for this purpose, it will be adequate to furnish from 670,000 to 1,000,000 tons of paper.