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Vol. 16, No.8
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Pulp and Paper Manufacture Flow Sheet
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HE essential steps in the manufacture of bleached wood pulp by the bisulfite process are as follows: the chipping of the wood, manufacture of cooking acid, preparation of bleach liquor, digestion of the wood chips, washing and bleaching of the pulp, and the thickening or drying of the fiber. I n the manufacture of fine papers there are two major steps: ( a ) the "making process," which includes the mixture, beating and refining of the
stock, and the formation and drying OF the sheet; and ( b ) the "finishing process," under which may be included not only the giving of the proper surface characteristics to the paper, but also the sorting, counting, and trimming of the paper preparatory to storage or shipment. The accompanying flow sheet outlines in convenient form the flow of materials through these steps. No attempt has been
August, 1924
INDUSTRIAL A N D ENGINEERING CHEiMISTRY
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made t o include complete details, nor does it necessarily represent ideal practice. It is merely an attempt to give a general picture of a representative American mill producing sulfite pulp and fine papers. A few points of interest regarding operation are worth considering. Practice differs considerably in regard to acid-making. Instead of the milk-of-lime system shown here, many mills use towers filled with limestone, over which water and sulfur dioxide pass countercurrently. The source of sulfur, the types of sulfur burners, gas coolers, and acid coolers are other features of variance. The spent sulfite liquor from the digesters is utilized by some mills, although not by the great majority of American mills. The chief uses of this waste include its use for fuel, as a binder, a tanning material, and as a source of alcohol by fermentation.
Local conditions very largely determine bleaching practice. Dry chloride of lime and liquid chlorine are widely used for preparation of the bleach liquor. It is principally an economic balance between the cost of power and transportation charges. Where fine writing papers are made it is very essential t o avoid all possible dirt, in the pulp-making process as well as in the paper mill. I t is interesting to note the number of steps taken to rid the pulp of foreign matter. Light cinders that float on the water are an especial nuisance, since they cannot be screened out readily. The relative labor requirements of a pulp mill and paper mill of this sort are much higher per unit weight of product in the latter, owing to the form in which the product is handled and the great care necessary to insure uniform high quality. This does not hold true to such an extent in mills manufacturing news and cheap grades of paper.
