DECEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
1389
Discussion of New Metals in the Pulp and Paper Industry J. D. MILLER, Continental Paper & Bag Corporation, York Haven, Pa.
T
HIS discussion will be confined to particularly severe service conditions, and to the most recent applications of new materials to old processes and of both old and new materials to new processes and equipment. Not only is the durability of equipment important in the paper industry, but in many cases the quality of the finished product is determined by the ability of the constructional materials to protect the product from harmful contamination. This is especially true of fine papers and with pulp to be used in the manufacture of rayon which is purchased on close specifications as to metal content. Probably the most important development in the field of materials has been the application of stainless steels of various types in the sulfite pulping process. Until about ten years ago the materials used were principally copper and various brasses and bronzes, which required frequent replacement at considerable cost. At present these materials are being displaced rapidly by chromium-nickel-iron and chromium-nickel-molybdenum-iron alloys. In the early days of the application of these new materials, occasional difficulties were experienced because the necessity for proper control of composition and heat treatment was not appreciated and the improper alloy was often chosen to suit the corrosion conditions encountered. After much study the Materials of Construction Committee of the Technical Association of the Pulp and Paper Industry set up a specification for castings, requiring a minimum alloy content of 20 per cent chromium and 9 per cent nickel, and providing for control of carbon content and heat treatment. The optional addition of molybdenum up to 4 per cent was provided for, with the suggestion that its presence would be beneficial where the amounts of nickel and chromium were near the minimum allowed. At present the most serviceable and popular alloys for castings are those containing about 28 per cent chromium and 10 per cent nickel plus (in some cases) 3 per cent molybdenum, and those containing about 20 per cent chromium, 10 per cent nickel, and 3 per cent molybdenum. In the wrought form the best allround material is the alloy containing approximately 18 per cent chromium, 8 per cent nickel, and 3 per cent molybdenum, although an alloy containing about 27 per cent chromium] 4.5 per cent nickel, and 1.5 per cent molybdenum is/ coming into use, especially for piping, because of the difficulty of obtaining seamless tubes of the former alloy. Cast pipe in long lengths of various compositions is being produced by the flash butt welding of centrifugally cast tubes. An interesting comparison of the various alloys was made by exposing specimens in contact with relief gas from a digester. This gas consisted principally of sulfur dioxide saturated with water vapor and containing organic compounds a t temperatures up to 150” C. and pressures up t o 40 pounds per square inch. Of the straight chromium alloys, one containing 12 per cent chromium was not resistant to corrosion, the 18 per cent chromium alloy was about as good as bronze, and the 27 per cent chromium alloy was highly resistant. Alloys containing 18 per cent chromium-8 per cent nickel and 27 per cent chromium-4.5 per cent nickel-1.5 per cent molybdenum were practically free from attack. The results of these tests were confirmed by practical use of the resistant materials in the same digester.
SOME trouble has been experienced from galvanic corrosion where the stainless steels have been used with bronzes. Both materials have suffered premature failure, and in general it is best to avoid such combinations of materials if possible. The accumulation of scale, such as calcium sulfite or sulfate, on the metal surfaces brings about particularly severe corrosion
conditions. To avoid such accumulation in pipe lines and heater tubes, it is desirable to keep the total sulfur dioxide content of the cooking liquor as high as possible. In addition to the economies in maintenance expense derived from the use of the stainless steels in the sulfite process, they have contributed something more by making feasible the development and use of equipment for indirect heating and circulation of cooking liquors, and of what is known as the hot acid system in which heat units formerly lost in the relief gaqes are saved and used to preheat the cooking liquor in an accumulator, so that hot acid may be used a t the beginning of the cook. I t is claimed that this process provides a pulp of superior characteristics. Prior t o the advent of the stainless steels, equipment of this sort could not have been used because of the expense of frequent replacement of parts subjected to corrosion. In the alkaline pulping processes, corrosion is much less severe, and ordinary iron and steel are adequate for most of the equipment. Stainless steels, Monel metal, nickel, and Inconel are coming into extensive use for valve trim and for tubes in “black liquor” evaporators used to concentrate the spent cooking liquors in the alkali recovery processes. Their extra life over ordinary steel tubes more than pays for their additional cost in most cases. There is also a trend to the use of welded steel construction instead of cast iron for the bodies of black liquor evaporators. Soapstone and chromium brick linings of furnaces used to smelt the black ash in the alkali recovery process have usually been short-lived. An interesting approach to a solution of this problem is the use of water-jacketed sections for the smelter bodies. Although only a few installations have been made, reports are that after 18-month operation of the first unit there was no maintenance expense, whereas the useful life of ceramic linings used to be about one year. In addition, the use of water jackets has provided an additional source of hot water equivalent to about 3000 pounds of steam per ton of pulp. T H E introduction of new bleaching processes involving direct treatment of the pulp with chlorine, without a neutralizing alkali, has resulted in more severe corrosion problems in the pulp-washing equipment. A considerable amount of hydrochloric acid is formed in the process, and the pulp usually contains some free chlorine as well. Various materials are being used, including Hastelloy C for parts subjected to free chlorine. Monel metal, Hastelloy A, and stainless steel containing 18 per cent chromium, 8 per cent nickel, and 3 per cent molybdenum are being tested for wire cloth and winding wire on pulp washers. The proper choice of material for this service has not yet been established and probably will vary from plant to plant. Stock lines and white water lines, formerly wood or cast iron, are being replaced to a considerable extent by copper. Copper has the useful advantage of keeping itself clean and free from slime by reason of its toxic effect on the organisms responsible for sliming. Monel metal is being used successfully for stock lines handling bleached pulp. Stock chests and vats are generally made of wood or lined with tile. There is a trend towards the use of metallic linings of stainless steel or Monel metal, and some thought has been given to the fabrication of such equipment from nickel-clad or stainlessclad steel. There has been a considerable increase in the use of chromium plating, especially in connection with plates for flat wreens. Chromium-plated bronze screens are said to maintain the desired slot width longer than unplated screens, and the smooth surface is useful in connection with avoiding accumulation of stock and pitch. However, chromium-plated weens have not been uni-
.fuI. and there i s a growing intervst in relatively thin screen plates fabricated from stainless steel and Mono1 rn
