i
PREPARATION
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I n
lf
CHEMICAL RECOVERY
PREPARATION
HOWARD B. BROWN National Council for Stream Improvement, Inc., Engineering Experiment Station, Louisiana State University, Baton Rouge, La.
Conservation of Water in the Pulp and Paper Industry through Recycle, Re-use, and Reclamation The importance of water to the expanding pulp and paper industry has fostered the development of conservation practices and a trend toward lower consumption per unit of production
'TO pulp and paper industry, water is one of the three basic raw maTHE
terials necessary in production. Water is an essential to many intermediate operations, in which the end product is no longer desirable as a raw material. The pulp and paper industry ranks third in the demand for water by manufacturing industry; its estimated with.drawal is 4 billion gallons per day or
approximately 401, of the industrial usage in the nation. The very nature of volume demand of expanded operations in our phenomenal economic growth has in many instances necessitated the integration of all individual requirements into one gigantic water conservation program. This program has developed through in-plant conservation practices associated with
good business and by improved process and reclamation equipment, further stimulated by increased production with the same or decreased availability of supply and emphasis on stream improvement programs. The current trend of water usage has been in the direction of lower water consumption, with usage requirements on the order of 400 to 600y0 of the actual VOL. 48, NO. 12
DECEMBER 1956
21 51
LO5 FLUME
k& CHIPPER
SCREEN
STORAGE
Vent
4
& t r T a a t p l p n u l a&
rTolWo SEPARATOR
intake water or demand requirements. This figure is indicative of the progress made; however, the increase in production units demands continuing' improvement of conservation practices. The major processes in the pulp and paper industry (Figure 1) are pulping, the freeing of cellulosic from noncellulosic materials, papermaking, and the forming and drying of cellulosic pulp into a sheet of finished product. Operations primary to pulping include preparation of wood and of the cooking liquors; intermediate processes are pulp washing, screening, bleaching, and stock preparation. Allied operations, included in all t>pes of pulping processes, require water for equipment operations and heat, light, and power production. An integral part of the kraft process is the water-consuming chemical recovery operations. Process water. that which is in intimate contact with the product or intermediate raw materials, serves many uses.
Uses of Process Water
Chemical
I
I
I
!
I
I
WastA
L
I
I. Reactant A. Pulping B. Liquor making C. Chemical recovery D. Bleaching E. Additive preparation 11. Solvent
A. i V o o d preparation B. Pulping C. il'ashing
D. Screening E. Chemical recovery
F. Bleaching
G. Additive preparation Physical
id 4
p l o o k Llau6r From Seal Tank
-Low
.
A
TANK
GAS -OFF LIQUOR COLLECTOR
Stock To Woahrrs
--
L
A. B.
-EFFLUENT WATER RAW WATER L BLACK-
WHITE ~ LIQUOR
~
~
PRODUCT
Figure 2.
2 152
b
11. Dilutent or heat exchange medium
LEGEND
I
I. Conveyor A. Of uroduct or intermediate material 1. Wood oreuaration 2. Puipini 3. Liquor making 4. Screening 5. Stock preparation B. Of undesirable material 1. Wood preparation 2. Washing 3. Screening 4. Bleaching
111. Lubricant A.
Unit process flow diagram for pulping
INDUSTRIAL AND ENGINEERING CHEMISTRY
Within unit processes Between unit processes
B.
Raw material processing 1. Wood preparation 2. Mechanical pulping Production processing 1. Screening 2. Fiber conditioning 3. Fiber classification
RE-USE OF W A T E R BY I N D U S T R Y Cooling water demands are high, and major usages in condensing, operation of vacuum-producing devices, and general-purpose cooling are inherent to equipment operation. Requirements of water for heat, power, and light production, and for wash-up, fly ash control, and similar purposes are not greatly different from those in other process industries. Although water demands vary considerably with the specific situation and types of product, average figures (Table 11), based on good operational practices, are indicative of the volume required in integrated pulping, bleaching, and papermaking. The operations requiring the greater amount of water per unit of production are washing,
stook From Blow Tonkr
Table 1. Requirements for Unit Process Water in the Pulp and Paper Industry Gallons
Unit Process
per Ton
Wood preparation Pulping Mechanical Chemical (9) Pulp washing (2, 9) Bleaching (9) Full SemiScreening (2) Stock preparation and papermaking Paper (2, 9) Newsprint (2) Chemical recovery
500-10,000
40-50,000 20-30,000 50,000 50-60,000 25-30,000 0
140,500 180,500 109,900
Table II. Requirements for Process Water in the Pulp and Paper Industry (Fresh water demand) Gallons per Ton
Product
Bleached kraft paper Mill A Mill B Average (1951 survey)
or
Digeeter
OIIution.
Based on minimum figures.
Unbleached kraft paper Mill A Mill B Mill C Mill D Average (1951 survey)
Blow T a n k ,
6,500 30-35,000 10-35,000
Totala Unbleached paper Bleached paper Newsprint a
---1, To Weok Liquor Storage,
(4)
21,000 24,000 29,000 29,000 25,000
(4)
50,000 47,000 75,000
Newsprint Mill A Mill B Mill C
28,000 39,000 32,000
Average (1951 survey) (4)
25,000
i L A LA L - i ________ - W- A-S-H- WATER - -.- C H E M I C A L LIQUOR __.
PRODUCT
Figure 3.
Unit process flow diagram for pulp washing
screening, and papermaking. The recycle, re-use, and reclamation practices established as standard procedure over the years in these operations represent a major portion of the conservation practices in the industry. The net effect of these practices may be seen in actual mill operations (Table 11). Water is required in the pulping process (Figure 2) for cooling; surface condensers are employed for the recovery of vapors from the digester and pulpreceiving blow tanks. Water demand in wood preparation may be for flume or log washing. Mills utilizing these primary processes normally recycle this water extensively, and make-up water usually represents a waste effluent from another in-plant use. Pulp washing, whether brown stock washing or washing stages within the bleaching process, is of a similar nature, with countercurrent flowage employed as much as possible. I n brown stock washing (Figure 3) the objective is to recover maximum liquor solids a t highest concentration, and careful control is necessary to accomplish this while still obtaining a thoroughly washed pulp for the succeeding steps. Because thoroughly washed pulp is needed, this demand is generally satisfied with fresh water with practically no flow to waste. Pulp screening, which may follow both
brown stock washing and bleached pulp washing, provides fiber classification, rejection of incompletely digested wood particles, and further removal of soluble and insoluble foreign materials. The operation (Figure 4) is to a great extent a closed system, and the water from the dewatering step is recycled. Make-up water may be provided by re-used water from an advanced operation, and excess effluent overflow water can be recycled for use in other processes. The bleaching process represents further purification of the cellulosic material. Here the main uses of water are : to remove dissolved matter in washing, to control pulp consistency, and to add heat to the system. The amount of water required is a function of the type and number of stages involved, and the amount of recycle is limited by the removal of materials rendered soluble in water. From the production standpoint, bleaching requires as much fresh water as is economically possible with countercurrent operation. General practice (Figure 5) is to use fresh water for the washer showers, and reclaimed “white water’’ from the drying or sheeting machine for dilution. Usually only the highly contaminated filtrate from the chlorination and alkalinic extraction steps is sewered, and the other filtrates are countercurrently re-used for interVOL. 48, NO. 12
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DECEMBER 1956
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mediate dilution and regulation of consistency of incoming stock. White water may also be used for regulation of consistency of fully bleached stock sent to storage. The greater amount of recycle and reuse water is used in the papermaking process, and in continued operation on the same type of paper this may virtually approach a closed system. Limiting factors of re-use are primarily economic; too tightly closed recirculatioq results in lower quality, reduced quantity and machine speeds, bacterial sliming, corrosion, foaming, and other operating difficulties. Re-use is further complicated by different types of production, which may involve colored products or special additives. The recycle and reuse practices (Figure 6) established have been made possible through development of newer types of save-all equipment, which recovers fiber and yields a quality effluent for re-use within the process or in excess water, replacing fresh water demands in other departments. I n the chemical recovery and cooking liquor operations of the kraft process (Figure 7) further re-use, recycle, and reclamation of process waters are practiced. The bulk of the water demand in the department is supplied by used or reclaimed flow from other departments and recycle operations represent an essentially closed system. Water conservation practices beyond those accepted as standard practices vary from mill to mill, the extent of reuse being a function of the type of production, availability of supply, effluent disposal, available equipment, and economics. A current activity of the southern region of the National Council for Stream Improvement is a survey of mill practices in the field of water re-use. A summary of this information is to be made available for use of the member mills in extending their conservation programs. The present study has to date shown many sometimes ingenious water conservation practices resulting from complete knowledge of the over-all operation and fostered by the necessity of the existing situation. Water Conservation Practices
EFFLUENT WATER
---__----- REUSED
WATER
PRODUCT DECKERED STOCK CHEST
r
I I
.
I
To Paper Yochina o r B l e a c h Plont
Figure 4.
I
Unit process flow diagram for screening
I
Stock From Pulp Mill
TO
i sewer
I I
CONSISTENCY
I
Machine Waters
A. Stock dilution and showers B. Screening C. Knotters D. Broke beaters E. Repulpers F. Groundwood grinders G. Condenser water H. Bearing seals I. Fly ash control
I
I
J
_ _I
II
BLEACHED STOCK STORAQE
t
A. Other white waters 1. Stock dilution
2 154
INDUSTRIAL AND ENGINEERING CHEMISTRY
I I
ll
I
I
c- -
TOWER
_ _ _ _ _ _ _-__
----FRESH
Miscellaneous Effluents
--pJ---bo I!
WATER
REUSED WATER
L.D. HYPO
TOWERS
PRODUCT
Figure 5.
Unit process flow diagram for bleaching
RE-USE O F WATER B Y I N D U S T R Y 2 . Shower water 3. Warm water system
B. PRESS ROLL
Cooling and sealing waters 1. Brown stock washing 2. Log flume water 3. Recycle 4. Mud washing
Combined Condensates A. Caustic room make-up 1. Mud washing 2. Mud filter 3. Dregs washing B. Brown stock washing C. Condenser water make-up Decker Filtrates A. Stock dilution B. Screening C. Brown stock washing D. Repulpers E. Groundwood grinders F. Condenser water
REFINED STOCK CHEST
I
L
a
---FRESH
WATER
- - - - _ -- - - - REUSED
OR WASTE WATER PRODUCT
Figure 6.
Unit process flow diagram for papermaking EVAPORATOR CONDENSER WATER SUMP
References
I
I I
I EVAPORATOR
Figure 7.
c
--_ - ----_- - - -
The key to past and future accomplishments of water conservation through recycle, re-use, and reclamation is making the best possible use of the supply available. In a tight water situation the solution lies in taking a general view of the over-all operation and doing “what is best for the most” in regard to the function of the individual operating departments. The. pulp and paper industry has made much progress in the field of water conservation and will, as required by continued expansion, further develop means for the maximum usage of this essential raw material.
EFFLUENT WATER RAW WATER 0LACU-GREEN
(1) Amberg, H. R., Tappi 38, No. 11, 154A-5A (1955). Brown, H. B., ‘(Water Usage Practice in the Southern Kraft Industry,” Tech. Assoc. Pulp and Paper Industry, Water Committee Program, February 1956. Gehm, H. W., Moggio, W. A., Louisiana State University Engineering Experiment Station, Bull. 38 (1953). Mussey, 0. D., U. S. Geol. Survey, Water-Supply Paper 1330-A (1955). (5) Natl. Council for Stream Improvement, Inc., Tech. Bull. 72 (1954). (6) Zbid., 73 (1954). ( 7 ) Natl. Council for Stream Improvement, Inc., unpublished data. (8) Rudolfs, W., “Industrial Wastes,” Reinhold, New York, 1953. ( 9 ) Stephenson, J. N., ed., “Manufacture of Pulp and Paper,” Vol. 111, IV, V, McGraw-Hill, New York, 1937, 1938, 1939. (IO) Sutermeister, E., “Chemistry of Pulp and Papermaking,” Wiley, New York, 1941. (11) Tech. Assoc. Pulp and Paper Industry, “Bleaching of Pulp,” Monograph 10, 1953. (12) Tech. Assoc. Pulp and Paper Industry, “Industrial Water for Pulp, Paper and Paperboard Manufacture,” TAPPI Monograph 1,1942.
LlPUOR
OASES-STEAM
Unit process flow diagram for chemical recovery
RECEIVED for review April 10, 1956 ACCEPTED July 20, 1956 VOL. 48, NO. 12
DECEMBER 1956
2155
