Method for reducing contamination in pulp processing

Method and apparatus for reducing contamination in processed pulp and in white water effluent discharged from a pulp dryer or paper making mill. The system includes multiple pulp bleaching and washing stages wherein white water effluent discharge from the dryer or paper making mill is fed to the next-to-last washing stage and fresh water is supplied to the last washing stage.

BACKGROUND 
This invention relates generally to pulp processing and, more particularly, 
to the final bleaching and washing stages of a multiple stage pulp 
processing system. 
In the pulping industry, environmental and economic concerns dictate that 
water be conserved and effluent volumes be reduced. Reductions in the 
amount of fresh water consumed, as well as in effluent volumes may have 
additional beneficial consequences in the form of reductions in amounts of 
fibers and chemicals used, reductions in heat losses, and overall 
reductions in operating costs. To achieve these benefits, however, it is 
critical that these matters be handled correctly and efficiently. 
One area of conventional pulp processing which utilizes considerable 
amounts of fresh water is in the washing operations following the last two 
pulp bleaching stages. Another area where large amounts of fresh water are 
used is in pulp drying apparatus. 
In the prior art, one current practice is to conserve water by extensive 
recycling in the pulp dryer, and by using excess white water effluent 
discharge from the dryer to wash pulp in the washer following the last 
bleaching stage. In this manner, effluent from the pulp dryer can be 
substantially reduced. However, fresh water is still required in the 
next-to-last washer accompanying the next-to-last bleaching stage. The 
prior art arrangement described above causes corrosion problems in the 
dryer due to the build up of salt, pitch and resin concentrations which 
will eventually effect pulp quality. In addition, this configuration can 
result in build up of other potentially harmful substances that could 
cause injury to intermediate or final users of the product. 
It has now been discovered that the problem of build up of various harmful 
deposits in the white water system of the pulp dryer (or other pulp 
processing apparatus such as a paper making mill), may be effectively 
dealt with by using the white water effluent discharge as the wash water 
in the second-to-last washer stage, and by using fresh water in the last 
washer stage. In this way, most of the harmful white water compounds will 
be washed out in the last washer. As a result, it has been found that 
fresh water consumption and effluent volumes may be maintained at a lower 
level, and contamination of the white water discharge as well as the pulp 
leaving the dryer or other processing apparatus may be reduced up to about 
40% of the contamination level currently experienced in the prior art 
systems. 
Another advantage of the arrangement described herein is a reduction in 
corrosion within the pulp dryer which, normally, is not made of the higher 
grade alloys used in the construction of, for example, the pulp bleaching 
apparatus. 
While there may be other ways to reduce the concentrations of various 
harmful compounds in the white water system, the only one that has been 
found to have reduced water consumption and effluent volume is one which 
requires the addition of still another washer stage between the last 
bleaching stage and the pulp dryer or other pulp processing equipment. 
However, the cost of this measure is substantially higher, both from an 
investment standpoint as well as an operational standpoint, particularly 
since this additional washer would have to be made of high grade steels to 
prevent excess corrosion. 
Additional details and objects of the invention will become apparent from 
the detailed description of the invention and claims which follow.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, the final stages of a multiple stage pulp 
processing system 2 are shown in schematic form. Pulp from a preceding 
processing stage 4 is pumped through a conduit 6 and, after mixing with 
chemical additives at 8 is fed into a bleaching tower 10. The bleaching 
tower 10 represents the next-to-last bleaching stage in the process. From 
the tower 10, the pulp flows through conduit 12 to a washer stage 14, 
representing the next-to-last washer stage. Discharge from the washer 14 
flows by way of conduit 16 through a filtrate tank 18 from which it is 
recycled to preceding process stages via conduit 20. 
Pulp is fed from the washer stage 14 through conduit 22 which receives 
further chemical additives at 24 prior to entering the last bleaching 
tower 26. The finally bleached pulp is then passed by way of conduit 28 to 
a final washer stage 30. Washer discharge flows through line 32 to a 
filtrate tank 34 from which the filtrate is recycled to preceding 
processing stages through conduit 36. The finally washed pulp is then 
transported through a conduit 38 to a further pulp processing device 40 
which could be, for example, a pulp dryer, paper making mill or the like. 
White water effluent discharge flows from the pulp dryer, paper making 
mill or the like, through a discharge conduit 42 to a white water holding 
tank 44. It is conventional in the prior art to feed the white water from 
tank 44 through a conduit 46 to the last washing stage 30 where it is 
dispensed through nozzles 48. Utilization of white water in the last 
washer stage 30 is said to substantially eliminate effluent from the pulp 
dryer, paper mill or other pulp processing device. 
In this prior art system, fresh water alone, or fresh water mixed with 
filtrate from the last washer stage, or fresh water with minor amounts of 
white water from the white water holding tank 44 are utilized in 
conjunction with the next-to-last washer stage. Specifically, the water or 
water mixture is added by way of conduit 50 to dispensing nozzles 52 
associated with washer stage 14. 
As previously stated, the above described prior art arrangement eventually 
leads to corrosion problems in the dryer or other pulp processing 
apparatus due primarily to the build up of salt, pitch, and resin 
concentrations that eventually effect the quality of the final pulp 
product. Build up of other substances which are potentially harmful to 
users of the end product may also occur. 
Turning now to FIG. 2, illustrated therein is a schematic diagram of a 
contamination reduction system in accordance with the present invention. 
Like numerals, with prime characters added, are utilized to refer to 
elements in common with the prior art system illustrated in FIG. 1. Thus, 
pulp from a previous processing stage 4' in the bleaching plant is 
transported through conduit or pipe 6', to which chemical additives are 
supplied at 8', and fed through the bleaching tower 10' to a washer stage 
14'. 
Pulp leaving washer stage 14' is then passed, via line 22', to the final 
bleaching tower 26', and thereafter, through conduit 28' to the final 
washer stage 30'. While the washers are illustrated as outside the towers, 
they may be within the towers (e.g. diffusion washers). Subsequently, the 
pulp is delivered via conduit 38' to a pulp dryer, or paper making 
machine, or the like 40'. As in the prior art system, discharge from 
next-to-last washer 14' is passed through a filtrate tank 18' and the 
filtrate is returned to preceding processing stages via conduit 20'. 
Similarly, discharge from the last washer 30' is passed through a filtrate 
tank at 34' and returned via line 36' to preceding processing stages. It 
will be understood, of course, that line 36' may be operatively connected 
to the next-to-last washer stage 14'. It is at this point that the present 
invention departs from the prior art arrangement. 
In accordance with an exemplary embodiment of this invention, white water 
effluent discharge from the holding tank 44' is transported through a 
conduit 54 to the dispensing unit 52' at the next-to-last washer stage 
14'. It will be appreciated that the white water may be utilized in the 
washer stage 14' alone or in combination with filtrate from the last 
washer stage 30'. 
At the same time, fresh water, or fresh water with minor amounts of white 
water from the holding tank 44 (represented by phantom conduit 58), is fed 
via conduit 56 to the dispensing unit 48' at the last washer stage 30'. 
By using the white water as wash water in the next-to-last washer stage 
14', it has been found that most of the harmful white water compounds will 
be washed out in the last washer stage 30'. By this arrangement, not only 
is the fresh water consumption and effluent volume maintained at a low 
level, but in addition, the contamination of the white water and the pulp 
leaving the dryer, paper making mill, or the like may be reduced up to 
about 40% of contamination levels currently experienced in the prior art 
system. Thus, the present invention solves, to a large extent, a 
potentially damaging and harmful contamination problems presently 
experienced in the pulp processing industry. 
While the invention has been described in what is currently regarded as its 
most practical embodiment, it will be apparent to those of ordinary skill 
in the art that many alterations may be made without departing from the 
spirit and scope of the invention as defined in the claims which follow.