The Pope & Talbot, Inc. Halsey mill is a 500 admt/d market pulp mill located in the Willamette valley of western Oregon approximately 80 miles south of Portland. The mill is unique in that three M & D continuous digesters produce either softwood chip or sawdust pulp in campaigns lasting one to five days on each furnish. The softwood chip pulp is primarily Douglas Fir and the sawdust is a 50 % blend of Douglas Fir and Western Hemlock. The three digesters discharge into a common blow tank forming a single fiber line from that point on. Pulp from the blow tank is course screened with knotters then washed with three stages of vacuum washers, fine screened and washed a fourth time. After the fourth washing stage the pulp is further delignified in a medium consistency oxygen reactor. Dissolved lignin and spent caustic from the oxygen reactor is removed from pulp slurry via two twin roll wash presses operating in series. Fresh wash water is applied to the showers of the last press and all filtrates are cascaded forward from stage to stage in counter current fashion. After washing, the pulp is bleached in a four-stage bleach plant to 87 ISO and dried with a Flakt dryer.

The mill installed an oxygen delignification system in 1993 to reduce bleach plant chemical consumption and effluent color. The mill selected wash presses for the post O2 washing because of the high temperature of the stock exiting the reactor and the flexibility in consistency delivered to the subsequent stage. Prior to this time, the mill operated a conventional four-stage vacuum drum washing system. The solids in the first stage filtrate were controlled by adjusting the shower flows to the washers. The soda loss from the last washer was measured once per shift to monitor the washer performance. Conductivity and consistency measurements of the mat discharge were taken and the soda loss prediction made from a conversion graph.

After installing the oxygen system and wash presses, obtaining mat "squeezings" for measuring conductivity was made more difficult due to the high discharge consistency. The mill also questioned the relevancy of the conductivity test since caustic was added to the oxygen reactor and the associated filtrates became enriched in sodium. About the same time, controversy arose in the industry as to whether the soda loss test should be replaced by a more meaningful test like COD or TDS as the manufacturers engaged in debates as to who had the best washing device. Given the considerations mentioned above, the mill lost interest in soda loss as a process control test and relied on periodic but infrequent COD tests performed by the technical department to assess washer performance. After a period of time it became evident that the mill was experiencing large swings in the bleach plant chemical consumption which could be attributed to carryover from the brown stock washing system. The need to better control and monitor the performance of the washing system became self-evident. The following work is part of an on going effort to improve and control the washing losses.

This graph displays the COD and conductivity for the last two stages of brown stock washing along with the two stages washing following the oxygen delignification reactor. The relative drop in concentration from stage to stage can be seen and the variability appears to decrease as we follow process towards the final stage of washing.

This relationship is linear with a high correlation coefficient. This is not surprising since the COD and color are both a strong function of the organic or dissolved lignin content.

This relationship shows a slight curve and the best fit was obtained with a polynomial equation. We believe the slight curve is due to the oxidized and non-oxidized black liquor solids meeting at the 4^th filtrate tank. This is the filtrate tank associated with the last washing stage ahead of the oxygen delignification system.

This relationship also displays a slight curvature. Once again we believe the curvature is due to blending of oxidized and non-oxidized black liquor solids.

This relationship shows the same curvature seen when comparing COD to BLS. The regression coefficient of 0.97 suggests a strong relationship at our mill between COD and sodium or soda loss expressed as equivalent saltcake.

This relationship is linear with a very high regression coefficient of 0.98. If we shift the decimal point, the slope of this line represents the weight fraction of sodium on a dry black liquor solids basis. Horizontal deviations from the regression line in the lower quadrant are likely due to variations in the caustic charge applied in the oxygen delignification system. Horizontal deviations in the upper quadrant can be a result of variations in the white liquor or % AA applied at the digesters. In our case, the largest change in % AA is due to changing furnishes between softwood chips and sawdust.

This relationship is linear. In years past, this relationship was used as the basis to develop the quick test to predict the soda loss from conductivity measurements and washer mat discharge consistency.

This brings us back to COD versus conductivity, which was the first graph displayed. The relationship is shown again, but this time the regression equation and coefficient have been added.

COD and Color are linear in relationship across the four brown stock stages studied.

Total Dissolved Solids (TDS); sodium and conductivity are linear in relationship across four brown stock washing stages studied.

COD and Color display a polynomial relationship with respect to TDS, sodium and conductivity across the brown stock washing system This non-linearity is thought to result from the oxidized solids traveling towards the front end of the washing system and the non-oxidized solids traveling towards the back end. These two types of solids meet and blend at the washing stage immediately preceding the oxygen delignification system.

Although COD may be one of the better tests to predict and compare the impact of washing losses on bleaching chemical demand from one mill or fiber line to another, the test is complicated and time consuming for operating personnel to run. Given the normal variability encountered within our fiber line, conductivity was found to correlate very well with COD. Conductivity has the advantage of being a simple and quick test to perform manually and can be measured real time with an in-line sensor and the data stored and analyzed in the DCS and MIS.

Color is an excellent predictor of COD and a quick and simple procedure to perform with relatively inexpensive test apparatus.

Based on the findings included in this report, the Halsey mill installed a conductivity probe in the filtrate from the second wash press and is monitoring and trending this data in the DCS. Significant changes in the conductivity level have been observed when the mill swings between chip and sawdust production. The sawdust pulp is lower freeness causing slower drainage on the washers and increased carryover. The effects of changing production rate or shower flow rate have been readily observed and found to occur much faster than had previously been thought.

Having established the relationships between these different parameters, the mill is better able to predict what impact washing changes have on other areas of the mill with out physically having to run the additional tests. For example, if environmental impact is being assessed then BOD5, COD, and color are the important parameters to consider; from a bleaching chemical demand perspective COD, BLS and soda loss are the most important, from a chemical recovery perspective BLS and soda loss are the most important.