Laboratory Analysis of Mechanical Pulp - The Effects of Storage Conditions and Time on Measured Technical Properties of Pulp, 2000 Papermakers Conference Proceedings

Tuomas Stoor, Heli Rautjärvi, Jouko Niinimäki--Errors in pulp sampling and analyze procedure itself provide most serious source of error for laboratory analysis of pulp properties, but also storage of samples may cause changes in pulp properties. Usually, the time between sampling and laboratory analysis is not noticed as a potential source of error, even though it is well known that many processes such as extraction and biodegradation take place especially in water suspensions at pH close neutral. Experiments were carried out with thermo mechanical pulp (TMP) made of Norwegian spruce (Picea abies) produced at UPM-Kymmene Co pulp mill in northern Finland. Pulp was tapped prior to screen room from the feed line and the tapped pulp was divided into three samples to be stored in different conditions. One part of the pulp sample was stored in the cold storage room where temperature was +3°C. Second part of the pulp was stored in room temperature (21°C). The third part of pulp was stored in freezer temperature of -18°C. All samples were tightly sealed in plastic (LD-PET) buckets with plastic lids. In the cold-storage room and in the room temperature stored samples were analyzed after 1, 2, 3 and 6 weeks storage times. Frozen pulp sample was defrosted, analyzed and congealed again three times in the period of 6 weeks. Clear decrease in pH was observed in pulp suspension stored in room temperature, whereas negligible changes took place in case of the pulps stored in cold storage room or in freezer. Drainage of pulps was examined both with CFS and measuring drainage time in sheet mould. CSF increased during the storage period in room temperature and along freezing cycles. Change in freeness is insignificant when pulp is stored in cold room. On the other hand, drainage time of the pulps in a sheet mould stayed constant with pulps stored in freeze and cold room but clearly increased in the case of pulp stored in room temperature. There are many factors affecting the result of freeness measurement. Because there is difference in aperture size of screens (CSF 0.5 mm and sheet mould 0.15 mm), we can assume, that the main reason for increased drainage time is agglomeration of colloidal material. The strength properties stayed relatively constant over six week testing period in that pulp stored in freezer either cold room, but in room temperature those properties collapsed at the second week of storage period. Optical properties remained relatively unchanged in all the storage methods. Microbiological cultivation indicated variety of micro-organisms also in the original mill pulp. Especially pulp sample stored in the room temperature proved its suitability for growing base for bacteria. Considering the volume of micro-organism, it is due to affect the quality of pulp and it can even be considered as the primary reason for deterioration of the strength properties of pulp samples. Cold room showed its compliance as a mechanical pulp storage place when it was compared with freezer and room temperature storage. In cold room of +3 °C, pulp samples remained relatively unchanged over the testing period of six weeks. Freezer is the best storage in the case of long storage times, even if slight changes in drainage properties may occur already during the first freeze-defrost cycle. Higher number of the cycles are not recommended. The storage time in room temperature should be kept in minimum, because fast changes may occur due to biodegradation. Definite maximum time at room temperature is one week and after that samples do not represent adequately the original sample properties.