Dry matter content, chip size distribution and bark content are determined with SCAN-CM tests. Standards used in the testing are:
• CM 39:94 Wood chips for pulp production – Dry matter content
• CM 40:01 Wood chips for pulp production – Size distribution
• CM 41:94 Wood chips for pulp production – Sampling
• CM 42:06 Wood chips for pulp production – Bark content
The quantities used
When sawmills sell the chips to pulp mills, it is based on price tables which are calculated from the dry matter content. The price is given as euros against a ton of dry matter.
Dry mass is calculated by measuring the wet chip weight and then multiplying it with dry matter percentage. Dry matter content is derived from weight of dried chips versus weight at the time of sampling.
Measuring units vary in practice at sawmills from loose m3 to solid m3 to fresh matter ton. Often the solid m3 measurement, which is the basis for the trading, is calculated by using a computational multiplier against the loose m3 volume. Dry matter content is in practice determined by the seller, and it is reported back to the seller on a monthly basis.
Quality requirements
The chip purchasing companies determine the quality definitions of chips individually. Factors affecting the final price are the dry matter content, size distribution of chips and the bark content, all of which are measured by using the sampling and test screenings.
The quality of chips deteriorates with too big chips, sticks, sawdust and bark.
From the quality yield of chips point of view the proportion of so called optimum fraction of over 13mm is most important. The bigger the content of this, the better the price multiplier.
Oversized fractions’ (oversized hole of >45mm and oversized gap of 8mm) maximum allowable content – after which the quality deteriorates – is defined individually for each supplier. Oversized chips have to be rehandled in the pulping process, reducing their size in order to make them suitable for the process.
Also small particle (hole >7mm, stick particle hole >3mm and sawdust) maximum contents have been agreed individually with each supplier. In pulping process the small particles reduce the value of pulp.
Sawmill optimize the quality yield of chips by modifying the speeds of chipping units against the sawn goods size production’s sawsets to meet buyers optimum chip requirements.
Furthermore, the chip sizes can be affected with various technical means, such as different cutter heads and cutter head angles. Sawmills can also screen to reduce the unsuitable particle content of the chips.
In practice, the biggest challenge comes from the minimising of small particles, which is reduced with screening. Another seasonal challenge is in winter, in keeping the bark content within agreed tolerances.
