Wood content in the bark fraction - PFI

SCAN-CM 53:94Accepted 1994Wood chips for pulp productionWood content in the bark fraction1 ScopeThis SCAN-test Method describes the procedure fordetermining the wood content in the bark fraction from abarking drum or from other machines for barking.The Method is applicable to the determination of thewood losses in the barking process. The total wood lossalso includes breakage. The determination of breakage isdescribed in the Appendix.2 ReferencesSCAN-CM 39 Wood chips for pulp production −Dry matter contentSCAN-CM 40 Wood chips for pulp production −Size distributionSCAN-CM 41 Wood chips for pulp production −Sampling3 DefinitionsFor the purpose of this Method the following definitionsapply:3.1 Bark fraction − The mixture of bark, wood,branches etc, which is separated from the pulp wood in abarking drum or in other machines for barking. Theexpression "bark" refers to the outer bark as well as theinner bark.For the purpose of this Method, needles and smalltwigs are regarded as bark whereas the cambium isregarded as wood.3.2 Wood content in the bark fraction − The totalwood content in the bark fraction excluding breakage.3.3 Wood loss − The part of the total wood input tothe barking operation that is not used for the productionof pulp.3.4 Breakage − Piece of wood, from the trunk of atree, of oven-dry mass exceeding 20 g, present in thebark fraction.Note − Breakage is normally formed in limitedamounts when processing wood, for example in abarking operation. It is part of the wood loss.

SCAN-CM 53:94Page 24 PrincipleThe breakage and the fines are removed from the barkfraction sample. The wood in particles > 3 mm isseparated manually and its total mass is determined byweighing. The dry wood content is reported as apercentage of the dry sample, excluding breakage andfines.5 Apparatus5.1 Containers, of aluminium foil, for weighing thewood and the wood-free bark.5.2 Chip classifier, as described in SCAN-CM 40.5.3 Drying oven, capable of being controlled at(105 ± 2) °C, and suitably ventilated.5.4 Balance, accurate and readable to 0,1 g.6 Sampling and preparation of sampleThe sampling procedure is not covered by this Method.Make sure that sampling has been carried out in amanner that ensures representative samples. A suitablesampling procedure for wood chips is described inSCAN-CM 41 and can be used for the bark fraction, asfar as it may be applicable.The test portion for each determination is 4 to5 litres. If the sample has to be subdivided to obtain testportions of that size, take precautions to avoid anyfractionation of the material. A suitable procedure forsubdividing a sample is described in SCAN-CM 41.If the sample contains breakage, the breakage has tobe removed from the sample before the determination.Determine the dry matter content of the bark fractionsample as described in SCAN-CM 39.The dry matter content of the sample should exceed50 %. If the sample has a dry matter content below 50 %the separation of the fines fraction will be incomplete.Let the sample dry before screening.Screen the bark fraction sample in a chip classifier(5.2) as described in SCAN-CM 40 without using thesecond screen (mean free distance between adjacent rods8,0 mm). Discard the fines fraction and use the otherthree fractions for the determination of wood content.Note − The fines fraction is discarded because thewood particles in the fines fraction are too small tobe separated from the bark.The fact that the fines are left unconsidered inthis test introduces an error in the result if the woodcontent of the fines fraction deviates from theoverall wood content. Normally the wood contentof the fines is slightly lower than that of the wholesample and the result of the test will be somewhathigher than the true value.For the purpose of this Method, the error isdisregarded.7 ProcedureWeigh the dry aluminium containers (5.1).Spread the sample in a thin layer over the top of alarge table. Remove all wood particles and collect themin an aluminium container. With the aid of a sharp knifesplit those pieces in which wood is attached to the barkso that the wood can be removed. Put the remainingwood-free bark into a separate aluminium container.Remove and report separately all stones, plastic andother objects belonging neither to the wood nor to thebark fraction.Dry the wood and the wood-free bark separately incontainers in a drying oven (5.3) at (105 ± 2) °C. After aperiod of not less than 16 h and not more than 24 h,remove the containers with wood and bark and weighthem immediately on a balance (5.4).Note − Weighing the container before it reachesroom temperature causes a small weighing error.Another error may arise because the sample is incontact with the surrounding air, and it may gain orlose moisture during the weighing. For the purposeof this Method, these errors are disregarded andany prolonged waiting for the reading to stabilizeshould be avoided.8 Calculation and reportCalculate the wood content in the bark fraction from theexpression:aX = 100 [1]a + bwhereXabis the wood content in the bark fraction, as apercentage;is the mass of the oven-dried bark-free wood, ingrams;is the mass of the oven-dried wood-free bark, ingrams.Report the wood content with one decimal.

SCAN-CM 53:94Page 3The test report shall include reference to this SCAN-testMethod and the following particulars:(a)(b)(c)(d)(e)(f)date and place of testing;the sampling procedure;identification mark of the sample tested;the result;the mass of stones, plastic and other objects notbelonging to the wood or to the bark fraction, as apercentage of the dry sample excluding breakageand fines;any departure from the standard procedure andany other circumstances that may have affectedthe test results.9 PrecisionThe method error has been calculated as:s =2∑d2n[2]The repeatability and the reproducibility, calculated asthe method error, are given in Table 1.Table 1. The repeatability and the reproducibility,calculated as the method error (s), within and betweenlaboratories for two different types of bark fractionsamples (the samples contained no breakage).Repeatability for two different laboratories:BarkfractionsampleLab AMethoderrorwithinlabNumberofduplicatesamplesLab BMethoderrorwithinlabNumberofduplicatesamplesBirch 0,6 13 0,4 20Spruce 0,6 10 0,3 36Reproducibility between two laboratories:wheres is the method error;d is the difference between the results ofduplicate determinations;n is the number of duplicates.Bark fractionsampleMethod errorbetween labBirch 1,0 18Number ofduplicatesamplesTwo laboratories made duplicate determinations on twodifferent types of bark fractions samples, namely birchwood and spruce wood bark fractions. For some of theduplicates, the laboratories each tested one of them.

SCAN-CM 53:94Page 4Appendix - Determination of breakageWhen calculating the total wood loss, wood loss causedby breakage should be included. A suitable procedure todetermine the breakage in the bark fraction is to removeall breakage in the out-flow from the barking processduring a specified time.Calculate wood loss caused by breakage from theexpression:100cY =[A.1]dwhereY is the wood loss (oven-dried) caused bybreakage, as a percentage;c is the flow of bark-free breakage out of thebarking process, in kilograms oven-driedbreakage per hour;d is the flow of wood without bark into the barkingprocess, in kilograms oven-dried wood per hour.SCAN-test Methods are issued and recommended byKCL, PFI and STFI-Packforsk for the pulp, paper andboard industries in Finland, Norway and Sweden.Distribution: Secretariat, Scandinavian Pulp, Paperand Board Testing Committee, Box 5604,SE-114 86 Stockholm, Sweden.