1) Mix the pulp sample thoroughly either in slurry form or shredded pulp cake (see figures 1 and 2). If it is a slurry form, use a plastic beaker or a wide spatula to mix it thoroughly ensuring homogeneity. It settles quickly, therefore it is essential that you mix often before and while taking your sample. If it is a vacuum de-watered pulp cake, either use a machine such as a pulp shredder or Hobart Blender or tear it by hands to small pieces (see figure 1). Never grab a big chunk from the de-watered cake as your testing sample. Note: After you mix the slurry, it quickly becomes inhomogeneous again, evident from the pools of supernatant that form rapidly after mixing (see figure 2).
Figure 1: The bucket on the left contains 4.5% pulp in slurry form. The pulp on the right is in shredded cake form which is at a consistency of ~20%.
Figure 2: A few minutes later after mixing. Notice how the water emerges in the right-hand picture while pulp is settling. Mix thoroughly often to ensure you collect homogenous and representative samples.
2) Pre-dry some filter papers that will fit your Büchner funnel. Dry them in the speed dryer and obtain their oven dried (OD) weight. Write this weight on the back of the filter paper. Note: Pre-dried tared filter papers should already be near the speed dryer. Before you deplete that supply, pre-dry more, keep at least half a box full for future use.
3) When taking your sample, take from different parts of the container or bag, e.g. from the left, top, bottom, right, middle such that you get a representative sample. For the sake of time, it is best to keep the amount of pulp low, such that ultimately, you will have less than 3 g of oven dried (OD) pulp. The pulp cakes have a higher consistency than the slurries. The slurries from the low consistency refiners (LCR) are at ~ 4% consistency. This means that there are 4 g of OD pulp for every 96 g of water, total 100 g of slurry. We would target for 2-3 g of OD pulp. Let’s say we want 3 g of OD pulp, therefore the amount of 4% wet pulp needed is:
Aim for around 75 ± 5 g of slurry as the slurries will vary in consistency and you don’t actually know the consistency just yet. It is strongly recommended to take at least two samples for consistency determination. Remember to immediately seal and put the sample back to the refrigerator after taking some for testing. This prevents changes in consistency and rotting of your sample.
4) After you have weighed out the slurry into a tared beaker, add some water (~300 ml) to disperse the sample. Hook up the vacuum apparatus, place your filter paper in the funnel with your writing (weight and sample name) facing the porous bottom of the funnel. Use squirt bottle to pre-wet the filter then apply vacuum slightly to pull it against the porous bottom. Pour the suspension in slowly. Remember to quantitatively transfer your sample!
5) Depending on the type of pulp used, the filtrate that seeps through contains plenty of fines and DCS (dissolved and colloidal substances), which make up approximately 25 to 30% of the OD weight of TMP (see figure 3). Release the vacuum and pour the filtrate to the beaker. Take the filtrate and re-filter it through the pulp pad. Repeat another two times for a total of three times. Once this is done, remove pulp pad together with filter paper on the bottom side of the pad, and rub the bottom and inside wall of the funnel with your finger to pick up any pulp left behind. Transfer these fibres from fingers onto the pulp pad. Fold the pulp pad inside filter paper in half and dry using the speed dryer (see figure 4). Note: Re-filtering helps reduce the loss of fines.
Figure 3: On the left, is the initial filtrate. The cloudiness comes from the many fines and DCS in the suspension; in the middle, is the second pass of filtrate that has been re-filtered through pulp pad; on the right, the third time filtrate has been re-filtered through the pulp pad. Notice the increase in transparency signifying that fines are being retained.
6) Check the weight one hour later until constant weight is reached. Once the sample is completely dried, weigh it, being careful to record the weight right when it comes out of the dryer. Wait for it to stabilize (two seconds is good, don’t wait long because the pad rapidly absorbs moisture) and record the weight.
Figure 4: Semi-circle pulp pads being dried at speed dryer for consistency determination.
7) To find the consistency, divide the OD weight (subtract the filter paper weight first) by the wet pulp weight, that gives a fraction, multiply by 100 to get %. For example, original pulp slurry is 76.45 g; filter paper is 1.23 g; total weight of dried pulp pad with filter paper is 4.35 g.
4.352 g of dry pad– 1.232 g of filter paper = 3.12 g of OD pulp
8) Perform this entire process at least twice and average your results to finalize the consistency determination of the bulk pulp sample.
