Fines fraction of paper stock by wet screening

Transcription

1 T 26 cm-94 PROVISIONAL METHOD 979 CLASSICAL METHOD 990 REVISED TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability or responsibility under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published. CAUTION: This method may require the use, disposal, or both, of chemicals which may present serious health hazards to humans. Procedures for the handling of such substances are set forth on Material Safety Data Sheets which must be developed by all manufacturers and importers of potentially hazardous chemicals and maintained by all distributors of potentially hazardous chemicals. Prior to the use of this test method, the user should determine whether any of the chemicals to be used or disposed of are potentially hazardous and, if so,must follow strictly the procedures specified by both the manufacturer, as well as local, state, and federal authorities for safe use and disposal of these chemicals. Fines fraction of paper stock by wet screening. Scope. This method is designed to measure the fines content of paper stock or of pulp samples by means of a single-screen classifier. A modified procedure permits the use of the apparatus to measure the tendency of the fines fraction to be retained by the fiber fraction under graduated turbulent conditions..2 Separation of suspended particles of different shapes and sizes by wet screening may be accomplished using various devices. The apparatus described below is recommended for convenience and accuracy. 2. Significance 2. Samples of paper stock, particularly those taken from the paper machine circulating system, are characterized by having different proportions of particles generally classified as "fibers" and "fines." The proportions of these two components may vary greatly, depending upon the furnish, upon stock preparation, and whether mineral fillers are added to the stock. The greater tendency of the fines fraction to pass through the wire during sheet formation and to be recycled leads to an accumulation of fines in the headbox. The extent of this buildup is an indication of the retention performance of the machine and affects, for example, drainage, felt filling, and saveall loading. 2.2 A suspension such as paper stock contains a range of solid particles that differ in size by small increments. Any dividing line between what is defined as fiber and as fines is an arbitrary decision. From review of published literature and microscopic examination, it is recommended that the dividing line should be particles that will pass a round hole 76 µm in diameter or a nominally 200 mesh screen. Other hole sizes may be chosen as discussed under Section This technique may be used to measure the relative tendency of the fines fraction to stay with the fiber rather than to follow the water into the filtrate under graduated turbulence in simulation of paper machine conditions. The determination of fines retention is of particular importance in determining the effect of graduated turbulence in the evaluation of additive materials such as retention aids. Approved by the Pulp Properties Committee of the Process and Product Quality Division TAPPI

2 T 26 cm-94 Fines fraction of paper stock by wet screening / 2 3. Apparatus and materials 3. Fiber classifier. The single-screen classifier consists of a cylindrical sample holder equipped for the insertion of a bottom screen and air chamber. 3.2 Stirrer. The 50-mm stirrer shaft carries a three-blade, 5 x mm (0.2 x 0.04 in.) propeller. The blades are circular, 7.5 mm ( / 6 in.) in diameter and with a pitch of 30. The stirrer must be centered in the jar and be free of wobble or vibration. The distance between propeller and screen is 3.2 mm ( / 8 in.), set with a shim. The direction of rotation of the motor shaft must be counterclockwise so that the push of the propeller blades is downward toward the screen. The stirrer motor and control shall maintain 99% of set speed within the range of speed ( rpm) and torque used in the test. An integrally-wound motor-generator with servo feedback amplifier type motor drive has been found to be satisfactory. 3.3 Screens. It is recommended that perforated metal plates supplied with equipment be used in this test. The holes in these screens have a conical cross section which is produced by electrodeposition of nickel. Screens furnished have a shiny side and a dull side. The shiny side of the screen is the designated hole size of the screen and is placed face up in the fractionator to minimize plugging. 3.4 Dispersant concentrate, 2.5% by weight Na2CO 3, 2.5% by weight sodium tripolyphosphate (Na5P3O 0), and 2.5% by weight Tamol 850 Dispersant by Rohm and Haas Co. 3.5 Wash water. Wash water which is used in the fractionation procedure is made by adding 5 ml of the above dispersant concentrate to 5 L of distilled or deionized water. 3.6 Other equipment needed for the performance of this method includes: TAPPI disintegrator, stock containers of various sizes, circulating oven, desiccator, fast-weighing balances, plastic weighing cups, timer, stirrers, Büchner funnel and flask, variety of beakers and graduated cylinders, and a wash bottle. NOTE : A forced-air oven (circulating oven) is preferred as it gives better control of air circulation, hence, shorter drying times as well as better temperature control and uniformity than a natural (gravity) convection oven. A dessicator should be used to allow the ovendried samples to cool to room temperature, in a dry atmosphere, prior to weighing. 4. Sampling and test specimens Samples for this procedure may originate as small-scale laboratory-prepared furnishes, or they may originate from various points in the paper mill system. A critical factor is that all sampling must be representative of whatever system from which it is withdrawn. 5. Procedure 5. Sample preparation 5.. The total solids in any sample must be measured accurately to ±0.0% consistency. The following stock preparation procedure is recommended: If the stock consistency is greater than 0.5%, it is diluted to approximately 0.5% with distilled or deionized water. If less than 0.5%, it can be used as is. A master batch of from 4 to 0 L is kept dispersed by stirring. For the consistency test, a sample is dipped from the batch, poured into a 00-mL capacity weighing cup and weighed on a fast-weighing balance, making sure that the outside of the cup is dry. The sample is filtered through weighed filter paper (Whatman No. 30 or equivalent), dried in an oven at 05 C and consistency is computed in the usual manner The accurate fractionation of suspended solids by size requires that the particles be discrete and act as individuals during the test. Agglomeration or flocculation of particles will interfere with the test, making the fines fraction appear too small and the fiber fraction too large. See paragraph 5..3 for elimination of flocculation If the sample has been treated with any material having a flocculant or has flocculated, add 0.5 ml of dispersing solution to the 500-mL sample for fractionation. Then place in a TAPPI disintegrator for min. This procedure eliminates any flocculation tendencies, without foaming and without generation of fines (see 3.5). 5.2 Fractionation procedure 5.2. From the sample of paper stock to be tested at a known consistency of about 0.5% (see 5..), withdraw a sample of net weight g, weighed to 0. g accuracy on the fast-weighing balance. Dilute this sample to 500 ml with "wash water" described above, and transfer to the drainage jar with bottom outlet closed. Set the stirrer with the propeller 3.2 mm ( / 8 in.) from the screen and with a speed of 750 rpm, and commence drainage. To maintain a steady, uniform flow, provide the stopper with a 6.35-mm ( / 4-in.) glass tube connected with a rubber tube and pinch clamp. Release the pinch clamp to start the flow. Allow the contents of the jar to drain into a beaker. Continue the

3 3 / Fines fraction of paper stock by wet screening T 26 cm-94 drainage until the sample is within to 5 mm of the screen, then add an additional 500 ml of wash water to the jar and drain. Add additional 500-mL portions of wash water to the jar and drain as before. Collect each 500-mL portion in a separate beaker and repeat the process until clear water is obtained in the final beaker. This normally occurs at the fourth or fifth beaker. Clarity is defined as not being over 0 ppm suspended solids; this can be judged visually. It is suggested that a dispersion of 0 ppm fines be prepared of the stock being tested. Use this sample to judge the clarity of the filtrate. Rinse the fiber fraction remaining on the screen onto a weighed filter paper (Whatman No. 30 or equivalent) on a Büchner funnel, transfer it to an oven, and dry to a constant weight. It is recommended that a fastweighing balance, located close to the oven, be used. The weighing should be accurate to mg. If desired, the filtrate containing the fines fraction may be filtered and the solids weighed, although normally this is not considered necessary. 5.3 Retention procedure 5.3. Place a 500-mL sample of known fines fraction and consistency in the retention jar. Fit the outlet of the jar with a rubber stopper containing a 6.35-mm ( / 4-in.) glass tube with a rubber tube with pinch clamp connected to a medicine dropper. This arrangement is designed to control the downflow to a point that a mat does not form on the screen, even at relatively low stirring rates. The principle of the apparatus is that the flow of fine particles through the holes of the screen is not impeded by formation of a mat. Set the rotation of the stirrer at a selected rate. For headbox samples, it is common to make retention measurements at several speeds of the stirrer, commonly 500, 000, and 500 rpm. When all is in readiness, release the pinch clamp and collect the filtrate in a weighing beaker for 30 s. The sample collected should be in the range of 80 to 00 ml and weighed to 0. g. Filter this sample on a weighed filter paper (Whatman No. 30 or equivalent), dry in an oven at 05 C to a constant weight. Record the weight to the nearest mg In the fractionation technique, the dilute sample of stock in the jar is washed with successive portions of wash water until visually free of fines. In the retention procedure, a higher consistency sample (about 0.5%) is allowed to drain briefly, and an aliquot of the filtrate is used to measure the distribution of fines between the fiber left in the jar and the filtrate passing through the screen. Normally, retention is performed at several levels of turbulence by controlling the rpm of the stirrer as a simulation of various degrees of turbulence on a paper machine. 6. Calculations 6. Fractionation. Determination of percent fines in diluted stock sample fines content of the sample is calculated by the following equations: F = {[(A C) B] / (A C)} 00 or F = [D/ (A C)] 00 where F= fines content, % A= weight of original sample, g B= weight of fiber pad, o.d., g C= consistency, g/g D= weight of fines collected on filter paper,g Example: if the fractionated sample was exactly 0.% consistency, then the 500-mL-diluted sample contains 500 x 0.00 = g total solids. The residual fiber on the screen weighed g; the % fine equals: [(0.500 g g) / g] x 00 = 26% fines in headbox (total solids basis) 6.2 Retention. Retention is calculated as follows, where C and F are the quantities defined in 6.. In this case, retention is defined as the percentage of the total fines retained: S = V C T = (S F) / 00 R = [{T - [W (V/U)]}/ T ] 00 where S= total solids in sample, g T= total fines in sample, g R= fines retention, % U= weight of filtrate, g

4 T 26 cm-94 Fines fraction of paper stock by wet screening / 4 V= volume of original sample, ml (assumed numerically equal to the weight of original sample in g) W= weight of fines in filtrate, g Example: Suppose the headbox consistency was determined to be 0.57% ( g/g). Then the total solids in 500 ml of furnish would be 500 ml x = g total solids. Suppose that the percent fines as determined by fractionation (Step ) was 26%; then the total fines in the sample would be: (2.585 g x 26%) / 00 = g headbox fines per 500 ml. Now suppose the weight of the drainage volume was 02.5 g and the solids in that sample were g; the percent retention of fines fraction would be: {[0.085 g (500 ml / 02.5 g)] / g} 00 = 6.7% retention of fines. 6.3 In the procedure described above, retention is calculated as a percentage of the fines fraction retained on the screen with the fiber. The reason for this is that the fiber fraction is almost completely retained under paper machine conditions. If the calculation is based upon total solids in the headbox, then the result will be biased by the ratio between fines fraction and fiber. This can be of great importance in comparing laboratory, pilot plant, and machine results. However, in making tests on machine samples, it may be preferred to make the calculations based upon total solids. The total solids in the sample is calculated as in 6.2 (2.585 g). If the solids (fines) in the filtrate is 0.45 g, then: Total solids retention, % = [( ) 00] / = 83.9% 7. Report 7. Fractionation is reported as percent fines fraction of 00 minus that value, which is the fiber fraction, both based upon total suspended solids. In systems containing mineral filler, this will normally appear in the fines fraction. Conversely, the fiber fraction is normally filler or ash free. 7.2 Retention is reported as percent fines fraction retained under the conditions of the test. Such conditions would include rpm of the stirrer, hole size and percent open area of the screen, temperature, and geometry of the jar such as insertion of vanes or baffles. If no conditions are specified, this should mean 000 rpm of the stirrer, hole size 76 µm, percent open area 4.5, and jar geometry as shown in the Appendix. 8. Precision 8. Experience shows that most errors relating to this method originate from errors in consistency determinations and in withdrawal or representative samples from batches. 8.2 Repeatability = 2.77% fines. 8.3 Obviously, where relative fiber length is not of interest, the greater convenience of the single screen, the avoidance of cumulative error, and the ease of collection and examination of the fines fraction make the single screen classifier advantageous. 9. Additional information 9. Effective date of issue: April 20, Related method: TAPPI T 233 "Fiber Length of Pulp by Classification." 9.3 This method was reviewed and in part updated prior to reclassification as a classical method in 990. This revision is to correct errors in calculations which appeared in the 990 revision. Appendix A. Jar assembly. A sketch of the apparatus is shown in Fig.. The swingout base support is attached by means of a short pipe nipple and tee to a pipe section, into which has been inserted the upright rod of a support stand. This arrangement fixes the elevation of the base support and by swing-out permits the insertion and removal of the jar with no change in elevation of the jar or the propeller of the stirrer. The distance between propeller blade and the screen must be constant for all tests, normally 3.2 mm ( / 8 in.). The jar itself consists of two parts: the upper chamber or barrel is formed from a section of clear plastic pipe 0.6 cm (4 in.) i.d., 6.35-mm ( / 4 in.) wall thickness, and 5.24 cm (6 in.) long. The total volume is about L and the normal sample volume is 500 ml. The lower chamber and the barrel are

5 5 / Fines fraction of paper stock by wet screening T 26 cm-94 threaded to fit each other and to hold the perforated plate between them. The perforated plate is shown as a dotted horizontal line in Fig.. This assembly is sealed with rubber gaskets. The lower chamber forms an air lock to hold the sample in the reservoir until the start of the test. The size of the aperture may be varied to control the rate of flow of the filtrate. There may be occasions when greater turbulence is required over that obtained in the sample jar in Fig.. This can be achieved by inserting plastic vanes 6.35 mm ( / in.) square in cross section cemented to the wall of the jar. 4 References. TAPPI CA Report No. 57, Chapter VIII, 975, TAPPI, Atlanta, GA. 2. Britt, K. W., Tappi 56 (0): 46 (973). 3. Britt, K. W., and Unbehend, J. E., Tappi 59 (2): 67 (976). 4. Frankle, W. E., and Sheridan, J. L., Tappi 59 (2): 84 (976). 5. Unbehend, J. E., Tappi 60 (7): 0 (977). Your comments and suggestions on this procedure are earnestly requested and should be sent to the TAPPI Technical Divisions Administrator.