US2502330A

US2502330A - Washing paper pulp to remove coloring matter

Info

Publication number: US2502330A
Application number: US477514A
Authority: US
Inventors: Clarence E Libby
Original assignee: Individual
Current assignee: Individual
Priority date: 1943-03-01
Filing date: 1943-03-01
Publication date: 1950-03-28
Anticipated expiration: 1967-03-28

Description

March 28, 1950 c. E. LIBBY wAsHmc PAPER PULP To movl; coLQRINc MATTER Filed man 1', 194s 5 Sheets-Sheet 1 Oh. m. N

INVENTCR Clarence ELibby MM dwfm.

March 28, 1950 c. E. LIBBY WASHING PAPER PULP To REMOVE coLoRING MATTER Filed March 1, 1943 5 Sheets-Sheet 2 QON 00m: COV 00m, 00W

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INVENTOR March 28, 1950 Yc. E. LIBBY WASHING PAPER PULP To movi: `coLoRmc; uAT'rER 5 Sheets-Sheet 3 Filed March 1, 1943 KEY PE RC EN 7' C HLOR/NE C` ONS UME D Mardi 28, 1950 c. E. |BBY .2,502,330

WASHING PAPER PULP TO REMOVE COLORING MATTER Filed March 1, 1943 5 Sheets-Sheet 4 BEA TER STRENG/-l CHA/PA CTEP/snes BURSTA/VD TEA/P FA C70/R9 TEAR FA CTO/i 75o 67o 4 /fo 3o CANAo/AN SMA/0,490 FREE/mss INVENTOR CVs/"ence L/Znby Ah IMQ.

c'. E. LIBBY WASHING PAPER PULP T0 REMOVE COLORING MATTER March 2s, 195o 5 Sheets-Sheet 5 Filed March 1,A 1943 wwwml QQ EQ2 vk@ QvvU om o@ 0mm. A oom, om@ ovv .nu SQ n.555@ MSR m6 Qmmu 86 SM En Sum DMW INVENTOR Patented Mar. 28, 1950 WASHING PAPER PULP TO REMOVE COLORING MATTER Clarence E. Libby, Syracuse, N. Y.

Application March 1, 1943, Serial No. 477,514

8 Claims.

This invention relates to a process of removing coloring matter from paper pulp by any of the commonly used pulping processes, such, for example, as the sulphate (Kraft) soda or sulphite process.

This application is a continuation-in-part of my application, Serial No. 334,178, illed May 8, 1940, now abandoned.

'I'he invention will be described rst as applied more particularly to the removal of coloring matter from sulphate pulp. In carrying out the sulphate or Kraft process for producing paper pulp, the chips of wood are digested under pressure with a liquor containing sodium hydroxide and sodium sulphide as the principal constituents and also containing smaller amounts of other materials including sodium carbonate, sodium sulphate, sodium sulphite and sodium thiosulphate. During the digestionor cooking process, the color bodies are to a considerable extent dissolved in the cooking liquor. Unless this liquor is removed from the exterior and interior of the bers of cellulose at the end of the cook, enough of the color bodies remain on and in the fibers to impart a dark brown color to the paper made from them. These color bodies are organic compounds, either present originally in the wood or derived from reaction of the bodies in the wood with the treating chemicals and are in the nature of tannin or lignin compounds. The constituents of water may contribute to the production of color bodies, either directly or in combination with inorganic compounds. For example, dissolved iron or manganese willbe oxidized and precipitated respectively as red hydrous ferrie oxide and black manganese hydroxide, or may combine with the organic bodies to produce objectionable color.

Fig; 1 is a graph showing the reilectance values and chlorine consumed in bleaching pulps which had been previously washed with different chemicals while still in the digester and without access of air;

Fig. 2 is a graph showing the bursting strength of bleached sulphate pulps which had been previously washed with different chemicals while in the digester and without access of air.

Fig. 3 is a graph showing the reflectance values and chlorine consumed in bleaching pulps which had been previously washed outside of the digester in the presence of air.

Fig. 4 is a graph showing the beater strength characteristics, (burst and tear factors) of the sulphate pulp prepared according to Example 1.

Fig. 5 is a graph showing the burst factor o1' sulphite pulps which were washed prior to bleaching with water or with sodium hexametaphosphate solution.

I have found that the color of the pulp may be materially improved, if it is washed after the digestion step, but before bleaching, with an aqueous solution containing a molecularly dehydrated alkali-metal phosphate. The washing process may be carried out in the digester without allowing access of air to the pulp; or it may be carried out in the presence of air, for example, in washing pits or upon continuous rotary iilters.

The preferred molecularly dehydrated alkalimetal phosphate is the glassy phosphate commonly known as Grahams salt or sodium hexametaphosphate, which has a mol ratio of Na20 to P205 of 1:1. It will be understood that thissodium phosphate glass is given as representative of the family of glassy molecularly dehydrated phosphates and that other glassy molecularly dehydrated phosphates having a mol ratio of Na20 to P205 between 0.9:1.0 and 1.7:1.0, such as the so-called sodium tetraphosphate NasPiOn. with a mol ratio of Na20 to P205 of 1.5:1.0 or the so-called sodium decaphosphate, Na12P1o0s1, with a mol ratio of NazO to P205 of 1.2:1.0, may be used in place of or in addition to sodium hexametaphosphate. All of these glassy phosphates have properties which are similar to one another, although differing somewhat in their effectiveness.

I may use in place of or in addition to the glassy molecularly dehydrated alkali-metal phosphates the alkali-metal tripolyphosphates, for example. sodium tripolyphosphate, NasPzOio, or

the alkali-metal pyrophosphates, for example. tetrasodium pyrophosphate, Na4Pa0'z.

'I'he common practice in counteracting the bad eiects of the color bodies in paper pulp is to wash the pulp with water or with weak black liquor followed by water and then to bleach the pulp. I have found that greatly superior results are obtained by washing the pulp with water containing molecularly dehydrated alkali-metal phosphates, as compared with the results obtained in using the commonly known washing methods. Ialso have found that the molecularly dehydrated alkali-metal phosphates produce materi-ally superior results as compared with the use of caustic soda or other ordinary detergents. These superior results are shown by the lower chlorine demand in bleaching the pulp, the higher reflectance of the bleached pulp and the higher retention ofstrength in the bleached pulp.

Figs. 1 and 2 show the results obtained by washing a sulphate pulp after digestion but before contact with air with water alone or with aqueous solutions containing various chemicals. The washed pulp was then bleached with a chlorine bleaching agent and reflectance values and bursting strength values for the different samples were obtained. The bleached pulp which had been washed with sodium hexametaphosphate solution attained higher reectance values than the pulp washed with any of the other chemicals.

In carrying out 'the process, the pulp was treated in the digester after the digestion step had been completed.` The cooking liquor was drained from the dlgester and a washing solution was circulated through the digester while excluding air.' The circulation of the washing solution was continued for approximately thirty minutes per wash and the washing liquors were maintained at about100 C. '.lable I lists the com-pounds which were investigated, the ratio by weight of compound to wood, and the number and length of the washes.

TABLE II BLnAcHAsrLrrY Txsrs or WAsm Kam PULPs Cook Percent C] Percent Cl Higgins Natural Ci No. Added Consumed Reflectance Consumption 7 6. 93 57 9 8. 54 58. 3 1 ll 10. 18 60 l2. 5

7 6. 53 (il. 5 9 7. 86 63 2 11 9. 34 64 1l. 5

13 1l. S9 75 15 13. 73 77 7 6. 89 58 9 8. 49 65 5 11 l0. 38 77 ll. 0

It should be noted that while all pulps are difficult to bleach, requiring from 9 to 12.5% of pulps.

TABLE l DETERGINT WASHING 0F KRAIT PULP C00 W t Grams Schedule Com und eshing Agen r on ven No. Wgsh Dry Chips Per cent i Water Continuous lb.

rem. 2 Na,o3.22 sio, 15o 1 gaat, so 111111---- 5 a dn 2 wacsllies, 30 min. l0

ea 4 Sodium Hemmetsphosphate 150 1 wash, 30 min... 5 5 Naas 15A) 1 wash, 30 mim.-- 5 6 NaOH 150 1 wash, 30 min.-.- 5 7 Oil gnap....t;;l;1......E 1 wash, 30 min um exame os a 8 Red ou soapm; so 1 mh 3 i 9 Na4Pg 1.-...--" 75 2 washes, 30 min. 5

1o Nessie. 75 c malien, ao min. 5

After the washing' had been' completed, the

'pulps Were bleached using calcium hypochlorite bleaching liquor and the bleached pulp vsamples by the Higgins color analyzer. 'Ijable II is a. tabulation of the bleaching data both as to quantity of chlorine added and consumed and the results obtained in terms of brightness of the nished chlorine, the results are strictly comparable and actually duplicate some commercial pulps which A 'are cooked 'primarily for strength and not for were made into sheets and tested for brightness 0 a high white color on bleaching.

Fig. 1 presents graphically ,the data tabulated in Table II.

The effect of washing with diierent chemicals before exposure to air on the bleachability of sulphate pulp is shown in Fig. 1, which gives the 2,502,aso

. 1 Higgins reilectance values and the per cent chlorine consumed in bleaching the different samples which had been previously washed in digester without access of air with aqueous solutions of. the dierent chemicals given in 'Table I. It will.

be seen from this figure and from Table II that sodium hexametaphosphatel (curve 4) and sodium hexametaphosphate plus soap (curve 8) when used as washing solutions gave higherreectance values of the bleached pulp than any of the other chemicals.

The following tests were made on the pulps before and after bleaching:

Beater strength tests were made according to 'I'. A. P. P. I. standard T-200 M-40.

Bursting strength tests were made according to T. A. P. P. I. standard T403 M-36.

Tearing strength tests were made according to w T. A. P. P. I. standard T-4 14 M-40.

Bleachability was determined by a single-stage hypochlorite bleach commonly known as the "Excess chemical method." The bleachability as recorded in the tables refers to the per cent of chlorine on the weight of oven-dry pulp that is required to give an 80% brightness reading on the Higgins photoelectric colorimeter.

Pulps washed with metaphosphate show higher physical properties than those washed with other chemicals. The most common test employed for the evaluation of pulps is the beating strength test in which a weighed quantity of pulp, e. g. 500 grams, is beaten in a small strength testing beater i'or a period of time or until the maximum bursting strength of the pulp has been developed. All pulps increase in strength as the beating progresses until a maximum is reached, after which the bursting strength decreases in value. As the strength increases the freeness, as measured by the amount of water which will drain from a weighed ,quantity of the pulp suspended in 1000 cubic centimeters of water in the standard freeness testing instrument, rapidly decreases. It is customary to beat pulps until the freeness decreases to 50 ml., at which time the maximum Ystrength will have been passed in almost all cases. The bursting strength is determined by making sheets of the beating pulp on a standard sheet mould, drying at a given temperature, conditioning the dried sheets in an atmosphere of 65% relative humidity and 70 F. for a period of 10 hours and then testing on a. Mullen bursting strength tester. This instrument consists of a chamber lled with glycerine into which a plunger is forced at a constant rate, this action forcing the glycerine against a rubber diaphragm over which the sample sheet is clamped.- The rubber diaphragm has an area of one souare inch so that the results are recorded in terms of pounds per square inch required to burst the sample, the instrument having a pressure gauge attached to the glycerine chamber. The sample sheets are usually made to some standard weight basis, e. g. 50 pounds/per ream, 25 x 40-500'. Since unavoidable variations in basis weight will occur, it is customary to divide the bursting strength test value by the basis weight of the sheet being tested, the result being reported as burst factor. A similar report is made for all other physical tests. This form of report compensates for irregular results. due to variations in the thickness and weight of the test sheets.

It should be noted that the burst factor is considered the most important criterion Voi? the quality of a pulp. Many mills make no other test, although some use a Tear test which always cially if the pulp has been bleached.

TableIII is a tabulation of the strength data obtained by a series of physical tests on the Kraft pulps which has been washed before bleaching with various detergent compounds.

, TABLE III SrnnNcm EVALUATION or Drrx-:ncaNr WAsHrn PULPs Bening Canadian Burst Tear Bulk Washing Agent bmg SFmrEEgg Factor Factor Factor H10 Wash 5 720 .71 :4.03v 2.57 Unbloached 2o 350 1. 07 2.75 1.56 #1 35 130 1. 03, 2. 7s 1. s1 25 1.49 2.28 1.74

BLEACHED PULPs H40 Wash 5 580 .71 1. 87 1.65 #1 o 20 160 .73 1.60 1.48 35 27 .70 1.30 1.25

Sodium 5 500 .53 1.16 1.73 silioaro 20 125 .53 1.04 1.66 #2 27 3s :57 .7s 1.70

sodium 5 575 ."60 1.68 1.73 silicate 20 102 .6a .97 1.31 #s 24 5s .64 .a0 1.17

sodium Hom. 5 560 .so 1.72 1.65 momphosphate 20 104 .s0 1.10 1.49 #4 24 50 .es 1.15 1.32

sodium 5 6% .57 1.62 1.76 saludo 20 a .s0 1.22 1.56 #5 35 e5 .s4 1.10 1.20 3s 56 .77 .95 1.21

- sodium 5 690 .64 1.66 1.82 B droxide 20 365 .88 1.28 1.60 16 a5 110 .9s 1.16 1.12 40 50 .87 1.06 1.00

sodium 5 730 .63 1.70 1.60 oleate 20 425 .76 1.38 1.51 #7 a5 140 .01 1.18 1.34 43 5s .70 1.12 1.24

soa 1% 5 600 .65 1.74 1.63 umHexai!) 425 .90 1.38 1.39 mempnospha 35 150 .90 1.37 1.16 -#s 44 56 1.04 1.05 1.0 2

Tem sodium 5 700 .61 1. 3s 1. 74 Pyrophosphafe 2) 320 77 l. 13 l. 4B #o 63 .71 .se 1.26 37 47 .70 .67 1.17

sodium Hydro- 5 700 .65 1.54 1.67 51111156 2o 350 .89 1.12 1.41 #10 55 73 .s1 .a6 1.16 as 47 .7 3 .90 1.14

An examination of the data in Table III shows that the only bleached pulps which even closely approached the ideal strength of .a point to a pound were the products of Runs Nos. 4 and 0 which had been washed with metaphosphate.

An inspection of the curves of Fig. 2 shows that Runs Nos. 4 and 8 not only have a higher maximum burst factor than any of the other pulps but that the strength values shown are not maximum factors, since the strengths are still increasing. This fact may be very important in that it indicates the probability that more fully cooked -pulps may be even proportionately stronger when washed with metaphosphate, since drastic bleaching treatment does not appear to be necessary to remove the colored impurities which remain after the metaphosphate treatment.

In the preceding tests, relatively large amounts of lmetaphosphate were employed in the washing of the pulp. It has been found, however, that advantageous saving in chlorine and conservation in strength of the pulp may be obtained, even when relatively small amounts of metaphosphate are present in the water used for washing.

The following is an example ofone method of treating sulphate pulp according to the present invention:

Example 1 Dry wood used grams 3000 Active chemical as Naso percent.- Sulphidity ..-do 33% Liquor ratio 5 to 1 The volume of black liquor employed was approximately 44% of the total volume of liquor in the digester and contained 7.5% of the total NaOH requirement calculated as NazO.

Cooking schedule:

11/2 hours to reach maximum temperature of 170 C. 3 hours at maximum temperature. Method of washing:

1. Weak black liquor at 75 C.20 minutes.

2. Fresh water+sodium hexametaphosphate at '75 C.-20 minutes. 3. Fresh water-l-sodium hexametaphospha at 75 C.-20 minutes.

compared with a maximum burst factor of 1.33 for the bleached pulp washed with water. In other words, the bleached pulp previously washed with sodium hexametaphosphate retains a considerably higher percentage of its unbleached strength as shown by the burst factor than does the bleached pulp previously .washed with water.

It can also be seen from Fig. 4 that washing with sodium hexametaphosphate and bleaching produces substantially higher tear factors than washing with water and bleaching of the pulp.

Washing solutions of molecularly dehydrated alkali-metal phosphates may also be employed to remove color in the treatment of soda pulps prior to bleaching. The following is an example:

Example 2 These pulps were prepared under the followin conditions:

1. Weak black liquor at 75 C.-20 minutes.

1 Based on weight oi wood. 2 Per cent chlorine to give 80 brightness in single-stage bleach.

3 The second stage of bleaching consisted of an alkaline extraction using 1% of N aOH ou the weight oi pulp, conducted at O. for one hour.

NorE.-Cook 27 was washed inside the digester in the absence of air, oook 48 was washed outside the digester in the presence of air.

Bleachobilitu Determinations Cook N o. 28 Water Washed Per cent chlorine consumed 7. 0 8. 81 l0. 40 12. 31 14. 16 16. Brightness 64. 0 75. 0 78. 5 80. 5 81. 0 8l.

5 Cook No. 27 Washed with 0.02% Sodium Hexametaphosphate Per cent chlorine consumed 6.93 8.71 10.48 12.29 14.23 16. Brightness 72.5 78.5v 81.5 82.0 83.0 84.0

Cook N o. 48 Washed with 0.02% Sodium Hexametaphosphate Per cent chlorine consumed 6.98 8.85 10.07 12.56 14.55 16.@ Brightness 72.0 79.0 81.5 szo seo 83.0

This example shows a very considerable saving in the chlorine consumption during bleaching as a result of using in the washing operation 0.02% of sodium hexametaphosphate based upon the weight of the wood. ln the single-stage measurement of the bleachability, the chlorine consumption to attain a brightness of 80 was reduced from 12.0% for pulp washed with water alone to 9.4 and 9.0 for the pulp washed with sodium hexametaphosphate solution. The overall chlorine consumption in a three-stage bleach corresponding more closely to common plant practice was correspondingly reduced from 5.57% for pulp washed with water alone to 4.92% and 4.90% for pulp washed with Water containing 0.02% of sodium metaphosphate. The bleachability determinations given in Example l for Cooks No. 28 and 48 are shown graphically in Fig. 3.

The beater strength characteristics (burst factor and tear factor) of the sulphate pulp prepared according to Example 1 are shown in Fig. 4. It will be noted from Fig. 4 that the burst factor for the bleached pulp made by washing with sodium hexametaphosphate solution prior to bleaching has a maximum value of about 1.44 as 2. Weaker black liquor at C.20 minutes. 3. Fresh water sodium hexametaphosphate at C."20 minutes.

Per cent Un- Actual chlorine (Ilqook h ur?a scxxeedld Sed Sign' conlsumptign in o exame 1 sin e-ste ophpnatel Per een: Per een* Per cem ehi'orite regg saving in chlorine on singles e hypochiorite b1each=1ao 1 Based on dry weight ot woog I To give 65 brightness.

As in the case of sulphate and soda pulps, the invention also is applicable to the treatment of sulphite pulps by the employment of molecularly dehydrate alkali-metal phosphate solutions to remove color from the pulp prior to bleaching in order to decrease the bleaching requirements and improve the strength retention of the pulps. The sulphite pulps were prepared under the following conditions:

' Example 3 Dry wood used 3000 grams Sulphite cooking liquor 5.0% Free SO2 1.3% Combined SO2 6.3% Total SO2 Liquor ratio 6 to l Cooking schedule: At the end of 2 hours, the temperature of the cook was C., and at the end of 6 hours it was 140 C. It was maintained These sulphite pulps were washed with cold water or sodium hexametaphosphate solutions. Each cook was given four washes of 18 liters each for a total of 72 liters, equivalent to about 50 pounds of solution to one pound of pulp.

The pulp was removed from the digester and placed in a disintegration tank and agitated for hexametaphosphate solutions were employed. In the two-stage bleach.the amount of chlorine required was considerably less for the pulp washed with sodium hexametaphosphate than in the case of the water-washed pulp.

Fig. gives the burst factor for bleached and unbleached sulphite pulps prepared according to Example 3, the bleached pulps having been washed with water or with a 35 P. P. M. aqueous 20 minutes in cold water The pulp was placed on l0 solution of sodium hexametaphosphate and therea at screen, excess water removed, and the yield after bleached to a' brightness of 80 Thls graph determined at this point. The pulp was screened againillustrates the improved strength characand all tests were carried out on the screened teristms of pulp Washed Wtth sodium hexamata' pulp. phosphate prior to bleaching.

From the bleachability determination it was Cupra'mmonmm disperse Visslty tests were to approximate the of chlorine made according to T. A. P. P. I- Standard needed to bleach the pulp using the two-stage M37. bleaching procedure. The equivalent of 550 grams Cuprammomum disperse viscosity tests of the of pulp was bleached to a brightness of 80 per cent sulphlte pulps are given in the following table in on the Higgins photo-electric colorimeter. The centipoises: first stage of the bleaching procedure was the TABLEV chlorination stage. Chlorine water was added to the pulp which was kept at C. and a ilnal conn bleached Bleached sistency of 3%. At the end of 90 minutes after determining the amount of chlorine actually con- 25 @ook No 5 Water washed 2 "2 sumed, the pulp was completely washed with Cook No 6 Sodium Metsphosphate water. The second stage of the bleaching pro- Washed P' P' M 85'5 48'9 cedure, or hypochlorite stage. was carried out at :ss-37 c. and at a consistency of 5%. Calcium The viscosity o1 the bleached pulp previously hypochlorite was added to the pulp, sample sheets 30 welShed With SOdillm heXametaDhOSPhate iS Conwere made at intervals, and when the brightness Slderably higher than that of the Water-Washed of the sheets, as determined by the Higgins photobleached pulp, Showing that less degradation 0f electric oolorimeter, was 80%, the ooi-,ual conthe pulp occurred in the case where it was treated sumption of chlorine was measured and the pu1p with the sodium hexametaphosphate solution. was'again washed with water. The pH of this 35 The temperature at Which the Washing With stage was maintained between 8.5 and 8.9. Total the molecularly dehydrated alkali-metal phoschlorine oonsumed was the sum of the chlorine phate is carried out is not critical but may be consumed in both stages, f varied over a wide range. Any washing tem- Freeness was determined by the Canadian pelellIeS 11D t0 the bOlIlg Peint 0f the Washing standard freeness tester, using a 0.3% consistency 40 solution might be employed.

at 20 C., and the results recorded in cubic centi- In Order t0 produce materially improved results meters. as compared with washing with water, about Prior to testing both the bleached and un- 0.02% of Sodium hexametaphosphete based 0n bleached sheets, the sheets were conditioned in the weight of the dry Wood ShOulCl be employedthe paper testing laboratory for 16 hours in an 45 AS the amount of sodium hexametaphosphate is atmosphere maintained at 73 F. and 50% relaincreased up to about 0.1% based on the Weight tive humidity and all tests were carried out under of the dry Wood. the results are improved Somethese conditions. what; but beyond .this concentration, the im- The results obtained are shown in the ronowmovement in results probably is not sufcient, ing table: from the standpoint of cost, to justify increasing TABLE IV Actual overall chlorine Sodium Hexametaphosconsum tion in 2 I phate Used Un Screened Screenltlse b mh pement cook No. gg Yield, ings, ggg;

Wnt percent percent 0011"' ggee'tn t 2nd Total RP. Drywood stage, stage so .o5 4s.75 47.22 1.5a ,4.3 5o .12 48.4 45.s1 1.59 3.7 45.7 47.11 1.59 5.5 1.46 1.20 acc a5 .oo 4&3 46.79 V1.51 3.2 1.25 .sc 2.11

l Percent chlorine to give 80 brightness in single-stage bleach.

The table shows that the chlorine consumpthe amount of the metaphosphate employed. tion of the pulp washed with sodium hexameta- Expressed in another manner, the washing soluphosphate was considerably less than that of the tion should contain at least about 20 parts of pulp washed with water. Whereas it required sodium hexametaphosphate per million parts of 5.5% o! chlorine to give brightness in a singlewater. As the concentration is increased up to stage bleach when the pulp was washed with` about parts per million, somewhat improved water, it required only 3.2% or 3.7% or 4.3% of results are obtained; but beyond this an increase chlorine. depending upon the concentration of in the concentration o! metaphosphate is probthe sodium hexametaphosphate when the sodium 75 ably not justified, from an economic standpoint.

1 1 The following table illustrates results which have been obtained with regard to bleachability, chlorine consumption and retention of strength, employing diierent temperatures and quantities of sodium hexametaphosphate, in the washing of various kinds of pulps:

stantial amount of cellulose constituent of the pulp, the step comprising washing the pulp after digestion but before bleaching with a solution consisting essentially of water and about 20 to 100 parts of glassy molecularly dehydrated phosphate having a molecular ratio of NazO to P205 1 Per cent on weight of dry wood.

2 Per cent chlorine to give 80 brightness in single-stage bleach, except in case of soda pulp, for which brightness of 65 was used.

3 As measured by burst factor, per cent oi unbleached strength.

In washing the digested pulp with the molecularly dehydrated alkali-metal phosphate solution prior to bleaching, the time of washing and other conditions will, of' course; be governed by the amount of coloring matter in the pulp and the desired whiteness of the bleached pulp. Multi-stage and countercurrent washing of the pulp are advantageous under many conditions.

The term molecularly dehydrated alkalimetal phosphate is intended to include any of the phosphate glasses previously referred to, as well as the alkali-metal trpolyphosphates or pyrophosphates, both of which are crystalline materials, or mixtures of any of these phosphates. All of these molecularly dehydrated phospates may be considered as derived from orthophosphates by the elimination of water of constitution. The molecularly dehydrated phosphates contain less water of constitution than the corresponding orthophosphates and have, therefore, come to be recognized as molecularly dehydrated phosphates, as pointed out more fully in the Hall and Jackson Patent 1,903,041, granted March 28, y

Although I have given specific examples and preferred embodiments of the invention, it will be understood that it is not limited thereto but may be otherwise embodied or practiced within the scope of the following claims:

I claim: l i

1. In the process of removing organic coloring matter in the nature of tannin or lignin compounds from paper pulp without removing a substantial amount of cellulose constituent of the pulp, the step comprising washingthe pulp after digestion but before bleaching with a solution consisting essentially of water and about 20 to 100 parts of molecularly dehydrated alkali nietalphospate per one million parts of water,

the concentration of molecularly dehydrated alkali metal phosphate in the wash water beingr substantially less than that which would be required to soften the water by sequestration of calcium.

2. In the process of yremoving organic coloring matter in the nature of tannin or lignin compounds from paper pulp without removing a subbetween 0.9:1 and 1.7:1 per one million parts of water, the concentration of molecularly dehydrated phosphate in the wash water being substantially less than that which would be required to soften the water by sequestration of calcium.

3. In the process of removing organic coloring matter in the nature of tannin or lignin compounds from paper pulp without removing a substantial amount of cellulose constituent of the pulp, the step comprising washing the pulp after digestion but before bleaching with a solution consisting essentially of water and about 20 to 100 parts of Grahams salt per million parts of water the concentration of Grahams salt in the wash water being substantially less than that which would be required to soften the water by sequestration of calcium.

4. In the process of removing organic coloring matter in the nature of tannin or lignin compounds from paper pulp made by the sulphate process without removing a substantial amount of cellulose constituent of the pulp, the step comprising washing the pulp after digestion but before bleaching with a solution consisting essentially of water and about 20 to 100 parts of molecularly dehydrated alkali metal phosphate per one million parts of water, the concentration of moecularly dehydrated alkali metal phosphate in the wash water being substantially less than that which would be required to soften the water by sequestration of calcium.

5. In the process of removing organic coloring matter in the nature of tannin or lignin compounds from paper pulp made by the soda process without removing a substantial amount of cellulose constituent of the pulp, the step comprising washing the pulp after digestion but before bleaching with a solution consisting essentially of water and about 20 to 100 parts of molecularly dehydrated alkali metaVphosphate per one million parts of water, the concentration of molecularly dehydrated alkali metal phosphate in the wash water being substantially less than that which would be required to soften the water by sequestration of calcium.

6. In the process of removing organic coloring matter in the nature of tannin or lignin com- 13 pounds from paper pulp made by the sulphite process without removing a substantial amount of cellulose constituent of the pulp, the step comprising washing the pulp after digestion but be- -iore bleaching with a solution consisting essentially or water and about 20 to 100 parts of molecularly dehydrated alkali metal phosphate per one million parts of water, the concentration of molecularly dehydrated alkali metal phosphate- V'stantial amount Yof celluloseconstituent of the pulp, the .step comprising 'washingthe'pulp after digestion but before vbleaching with a solution consisting essentially'of water and about 20 to 100 parts of alkali metal pyrophosphatefper one million parts of water, the concentration of alkali metal pyrophosphate in the Wash water. being substantially less than that whichwould be required to soften thevwater by sequestration of calcium. l

8. In the process of removing organic coloring "matter in the nature of tannin or lignin compounds from paper pulp-without removing a substantial amount of cellulose constituenttof the pulp, the step comprising washing the pulp after digestion but before bleaching with a solution consisting essentially of water and about 20 'to 100 parts of alkali metal tripolyphosphate per one million 'parts of water, the concentration of alkali Imetal tripolyphosphate in the wash water being substantially less than that which would be required to soften the water by sequestration of calcium.

CLARENCE E. LIBBY.

REFERENCES crrnn The`following references are of record in the tile of this patent:

. UNITED STATES PATENTS Number Name Date 1,435,332 Phair Nov. 14, 1922 1,584,902 Summers May 18, 1926 1,880,046 Richter Sept. 27, 1932 1,883,478 Bassett Oct. 18, 1932 1,924,815 Skolnik Aug. 29, 1933 2,030,383 Luth Feb. 11, 1936 2,030,384 Luth Feb. 11, 1936 2,031,827 Fiske Feb. 25, 1936 2,056,185 Greene Oct. 6, 1936 y 2,074,473 Jayme Mar. 23, 1937 2,219,432 Scheller Oct. 29, 1940 FOREIGN PATENTS Number Country Date 417,035 France June 11, 1910 358,144l Germany --.Sept 6, 1922 438,403 Great Britain 'Nov. 15, 1935 208,192

Switzerland .AprilI 1, 1940 OTHER REFERENCES Ser. No. 237,302, Passow (A. P. 0.), published May 4, 1943. v

American Dyestui Reporter, Jan. 28, 1935, pp.

and Paper, publ. by Mc

Claims

1. IN THE PROCESS OF REMOVING ORGANIC COLORING MATTER IN THE NATURE OF TANNIN OR LIGNIN COMPOUNDS FROM PAPER PULP WITHOUT REMOVING A SUBSTANTIAL AMOUNT OF CELLULOSE CONSTITUENT OF THE PULP, THE STEP COMPRISING WASHING THE PULP AFTER DIGESTION BUT BEFORE BLEACHING WITH A SOLUTION CONSISTING ESSENTIALLY OF WATER AND ABOUT 20 TO 100 PARS OF MOLECULARLY DEHYDRATED ALKALL METAL PHOSPATE PER ONE MILLION PARTS OF WATER, THE CONCENTRATION OF MOLECULARLY DEHYDRATED ALKALI METAL PHOSPHATE IN THE WASH WATER BEING SUBSTANTIALLY LESS THAN THAT WHICH WOULD BE REQUIRED TO SOFTEN THE WATER BY SEQUESTRATION OF CALCIUM.