US2786757A

US2786757A - Method for preparing a paper product

Info

Publication number: US2786757A
Application number: US594390A
Authority: US
Inventors: Louis B Taylor
Original assignee: Columbia Southern Chemical Corp
Current assignee: Columbia Southern Chemical Corp
Priority date: 1956-06-28
Filing date: 1956-06-28
Publication date: 1957-03-26
Anticipated expiration: 1974-03-26
Also published as: DE1015309B

Description

United States Patent 2,786,757 METHOD FOR PREPARING A PAPER PRODUCT Louis B. Taylor, Pittsburgh, Pa., assignor to Columbia- Southern Chemical Corp., Allegheny County, Pa., a corporation of Delaware No Drawing. Continuation of application Serial No. 352,295, April 30, 1953. This application June 28, 1956, Serial No. 594,390

9 Claims. (Cl. 92-21) This invention relates to a novel method for preparing a paper product.

Paper made according to this invention has high brightness and opacity, and is suitable for use as writing paper, or as top liner for boxboard cartons, or for other purposes where paper having high brightness and opacity is required.

According to this invention, the herein contemplated paper may be prepared by forming a paper pulp dispersion in an aqueous acidic material which forms a substantially water-insoluble (less than about 5 or 6 grams per liter) salt of an alkaline earth metal and adding calcium silicate or equivalent alkaline earth metal silicate to the acidic slurry.

Any type of paper pulp may be used in making paper according to this invention. Chemically-treated pulps such as sulfite and kraft may be treated according to this invention, which is also applicable to ground wood and semi-chemical pulps. Either bleached or unbleached pulps may be used. Rags may also be included in the pulp if desired. Broke or scraps and cuttings of finished paper may be used in addition to pulp, and the use of broke reduces the pulp requirements. Paper wasted in manufacturing operations is generally available and may be used as broke.

Brightness and opacity of paper made from any type of pulp are improved by practice of the present invention, but the improvement is most marked in paper made from low-grade pulps. Paper made according to the present invention compares favorably in brightness, opacity and strength to paper pigmented with titanium dioxide, and may be produced at less cost.

The process of this invention may be performed by mixing the acid or acidic material with the pulp with or without beating and then adding the alkaline earth metal silicate to the acidified slurry. An especially good product is obtained when sulphuric acid, sulphurous acid, or aluminum sulphate is used as the acidic material, and calcium silicate is the alkaline earth metal silicate. However, other acidic substances and other alkaline earth metal silicates may be used. In consequences of this process, a finely divided precipitate (probably a mixture of silica and calcium salts of the acid used) deposits on the surfaces and the interstices of the fibers and is distributed throughout the mass of fibers.

Either sized or unsized paper can be prepared according to this invention. If sized paper is desired, the size such as rosin size in soluble form is mixed with the pulp slurry and a precipitant for the size is added together with or prior to the addition of the calcium silicate. The precipitant is a compound of a trivalent or tetravalent metal, and is preferably aluminum sulphate.

As has been stated above, sulfuric acid and sulphurous acid (as well as their anhydrides S03 and S02 and the acidic salts thereof, such as aluminum sulphate, sodium bisulphite, ammonium sulphate, sodium sulphate and the like which hydrolyze to produce sulphuric or sulphurous acid, are particularly effective acidic materials for use according to this invention because of the high brightness and excellent opacity properties of paper produced when such materials are used in conjunction with calcium silicate. Other acidic substances which form insoluble alkaline earth metal salts and which are usable in preparing the product of this invention are phosphoric acid, phosphorous acid, oxalic acid, tartaric acid, and citric acid. Acid solutions which attack cellulose fiber should be avoided. Thus, where sulphuric acid is used, it is usually best to use relatively dilute acid.

The calcium silicate which has been found to yield superior results is a finely divided precipitated product which has the composition CaO(SiO2)m where x is 2 to 4 and which has an average ultimate particle size below about 0.1 micron.

A particularly suitable calcium silicate can be prepared by mixing flowing streams of calcium chloride and sodium silicate solutions together in a zone of high turbulence where almost instantaneous mixing can be effected. While considerable variation in the concentrations of the reactant solutions is possible without affecting the particle size of the calcium silicate produced, a calcium chloride solution containing 50 to 150 grams of salt per liter and a sodium silicate solution containing 5 to 15 percent by weight of sodium silicate with a NazOISiOz ratio varying from 1:2 to 1:4 are preferred. One way to produce the required turbulence is to introduce the two streams closely together into the central area of a centrifugal pump. Agitation of the mixture is effected as the reactant streams are thrown outwardly by the pump rotor. To effect maX irnurn agitation, the amount of reacting solution supplied to the pump is held below the capacity of the pump to deliver liquid, usually less than percent of the pump capacity, and preferably 65 percent or less. Running the pump in this way causes the reacting mass to remain in the zone of agitation for a longer period and insures the production of calcium silicate having the desired fineness. Calcium silicate which precipitates is separated from the slurry in which it is formed, and is dried at a temperature ranging from about C. to 150 C., generally at about C. The reaction whereby calcium silicate is precipitated may be represented as follows:

CELCi2+NH20(SlOZ)z- Ca0(SiO2)+2NaCl where x is between 2 and 4.

Calcium silicate prepared by the above-described method has an average ultimate particle size of about 0.015 to 0.04 micron and a surface area of about 50 to 100 square meters per gram. This calcium silicate produces a high-quality product when used in the performance of this invention.

Naturally occurring forms of calcium silicate, such as wollastonite, and calcium silicate prepared by methods other than that above indicated may be used provided the product is readily reactive with the acids herein contemplated. When precipitated calcium silicate is used, it frequently is preferred to dry the precipitate, for example, above about 100 C. but below the temperature at which the calcium silicate becomes crystalline (preferably below 800 C. and usually 100 to C.). The amount of calcium silicate added to the acid slurry should be sufficient to establish a substantial SiOz concentration in the fibers. Preferably the resulting product should contain at least 0.5 to 1 percent by weight of SiOz based upon the dry weight of the paper pulp. Much larger amounts rang ing as high as 30 percent SiOz or even higher may be used. For most purposes, the products produced contain 0.5 to 10 percent, usually in the range of 2 to 5 percent SiO2.

It will be understood that when calcium silicate is added to the acidic pulp slurry, acidic component is consumed and the pH of the slurry rises. If sufiicicnt calcium silicate is added, all of the acid is consumed and, upon addition of further calcium silicate, the slurry becomes alkaline. For most purposes, sufiicient acidic component should be present to react with at least 0.5 to 1 pound of calcium silicate per 100 pounds of dry pulp in the slurry and usually the amount of acidic component is enough to react with substantially all of the calcium silicate added. Excess calcium silicate may be added. However, this excess is rarely above that which will increase the pH of the pulp slurry above about 9 and usually the calcium silicate added does not exceed more than twice the amount which will be decomposed by the acid, i. e., usually 50 percent or more of the calcium silicate added is decomposed.

Aluminum sulphate reacts with calcium silicate in the proportion of about one mole of aluminum sulphate to three moles of calcium silicate.

In performing the process according to the present invention, pulp is slurried in any suitable agitating vessel, and is then beaten. The duration of heating is determined in accordance with the type of pulp being beaten and the type of paper product desired. Groundwond pulp requires longer beating than do the chemically treated pulps. Generally, long beating should be avoided as this results in the production of a transparent paper.

Size and coloring matter are added either at the beginning of or during the beating cycle. Rosin size in the form of a soluble alkali metal rosin soap is suitable. Coloring matter may be added if desired.

Toward the end of the beating cycle, say about 10 minutes to one-half hour before the end, a precipitant for the size and an acidic substance which does not harm cellulose fibers are added to the beaten stock. As above stated, aluminum sulphate is especially useful since it serves for both purposes. In a typical operation of the type contemplated, l to 10 percent of aluminum sulphate hydrate, Al2(SOi)a'l8H2O, based on the dry weight of pulp may be used. If desired, aluminum sulphate may be added together with another acidic substance to the beater. In that case, the amount of aluminum sulphate is sufiicient only to precipitate the size, and is about 1 to 3 percent of the dry weight of pulp, depending upon the amount of size to be precipitated. Moreover, where other acidic materials are used, other precipitants may be used to precipitate the size, e. g., salts of other trivalent or tetravalent metals such as Ti(SO4)2.

After the size is precipitated, the calcium silicate or other alkaline earth metal silicate is added to the beaten acidic pulp slurry to form a precipitate on the fiber surfaces. Severe dusting problems arise if water is agitated while silicate is being added; for this reason, it is desirable to add all the silicate before the commencement of agitation, or to add it in beater bags. Broke may be included in the suspension if desired. The suspension of calcium or other alkaline earth metal silicate, and broke if desired, in water is mixed with the slurry of beaten and acidified pulp with agitation, and the product of this invention is formed.

Following preparation of the pulp slurry, the pulp may be sheeted into paper in any conventional manner. Water discarded from the sheeting operation, commonly referred to as white water, has a pH of about 5 to 6. The paper sheet is pressed and dried, and tub-sized if desired. Drying at a temperature under 100 C. for a long period of time is desirable in order to obtain paper of high strength.

In certain tests described in the ensuing examples, pripcr produced according to this invention was tested for brightness, opacity, tear strength, and bursting strength. The testing procedures were as follows:

Brightness-The percentage reflectance of ten sheets of paper was measured on a Hunter multipurpose refiectometer with both green and blue light.

Opacity.The contrast ratio method (TAPPI Standard T42Sm44) was used. The reflectance of a single thickness Contrast ratio:

Retention= where:

Azpercent ash in air dry stock. Bzpereent ash in air dry paper.

=percent loss on ignition of lone dry filler. The factor 0.94 allows for 6% fiber loss through the wire of the paper-making machine. The formula is taken from Sutermeister, Chemistry of Pulp and Paper Making, third edition, New York, Wiley, 1941, page 341. I The following examples are illustrative of the present invention:

EXAMPLE I A pulp beater was charged as follows:

Grams Unbleached sulfite pulp Bleached kraft pulp Hard white envelope cuttings 90 This charge was dispersed in 23 liters of water and the resulting slurry was beaten in a Niagara beater for about 80 minutes to approximately 400 milliliters freeness, Canadian Standard (TAPPI Standard T227m50). One hundred milliliters of prepared rosin of about 5 percent by weight size was then added. Thereafter, 9 grams of pregelatinized starch was added and the pulp was circulated in the beater long enough to insure complete mixing. Aluminum sulphate in the amount of 3 percent by weight, based on the dry weight of pulp, was added as a solution containing 12.92 grams of aluminum sulphate octodecahydrate per liter. At this stage, the pulp consistency was approximately 1.1 percent. Stirring was continued for 2 minutes. Immediately thereafter, the amounts of calcium silicate having the composition CaO(SiO2)3.3 and titanium dioxide indicated in the table were added. The amounts are expressed as percentages of the dry weight of the pulp. Stirring of the mixture was continued for 10 minutes more, and the resulting stock was diluted to 8,000 milliliters, divided into ten SOD-milliliter portions, and sheeted into paper. The paper was tested for brightness, opacity, bursting strength, weight and thickness, and ash and moisture content, with the following results:

Table I Sample No.

Percent titanium dioxide byweight 1.0 0.5 None 1.5 None None Percent calcium silicate by wei ht. 1.0 2.5 4.0 None 4.0 None Brightness:

Green light. 70.8 70.0 72.9 73.2 72.2 70.5 Blueliglit 61.2 02.2 615.3 63.1 till] 50.3 Contrast ratio (green light): Reflectance, black bncking 56.9 57.0 57.0 57.9 57.3 13.0 Reflectance, white hackiug 79.0 80.1 81.1 80.0 70.7 70.0 Ratio 72.1 71.2 70.3 718 T1." tii'tti Contrast ratio (blur.- light):

Reflectance, black backing 53.7 53.9 54.5 54.9 54.7 49.0 Reflectance, white backint: 60.54 67.!) 69.3 08.6 68.1} 000 Ratio 70.2 75.3 74.5 75.9 70.0 70.0 Bursting strength, lb./

in) 32.0 34.6 33.0 33.6 85.0 35.0 Percent ash in dry paper 2. 63 2. H3 1.86 2. 84 1. 2i

Comparison of paper pigmented with the calcium silicate-aluminum sulphate product of this invention (samples 55 and 57) with paper pigmented with titanium dioxide (sample 56) shows that about the same brightness and opacity is imparted by both pigments, and that both papers have about the same bursting strength. Hence, the relatively expensive titanium dioxide may be replaced with a reaction product of this invention at considerable saving in cost and without diminution of physical properties such as strength, brightness, or opacity. Titanium dioxide may be used in the present process as in samples 53 and 54.

EXAMPLE II One thousand pounds of bleached kraft pulp and 1000 pounds of bleached sulfite pulp were slurried in roughly 6000 gallons of water. The pulp was agitated after which it was beaten in a stainless steel Jones beater at 3000 lb./ sq. in. pressure for 90 minutes. Twenty-four pounds of rosin size in aqueous solution was added to the beater at the start of the beating cycle, and 26 fluid ounces of Ultramarine blue, equivalent to 13 ounces of dry coloring matter, was added to the beater one-half hour after the start of the beating cycle. Fifty-one gallons of iron-free aluminum sulphate solution containing 3.5 pounds of AI2(SO4)3' 181-120 per gallon was added to the beater onehalf hour before the end of the beating cycle. While beating was in progress, 120 pounds of finely divided calcium silicate prepared by the reaction of sodium silicate with excess calcium chloride as described above was slurried with 5 pounds of broke and about 1000 gallons of water and agitated for one hour. This slurry and the beaten pulp were mixed in a stock chest. The stock thus formed, which had a solids content of about 4 to 5 percent, was pumped through a refiner and head box to a Fourdrinier machine. Free water containing small amounts of pigment and pulp was sucked through the wire and discarded, leaving a paper sheet which was dried and tub-sized.

For comparison of paper pigmented according to the present invention with paper pigmented with titanium dioxide, a sample of the latter was made as follows:

One thousand pounds of bleached kraft pulp and 1000 pounds of bleached sulfite pulp were slurried in roughly 6000 gallons of water. The pulp was agitated after which it was beaten in a stainless steel Jones beater at 3000 pounds per square inch roll pressure for 90 minutes. Twenty-four pounds of rosin size in aqueous solution was added to the beater at the start of the beating cycle. After beating had continued for one-half hour, 50 pounds each of titanium dioxide and Huber X-43 clay were added. Twenty-four gallons of iron-free aluminum sulphate solution containing 3.5 pounds of Al2(SO4)s' 18H20 per gallon was added to the beater one-half hour before the end of the beating cycle. While heating was in progress, 500 pounds of broke was slurried in about 1000 gallons of water and agitated for one hour. This slurry and the beaten pulp were mixed in a stock chest, and the resulting stock was pumped through a refiner and head box to a Fourdrinier machine, where the pulp was sheeted into paper as described above.

The two samples of paper were analyzed for ash composition, brightness, opacity (contrast ratio), pH, yellowing with age, bursting strength and tear strength, with the following results:

Table II Calcium 8111- Titanium Dieate Pigmented oxide Pigmented Before After Before After Tub Tub 'Iub Tub Size Size Size Size Weight, lb. per ream (500 sheets,

22 x 34") 19. 4 20. 0 l9. 7 20.0 Thickness, mils per sheet 3. 8 4. 1 3.8 3.9 Percent ash in oven dry paper 4. 42 4. 41 4. 55 4. 21 Analysis of ash:

Percent, S10, 55. 6 56. 3 20. l 19. 7

Percent R10; (including T10,) 36. 4 36. 2 74. l 74. 5 Brightness percent reflectance]:

Blue llg t, felt side 82. 7 82. 6 84. 5 B3. 1

Blue light, wire side 82.3 82. 9 84. 6 83.3

Green light, felt side. 83. l 83. 2 83. 9 S3. 4

Green light, wire side 82. 6 84. 0 84. 2 83. 0 Contrast ratio:

Blue light H 85. 8 84. 2 88. 3 85. 3

Green light 82. 0 80. 0 84. 2 80. 9

'White light 80. 6 79. 5 83. 6 80. 5 pH (TAPPI Standard T435-M42).-. 6.5 6. 6 ti 7 6. 7 Fadeometer Test:

Brightness before test 82. 7 82. 5 84. 5 83.0

Brightness after 24 hours exposure at F 78.3 77. 4 79. 4 78. 6

Change 4. 4 5.1 5. l 4. 4 Bursting Strength (Mullen)- 22. 7 32. 7 2'4. 5 32. 1 Tear Strength (Elmendorf):

Machine direction 55 50 48 43 Cross direction 57 51 52 49 The above table shows that a paper having high brightness and opacity and satisfactory strength can be prepared by the present invention and that the paper thus prepared is essentially equivalent in brightness and opacity to paper pigmented with titanium dioxide.

EXAMPLE III A beater was charged with 600 pounds of rag fiber, 600 pounds of bleached kraft pulp, 1000 pounds of bleached sulfite pulp, 100 pounds of titanium dioxide, 32 pounds of rosin size in aqueous solution, and about 6000 gallons of water. The charge was beaten for about an hour and a half, and 16 fluid ounces of blue dye solution, and 45 gallons of iron-free aluminum sulphate solution containing 3.5 pounds of Al2(SO4)a-l8H2O per gallon were dumped into the beater 15 minutes before the end of the beating cycle. While beating continued, 200 pounds of finely-divided calcium silicate prepared by the reaction of sodium silicate with excess calcium chloride was placed in a hydrapulper together with 1200 pounds of broke consisting of scrap paper pigmented with titanium dioxide and about 2000 gallons of water. The hydrapulper was then started, and agitation continued for one hour. The hydrapulped slurry was mixed with two beaterloads of pulp in a stock chest. The stock was then pumped through a refiner and head box to a Fourdrinier machine, where paper sheet was formed. The paper sheet was dried and tub sized.

For comparison, a paper sheet containing no calcium silicate was made. The procedure described in the previous paragraph was followed, except that pounds of titanium dioxide and only 24 gallons of aluminum sulphate were used. All the pigment was admixed with the Results of tub-sized plug and rag fiber 1n the beater. samples of the two papers are as follows:

Table III Calcium Titanium Silicate Dioxide Pigmented Pigment Only Weight, lb. per ream (500 sheets, 22" x 34") 19. 7 20. 6 Thickness, mils per sheet i. 4. 1 4. 2 Brightness (percent reflectance, blue light):

Felt side 85. 0 83. 3 Wire side 84. 3 84. 5 Contrast ratio (green light) 86. l 85. 7 )H (TAPPI Standard 1135-. 6. 5 6. 6 ursting Strength 30.2 32. 0 Tear Strength:

Machine direction 50 50 Cross direction 55 52 Comparison of the properties of paper containing calcium silicate with paper pigmented only with titanium dioxide shows no material difference between the two. About 4 to pounds of calcium silicate per 100 pounds of dry pulp, together with sufiicient acidic material such as aluminum sulphate to neutralize at least one-half of the lime content of the silicate, produces paper with similar optical properties to that produced by pigmentation with about 1 /2 to 3 percent of titanium dioxide based on the dry weight of pulp.

EXAMPLE IV Three hundred and eighty-five grams of kraft pulp was slurried in 23 liters of water, soaked two hours, and beaten for three hours in a Niagara beater at 6500 grams bar load. Fourteen hundred grams of pulp slurry was diluted to 5900 grams, and 16.8 cc. of 5 percent aluminum sulphate solution, which corresponds to 4 percent by weight of aluminum sulphate based on the dry weight of pulp, was added. The pulp slurry was stirred five minutes.

A slurry containing 1.05 grams of dried precipitated calcium silicate. CaO(SiO2)3.s, or about 5 percent by weight based on the dry weight of pulp, in 400 milliliters of water was prepared. This slurry was added to the pulp slurry and the mixture was stirred for five minutes.

This procedure was repeated in successive runs in which part of the aluminum sulphate was replaced by a l percent sulfuric acid solution. The amounts of each reagent in each run was as follows:

cc. of 5% Al: cc. 011% (SO4)3. 181110 H2504 16. 8 None Table IV I Run 1 Run 2 Run 3 Run 4 Brightness (percent blue reflectance). 73.9 75. 7 75. 3 75. 6 Contrast ratio (green. light) 66. 9 67. 2 6T. 3 66. ll Bursting strengtl1..... .1 2T. 2 27. 2 26. 8 26. 4

Replacement of part of the aluminum sulphate with sulfuric acid results in a paper of improved brightness, about the same opacity, and slightly decreased bursting strength compared to paper produced using aluminum sulphate. Since sulfuric acid is much cheaper than aluminum sulphate, these results show the further advantage which accrues by use of sulfuric acid as the acidic substance and using aluminum sulphate only in suflicient quantities to precipitate the size. Other sulphur-oxygen containing acids, notably sulphurous acid including its anhydride S02, may be used with the same results.

The above description has been directed primarily to embodiments of the invention wherein the calcium silicate is prepared by continuous mixing of flowing streams of sodium silicate and calcium chloride and the calcium silicate has the composition CaO(SiO2)1 where x is about 2 to 4. However, calcium silicate of somewhat lower surface area produced by a batch process in which the sodium silicate is added to a pool of calcium chloride solution or vice versa also may be used. Moreover, other water insoluble calcium silicates including mixed silicates of calcium and other metals may be used, particularly when the silicates have an average ultimate particle size below about 1 micron, preferably below 0.1 micron. Such silicates include calcium aluminum silicates, calcium sodium aluminum silicates, calcium potassium, aluminum silicates, calcium zinc silicates, and like silicates in which the mole ratio of total Slog to the total CaO in the product is not substantially greater than about 10. A typical mixed silicate of this type which recently has become available has the following composition:

Percent Ignition loss at 1000 C 16.7 Loss in drying at C 5.4 SiO2 41.1 R201; (almost entirely AlzOs) 10.88 F6203 0.24 CaO 11.82 MgO 0.38 NazO 10.04 Chloride 0.04 S03 3.8 C02 6.6

Some portion of the CaO content of this product appears to be present as calcium carbonate.

Magnesium silicate, strontium silicate, or barium silicate may be used in place of calcium silicate in any of the foregoing examples.

Other woven, felted, or slurried fibrous materials, such as cotton, silk, rayon, linen, nylon, fibers of polymeric acrylonitrile and acrylonitrile copolymers, dacron, hemp, sisal, etc. may be treated in the same manner by impregnating with an acidic material of the type described above and reacting the product with an alkaline earth metal silicate.

This application is a continuation of applicants prior application Serial No. 352,295, filed April 30, 1953, now abandoned.

While the present invention has been described with particular reference to illustrative examples of several embodiments hereof, the invention is limited only by the scope of the appended claims.

What is claimed:

1. A method of preparing paper which comprises forming an aqueous slurry of cellulosic paper pulp and an acid substance which forms a Water insoluble alkaline earth metal salt, reacting with said slurry finely divided alkaline earth metal silicate and producing paper from the resulting slurry, the amount of said acidic substance being sufiicient to react with at least half the alkaline earth metal silicate.

2. A method of preparing paper which comprises forming an aqueous slurry of cellulosic paper pulp and an acid substance which forms a water insoluble calcium salt, reacting with said slurry finely divided calcium silicate and producing paper from the resulting slurry, the amount of said acidic substance being sutficient to react with at least half the calcium silicate.

3. A method of preparing paper which comprises forming an aqueous slurry of cellulosic paper pulp and aluminum sulphate, reacting with said slurry finely divided calcium silicate and producing paper from the resulting slurry, the amount of said aluminum sulphate being sufficient to react with at least half the calcium silicate.

4. A method of preparing paper which comprises forming an aqueous slurry of cellulosic paper pulp and aluminum sulphate, reacting with said slurry finely divided calcium silicate having at least two routes of SiO2 per mole of CaO and producing paper from the resulting slurry, the amount of said aluminum sulphate being sufficient to react with a substantial portion of the calcium silicate.

5. A method of preparing paper which comprises forming an aqueous slurry of cellulosic paper pulp and aluminum sulphate, reacting with said slurry finely divided calcium silicate prepared by reaction of sodium silicate and a calcium salt and producing paper from the resultirig slurry, the amount of said aluminum sulphate being sulficient to react with a substantial portion of the calcium silicate.

6. The method of claim 5 wherein the calcium salt is calcium chloride.

7. A method of preparing paper which comprises forming an aqueous slurry of cellulosic paper pulp and aluminum sulphate, adding to and reacting with said slurry finely divided calcium silicate having from two to four moles of SiOz per mole of CaO and prepared by reaction of sodium silicate and calcium chloride, the amount of aluminum sulphate being sufiicient to react with a substantial portion of the calcium silicate and producing paper from the resulting slurry, the amount of said added calcium silicate being suflicient to establish in the paper an Si02 concentration of 0.5 to 30 percent by weight.

8. A method of preparing paper which comprises forming an aqueous slurry of cellulosic paper pulp and aluminum sulphate, reacting with said slurry finely divided alkaline earth metal silicate and producing paper from References Cited in the file of this patent UNITED STATES PATENTS 228,328 Ehrhardt June 1, 1880 1,345,317 Clapp June 29, 1920 2,237,374 Smith Apr. 8, i941 2,599,094 Craig June 3, l952

Claims

1. A METHOD OF PREPARING PAPER WHICH COMPRISES FORMING AN AQUEOUS SLURRY OF CELLULOSIC PAPER PULP AND AN ACID SUBSTANCE WHICH FORMS A WATER INSOLUBLE ALKALINE EARTH METAL SALT, REACTING WITH SAID SLURRY FINELY DIVIDED ALKALINE EARTH METAL SILICATE AND PRODUCING PAPER FROM THE RESULTING SLURRY, THE AMOUNT OF SAID ACIDIC SUBSTANCE BEING SUFFICIENT TO REACT WITH AT LEAST HALF THE ALKALINE EARTH METAL SILICATE.