US3709779A - Bleaching of mechanical pulps with hydrosulfite in the presence of an alkali metal silikate - Google Patents

Abstract

A PROCESS IS DESCRIBED IN WHICH MECHANICAL PULPS ARE BLEACHED WITH SODIUM OR ZINC HYDROSULFITE IN THE PRESENCE OF SODIUM SILICATE. THE ADDITION OF SILICATE INCREASES THE BRIGHTNESS GAIN OBTAINED BY HYDROSULFITE ALONE AND REDUCES THE GRINDING POWDER CONSUMPTION. SILICATE CAN THUS REPLACE SODIUM TRIPOLYPHOSPHATE FOR THIS USE AND THEREBY ELIMINATE PHOSPHATE POLLUTION DUE TO NEWSPRINT OR OTHER PAPER MILL EFFLUENTS.

Description

FIPBSU'? 3,709,779 Patented Jan. 9, 1973 ABSTRACT OF THE DISCLOSURE A process is described in which mechanical pulps are bleached with sodium or zinc hydrosulfite in the presence of sodium silicate. The addition of silicate increases the brightness gain obtained by hydrosulfite alone and reduces the grinding power consumption. Silicate can thus replace sodium tripolyphosphate for this use and thereby eliminate phosphate pollution due to newsprint or other paper mill efliuents.

The present invention relates to the bleaching of mechanical pulps, and more particularly to a method of bleaching such pulps without using sodium tripolyphosphate.

Wood which contains varying amounts of cellulose, hemicelluloses, lignin and small amounts of other materials can be defiberized into a mechanical pulp by two processes. In the first process the debarked logs of wood, softwood or hardwood, are ground against pulp stones to convert them into a fibrous form known in the industry as stone groundwood. In the second process debarked wood is converted into chips which are in turn converted into the fibrous form by rubbing between two metal plates; this type of mechanical pulp is commonly known as refiner groundwood in the pulp and paper industry.

In the preparation of the above types of mechanical pulp no major component of wood is lost, although a small amount of water soluble material may be dissolved. In general the yield of the mechanical pulps obtained by the above two processes varies between 95 and 100%. A small variation in the refining process is also practiced in the industry: the wood chips for preparation of refiner groundwood are first soaked in a solution of sodium sulfite or a weak aqueous solution of sodium hydroxide or mixtures of the two. This softens the chips to some extent and disc refining is facilitated. The pulp thus obtained is sometimes called chemi-groundwood and is obtained in slightly poorer yield than regular refiner groundwood. All the above types of pulp shown identical bleachability behaviour with hydrosulfites and peroxides.

A mechanical pulp obtained by any of the above methods is commonly used in combination with various chemical pulps for the manufacture of newsprint, catalog paper, book paper, etc For many of these uses the reflectivity, or brightness, of the mechanical pulps prepared from many species of wood is satisfactory, while pulps from other woods are too dark-to be used without further bleaching or brightening. For economic reasons it is necessary to bleach these pulps without removing substantial amounts of wood components, i.e., by use of the so-called lignin-preserving bleaching agents. The most common bleaching agents for this purpose are sodium or zinc hydrosulfite, also referred to as sodium or zinc dithionite, 'and the peroxides of hydrogen or sodium. Both these reagents are used commercially. Bleaching can borohydride and thiourea dioxide, but they are not commercially used for var-ions reasons.

Of the two common reagents used commercially, hydrosulfite and peroxide, the former is used in more mills than the latter. Hydrosulfites can be added to the mechanical pulp at any point between the grinders or refiners and the paper machine. Usually it is added at the deckers where the consistency and temperature is quite suitable for hydrosulfite bleaching. In addition, the pulp stays in the deckers long enough for hydrosulfite to act. Hence no special equipment is required. On the other hand, peroxide bleaching can be performed only in special bleaching towers where the consistency of pulp (about 25% for optimum results) is much higher than anywhere in the mill and where pulp is retained in contact with peroxide for about three hours. Because of these problems, peroxide bleaching is carried out less often than hydrosulfite bleaching. It must also be mentioned that in practical terms hydrosulfite bleaching is limited to increasing the brightness of a mechanical pulp by about 6 to 8 percentage points. If greater brightening is desired, a peroxide bleaching procedure must be followed. It is also common in the industry that'a peroxide stage is followed by a hydrosulfite bleaching stage to get higher brightness.

The hydrosulfite bleaching of mechanical pulps has to be regulated because of two characteristics of hydrosulfite. This material is a strong reducing agent and quite stable in an alkaline solution, but in weakly acidic or acidic solutions, it decomposes at a very fast rate. The bleaching of mechanical pulps has to be carried out in an acidic medium because of the darkening of mechanical pulps at pH above 6.5. Considering the above two factors, laboratory experiments have shown that the optimum pH range for hydrosulfite bleaching of mechanical pulps is 5.5 to 6.0. In the mills, therefore, where the pH of mechanical pulps is in the 4.0 to 4.5 range, the addition of a buffering agent is necessary to raise the pH although it normally does not reach the optimum range.

Another problem with hydrosulfite bleaching is the catalytic effect of heavy metal ions on the rate of decomposition of the bleaching agent. To counteract this effect, addition of sequestering agents to the mechanical pulp during bleaching is desirable. The most common ma= terial added for this purpose is sodium tripolyphosphate (STPP), as described in US. Patent 2,707,144. The use of some other materials has also been patented, e.g., ethylenediaminetetracetic acid (EDTA) and (EDTA) trisodium salt (U.S. Patent 2,707,145), sodium citrate with or without alkali tetraborate (U.S. Patent 2,826,478) and several inorganic salts (U.S. Patents 2,071,304 and 2,071,307). However, because of cost considerations, the

most common reagent used is STPP.

In a technical bulletin issued bydu Pont (vol. 11, No. 3, pages 13613-8, September 1955) a process is described in which a mechanical pulp is bleached with 1% sodium hydrosulfite on pulp in the presence of 0.5% STPP on pulp or 0.1 to 0.2% EDTA 0n pulp. The pH of the pulp is maintained at 5.5-6.0 using approximately 0.1% sodium silicate. They also mention that if sodium silicate is not required for pH control, 0.01% silicate is recommended for corrosion control.

Although STPP is quite satisfactory for the above use, it is lost in the effluent waters of the mill. On hydrolysis it yields the phosphate ion which, in combination with carbon and nitrogen, is believed to be responsible for algal growth in natural water systems, and the waters became fouled by a green scum. Hence, for ecological considerations it is essential to replace ST PP by a suit= able substitute. The cost of other satisfactory materials such as EDTA, sodium citrate, etc. is a hindrance to their use.

Therefore, it is an object of the present invention to provide a process for the bleaching of mechanical pulps.

It is another object of the present invention to provide a hydrosulfite bleaching process wherein sodium tripolyphosphate, which is conventionally used at present, is completely eliminated, and is replaced with a compound which will not result in the adverse ecological effects described above.

Other objects and advantages of the present invention will be apparent from the detailed description of the invention which follows.

It has been found that the ecological problems attendant the hydrosulfite bleaching of mechanical pulps in the presence of sodium tripolyphosphate can be avoided completely by eliminating the use of sodium tripolyphosphate and using in its stead an alkali metal silicate, such as, for example, sodium, potassium or ammonium silicate. Although the invention will hereinafter be illustrated by reference to sodium silicate, it will be appreciated and understood that the other alkali metal silicates can also be used. In the description of the invention the amounts of sodium silicate are expressed as 41 B. solution as commonly used in the pulp and paper industry. The results when using either solid sodium silicate or any of the other alkali metal silicates are identical.

Mechanical pulp of standard production, either stone or refiner groundwood, has a fiber concentration within the range of from about 1% to about 15%. Based upon equipment limitations and to expedite handling, it is preferred, however, to employ an aqueous pulp suspension having a consistency of from about 1% to about 6%, based on the weight of the moisture-free pulp, and even more preferably a pulp having a consistency of from about 3% to about 4%. The consistency of 3 to 4% mentioned as preferable is usually the consistency in the deckers where the hydrosulfite bleaching is normally carried out in the mill.

Either zinc or sodium hydrosulfite may be employed in accordance with the present invention, in a concentration ranging from about 0.05 to about 1.0%, based on the weight of moisture-free pulp. In most cases it is preferable to use from about 0.3% to about 0.5%, by weight, of sodium or zinc hydrosulfite.

In the preferred embodiment of the present invention, sodium silicate and the hydrosulfite are mixed with the aqueous suspension of mechanical pulp and heated for a period of from about 10 minutes to about 6 hours at from about room temperature to about 100 C. Prefferably, however, the reaction is carried out at a temperature of from about 50 to about 70 C. for a period of from about 1 hour to about 3 hours.

In another embodiment of the invention, the sodium silicate is first added to the aqueous suspension of mechanical pulp and heated for a short period of time, for example, minutes at 60 C. Then the hydrosulfite is added, followed by stirring and heating at, for example, 60 C. for about 1 hour.

When 1% sodium hydrosulfite is employed, less than 0.15% sodium silicate solution, which is equivalent to about 0.05% solid sodium silicate or 1 lb./ton of mechanical pulp, produces no effect. At at sodium silicate concentration of about 0.15% or above, based on the weight of moisture-free pulp, the brightness of the pulp is improved beyond what is observed when sodium hydrosulfite alone is used. The brightness continues to increase with increasing amounts of sodium silicate. If, however, more than 1% on pulp, i.e., 20 lbs/ton is used, the advantage achieved starts to decrease because the pH of the pulp slurry or suspension rises above the optimum range for mechanical pulp bleaching. This optimum range is from about pH 5.5 to about pH 6.0. While this optimum range produces the best results, a range of from about pH 4.0 to about pH 6.5 can be employed satisfactorily. However, when the pH is below 4 the hydrosulfite begins to decompose and when the pH is above 6.5 a darkening in pulp color occurs. Thus, when 1% hydrosulfite is employed the useful range of sodium silicate is from about 0.15% to about 1.5%, based on the weight of moisture-free pulp, with from about 0.5% to about 1% being preferred for optimum results.

In the Du Pont Technical Bulletin mentioned above, 0.1% sodium silicate on pulp is used only if necessary to adjust the pH between 5.5 to 6.0. Since other sequestering agents, STPP or EDT'A, are present in much larger quantities, silicate has no effect on the brightness improvement. In addition, the amount added was too small to have any effect. Because silicate is not known to be a good sequestrant, the results obtained in this invention are unexpected and unobvious. Because of this unobvious result, it is postulated that the sodium silicate sequesters the ions of iron, copper manganese, etc.

The use of sodium silicate in peroxide bleaching of mechanical pulps, along with sodium hydroxide and magnesium sulfate, is a well known process. In peroxide bleaching, unlike hydrosulfite bleaching, the pH of the pulp slurry is very high, and sodium silicate acts as a buffer to maintain the high pH. In addition, it reacts with magnesium sulfate and forms colloidal magnesium silicate; the latter adsorbs manganese ions which are ex" tremely harmful in peroxide bleaching. Since manganese ion has no effect on hydrosulfite bleaching of mechanical pulps, the use if sodium silicate in peroxide bleaching is not indicative of any advantage in hydrosulfite bleaching.

In addition to increased brightness in hydrosulfite bleaching, sodium silicate reduces pollution because it replaces the phosphates formed by the STPP. The effluent water in this case contains unchanged sodium silicate. If this effluent is treated with a mineral acid such as hydrochloric acid, sulfuric acid, nitric acid, etc., sodium silicate is converted into the sodium salt of the acid used and silica. If hydrochloric acid, which is the preferred acid, is used, the products are sodium chloride and silica, two innocuous materials. Simple calculations show that 1 pound of 41 B. sodium silicate will give a concentration of 1 p.p.m. sodium chloride in the etfiuent water if the volume of the efiluent is 20,000 gallons/ton, a reasonable average for a newsprint mill. Silica is not objectionable in any water system.

Use of silicate will also help in corrosion control of mill equipment made of alloys of iron, copper and small amounts of other metals. The silicates are well known corrosion inhibitors and probably do not require further discussion. This corrosion inhibition is further enhanced by the increased pH effected by silicate. Since the pH of the pulp slurry i 4.0-4.5, the solution should be corrosive to many metals. Any increase in pH should decrease the corrosivity of the solution. It may, however, be mentioned that phosphates also have a similar effect, although the corrosion inhibition of phosphates is less marked than that of silicates.

To assess the usefulness of the invention in a mill operation, a mill trial was run using 15 to 20 pounds of 41 B. sodium silicate per ton of stone groundwood. An unrelated and unexpected result was noticed. It was found that the energy required for grinding the wood with stone grinders was reduced by 5 horsepower day/air dry ton of groundwood, which corresponds to 6-7% of the total energy used. This is a substantial saving in the energy consumption of the mill. This advantage of power requirement reduction is opposed to what is observed with STPP. Use of 3-4 pounds STPP per ton of groundwood usually raises the power requirements by about 3-4 horsepower days/ air dry ton of mechanical pulp.

EXAMPLE 1 A 40 gram (25% solids content) sample of commercial stone groundwood prepared from a mixture of black spruce and balsam fir was suspended in 303 ml. of water by stirring to give a suspension of groundwood in water of 3% consistency. The slurry was heated to 60 C. in a constant temperature water bath during five minutes. The

heated slurry was treated with 0.1 gram sodium hydrosulfite and the closed jar shaken for 30 seconds to mix the pulp slurry with bleaching agent. An immediate brightening eifect was visible. To complete the bleaching the jar was then replaced in the 60 C. water bath and held there at 60 C. for one hour. After this interval, the pulp was filtered off and washed with water. Two handsheets weighing 5 grams each and 12.5 cm. in diameter were prepared from the original unbleached pulp and two similar sheets were prepared from the bleached pulp. The sheets were pressed between cellulose blotters and stainless steel plates for two minutes at a pressure of 240 lbs/square inch. After drying between rings for 16'hours, their brightness (reflectance) was measured using an Elrepho reflectance meter. Relative to the magnesium oxide standard with a reflectance of 100% the unbleached pulp had a brightness of 59.6% and the bleached pulp 64.2%. Hence treatment of this groundwood with 1% sodium hydrosulfite increased the brightness by 4.6 percentage points.

EXAMPLES 27 A 40 gram (25% solids content) sample of commercial groundwood (same as that used'in Example 1) was suspended in 303 ml. of water containing various amounts of 41 B sodium silicate (0.2 to 1.0% by weight on moisture-free pulp) by stirring to give a suspension of 3% consistency. This suspension was heated during five minutes to 60 0., treated with 100 mg. sodium hydrosulfite (1% by weight on moisture-free pulp), mixed and heated at 60 C. for one hour. After this interval the pulp was filtered, washed with water and two handsheets made as described in Example 1. The brightness of the unbleached pulp was 59.6%. Table I gives the brightness of the sheets after treatment with various amounts of 41 B. sodium silicate (or equivalent solid sodium silicate) from which the amount of brightness gain due to sodium silicate can be calculated knowing that 1% sodium hydrosulfite alone TABLE I Bright- 41 B. Solid ness of Brightsodium sodium bleached ness gain silicate silicate pulp (per- (percent- Bright- Example (percent (percent cent on a e ness number on pulp) on pulp) pulp points gain 1 1 Due to sodium silicate (percentage points).

EXAMPLE 8 A 40 gram sample of (25% solids content) groundwood (same as used in Example 1) was suspended in 303 ml. of water containing mg. of sodium tripolyphosphate (0.2% by weight on moisture-free pulp) to give a 3% consistency. It was bleached with 100 mg. sodium hydrosulfite as in Examples 2 to 7. The brightness of the bleached pulp was 64.8% showing a brightness gain of 5.2 percentage point due to sodium hydrosulfite and sodium tripolyphosphate. Since the same amount of sodium hydrosulfite alone produces a brightness gain of 4.6 percentage points, the brightness gain due to 0.2% sodium tripolyphosphate is 0.6 percentage points.

EXAMPLE 9 A 40 gram sample of (10 gram moisture-free weight) of groundwood was bleached exactly as in Example 8 but with 40 mg. sodium tripolyphosphate. The bleached pulp had a brightness of 65.5% which represents a total brightness gain of 5.9 percentage points. Since 1% sodium hydrosulfite produces a 4.6 percentage point gain (Example 1), 0.4% sodium tripolyphosphate produces an extra 1.3 percentage point gain in brightness.

6 EXAMPLES 1045 In Examples 2 to 7 the amount of sodium hydrosulfite was 1% on pulp. In Examples 10 to 15 experiments were carried out with 0.5% sodium hydrosulfite in the presence of 0 to 1.0% 41 B. sodium silicate on pulp. The advantage of adding sodium silicate is evident from the results shown in Table II.

TABLE II Bright- 41 Be Solid ness 0! Brightbleached ness gain pulp (per- (percent- Brightcent on age ness pulp points) gain 1 See footnote 1 bottom of Table I.

EXAMPLE 16 The same sample of groundwood was bleached with 0.5% sodium hydrosulfite and 0.2% sodium tripolyphosphate. The brightness of the bleached pulp was 63.2 which represents a brightness gain of 3.6 points. Since 0.5% sodium hydrosulfite produces a brightness gain of 3.1 percentage points (Example 10) the gain because of 0.2% sodium tripolyphosphate should be 0.5 percentage points.

. EXAMPLE 17 When the experiment of Example 16 was repeated with 0.5% sodium hydrosulfite and 0.4% sodium tripolyphosphate, the bleached pulp had a brightness of 63.9, i.e., a total brightness gain of 4.3 percentage points. Hence the gain in brightness due to 0.4% sodium tripolyphosphate is 4.3 minus 3.1, or 1.2 percentage points.

EXAMPLE 18 A sample of commercial refiner groundwood prepared by refining a mixture of chips of black spruce and balsam fir was bleached with 1% sodium hydrosulfite as in Example 1. The brightness of pulp rose from 58.5 to 63.0 indicating a gain of 4.5 percentage points.

EXAMPLE 19 When the experiment described in Example 18 was repeated with 1% sodium hydrosulfite and 0.5% 41 B. sodium silicate, the brightness rose from 58.5 to 63.5, a total gain of 5.0 percentage points. Since 1% sodium hydrosulfite producesa gain of 4.5 points (Example 18) thegain due to 0.5% 41 B. sodium silicate is 0.5 percentage points.

EXAMPLE 20 In a mill trial beginning with the 3rd day and continuing through the 6th day, 15 to 20 pounds (0.75% to 1.0%) of 41 B. sodium silicate per ton of groundwood pulp was added at the deckers. Sodium silicate addition was discontinued on the 6th day. During the trial 4 to 8 pounds of sodium hydrosulfite (0.2% to 0.4%) per ton of pulp was employed. The power consumptions were recorded every day.

Table III shows the advantage of adding sodium silicate in reduction-of power consumption. The data also indicate that'there is some time lag between the addition of silicate and appearance of the effect. Also, since silicate remains in the closed water system after its addition has been stopped, it advantageous effect continues for some time.

TABLE III Power requirements Day: v HPD/ADT 1 1st 82.5 2nd 80.1

1 Horse power days/air dry ton of pulp. 9 Silicate started. Silicate stopped.

What is claimed is:

1. A process for the bleaching of mechanical pulps consisting essentially of reacting an aqueous suspension of said pulp with a compound selected from the group consisting of sodium and zinc hydrosulfite in the presence of from about 0.05 to about 0.562% of a solid alkali metal silicate, based on the moisture-free weight of the pulp.

2. The process of claim 1 wherein the reaction is carried out at a temperature of from about room temperature to about 100 C. for a period of from about 10 minutes to about 6 hours.

3. The process of claim 2 wherein the reaction is carried out at a temperature of from about 50 C. to about 70 C. for a period of from about 1 hour to about 3 hours.

4. The process of claim 1 wherein the consistency of the pulp in the aqueous suspension is from about 1% to about 6%, based on the moisture-free weight of the pulp.

5. The process of claim 4 wherein the consistency of the pulp in the aqueous suspension is from about 3% to about 4%, based on the moisture-free weight of the pulp.

6. The process of claim 1 wherein the hydrosulfite concentration is from about 0.05% to about 1.0%, based on the moisture-free weight of the pulp.

7. The process of claim 1 wherein the alkali metal silicate is sodium silicate.

8. The process of claim 7 wherein the sodium silicate is 41 B. sodium silicate and its concentration is from about 0.15% to about 1.5%, based on the moisture-free weight of the pulp.

9. The process of claim 8 wherein the concentration of the 41 B. sodium silicate is from about 0.5% to about 1.0%, based on the moisture-free weight of the pulp.

10. The process of claim 1 wherein the pH of the aqueous pulp suspension is from about 4.0 to about 6.5.

11. The process of claim 10 wherein the pH of the aqueous pulp suspension is from about 5.5 to about 6.0.

References Cited UNITED STATES PATENTS 2,862,784 12/1958 Kise et al. 16280 X 2,187,016 1/1940 Craig 16271 X 2,071,304 2/1937 Hirschkind 16271 OTHER REFERENCES Du Pont Technical Bulletin, September 1955, p. 136.

S. LEON BASI-IORE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R. 8--Il0; 162-80, 83