US3397945A - Chlorination of caustic soda solutions used for textile processing - Google Patents

Description

FIE- 830? 3,397,945 CHLORINATION F CAUSTIC SODA SOLUTIONS USED FOR TEXTILE PROCESSING Harry G. Smolens, Penn Valley, and Oliver S. Sprout, In,

Glenside, Pa., assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Filed Feb. 28, 1963, Ser. No. 261,817

2 Claims. (Cl. 8-139) ABSTRACT OF THE DISCLOSURE This invention is directed to a process for preserving the useful properties and effects of caustic soda solutions employed for the preparation of cotton textiles.

Cotton textiles manufactured by weaving, knitting or other processes are not immediately useful for most purposes because they are unattractive and crude in appearance and contain extraneous materials such as sizing and dirt. One of the types of processes commonly employed after fabrication of the textile is referred to as a preparation operation and involves scouring the textile with a solution of caustic soda for the desired period of time so that the textile becomes soaked with the caustic solution. Heat may also be a part of the operation, the caustic soda solution being heated as desired. In addition, cloth removed from the caustic solution may be maintained in a heated environment, the cloth containing as much of the caustic solution as it is able to soak up. The action of the caustic soda on the cotton textile is such that much of the non-cellulosic material is removed toward the end that a more highly refined cellulosic textile is produced.

There are various degrees of scouring, depending on the final treatment which is intended for the cloth. Therefore, scouring cannot be defined unless the desired end treatment is known. Cotton goods are usually scoured with caustic soda solutions either in a boil-off machine or by treatment in a caustic saturator followed by steaming in a J-box.

The scouring accomplished in a boil-off machine is undertaken at high temperature in order to secure as great a removal as possible of fabric impurities. After the boil-off treatment the cotton fabric is usually mercerized and dyed. There is no bleaching of this cloth.

Treatment of cotton cloth in a caustic saturator followed by steaming is part of a continuous bleaching process. The temperatures used in a caustic saturator generally reflect local plant conditions and may be oper ated at 120l40 F. However, when operated in accord UHUUQ ted States Patent 6 Mini Ll tilnlthlin 3,397,945 Patented Aug. 20, 1968 ance with our invention, the caustic saturator must have a minimum temperature of F.

The materials which are removed from the cotton cloth by scouring are carried into the caustic solution and result in an increased amount of impurities, dirt and color in the caustic solution. These materials limit the useful life of the caustic solution because the impurities tend to redeposit on the cloth being processed. When the cloth is being scoured with caustic at a rate'of about 3000 yards of cloth per hour, 2400 gallons of 5% caustic solution will have a useful life of 6 to 24 hours, after which it becomes too dirty for continued use. The dirty caustic solution must then be discarded and in addition to the expense of replacing this caustic solution, there is the added problem of how to dispose of the caustic liquor.

We have now discovered that the useful life of a caustic scouring solution may be extensively prolonged by diffusing chlorine gas into the caustic soda solution under specified conditions. Quite unexpectedly, the useful life of the caustic scouring solutions may be increased as much as ten or twelve fold. Important to this process is the fact that only a very small amount of chlorine gas is required so that there is very little or no increase in expense accruing to the processing of the cotton cloth. The added cost of the chlorine used in this process is more than offset by savings in the cost of neutralizing the impure caustic solution and disposing of it.

Another advantage to our process is the fact that only very simple apparatus is required to introduce the chlorine. Generally no additional processing equipment is required other than installing in the boil-off machine or caustic saturator a chlorine diffuser and a chlorine flow meter. The cotton cloth which is processed in accordance with our invention may thereafter be used as such, or the textile may be washed as may be required by subsequent operations, if any, orit may be subjected to further processing such as by dyeing, mercerizing or bleaching, or any combination of one or more of these operations.

While the introduction of chlorine into the caustic scouring solution maintains the solution in a cleaner condition and extends its useful life for a much greater period, there are certain other unexpected advantages which accrue to the cloth by the use of our process. Depending on the product to be manufactured, subsequent operations may include mercerizing, dyeing and bleaching either singly or in various combinations. For example, if the cloth is dyed following the caustic scouring, there is a much greater uniformity of dyeing brought about.

If it is desired to bleach the cotton goods with peroxide following the caustic scouring operation, we have found that as much as one-third less hydrogen peroxide is required to obtain the same amount of bleaching of the cotton goods where chlorine treatment of the caustic solution was not employed. Moreover, since chlorine is a strong oxidizing agent, the presence of chlorine prevents the formation of a reducing condition in the saturator which condition is known to permit introduction of metal- I lie impurities in the fabric. These impurities may form holes in the cloth. With chlorinated caustic solutions, these metallic impurities are oxidized and do not harm the fabric.

As used in this specification and claims, cotton goods includes 100% cotton fabrics as well as blended cotton fabrics which incorporate various amounts of rayon, nylon, Orlon, Dacron and other synthetic fibers. All of these goods work equally well in the practice of our invention.

The first condition that must be observed in practicing our invention is that sufficient chlorine gas must be employed so that effective cleaning of the caustic scouring solution is obtained. The rate of chlorine addition to the caustic solution will naturally be dependent on the rate of throughput of the cotton cloth which is being processed in the solution. As the cotton cloth is processed at a faster rate, naturally more chlorine will be employed. The correlation between the cloth throughput rate and the chlorine addition rate will be realized if it is considered that the impurities carried into the caustic scouring solution 'are brought about by removal from the goods being processed. We have found that the minimum rate of chlorine addition is 0.1% by weight of the dry cloth being processed. Thus if 1000 pounds of cloth are being processed per hour, the minimum rate of chlorine addition will be one pound per hour.

For very dirty cloth or where a severe scouring operation is being obtained as in a boil-off machine, much greater quantities of chlorine are required and the upper limit of the chlorine addition is 2.0% of the weight of the dry cloth which is being processed. For a cloth throughput rate of 1000 pounds per hour, this would require pounds of chlorine per hour.

In addition to securing effective purification of the caustic scouring liquor, the upper limit of the chlorine addition rate is also governed by the fact that it is required that there be no hypochlorite present in the caustic scouring solution. The presence of any hypochlorite is detrimental for various reasons. Primarily, the presence of hypochlorite, if not removed will interfere with subsequent operations such as dyeing or peroxide bleaching. In addition to interfering with subsequent processing of the cloth, the presence of hypochlorite seriously corrodes subsequent rnet'al processing machinery, particularly where elevated temperatures are employed such as in I-boxes. On the average we have found a chlorine addition rate of 0.3 to 0.5 percent of the weight of the dry cloth will control the impurities of the average caustic saturator solution for a great many of the types of cotton goods being processed, and this is our preferred range of chlorine addition for a caustic saturator. In a boiloff machine wherein a much more severe scouring is obtained, the preferred chlorine addition rate is 1 to 2 percent of the weight of the dry cloth.

The second condition that must be observed in practicing our invention is that a minimum temperature of at least 160 F. must be maintained in caustic scouring solutions in order to prevent the formation of hypochlorite. As mentioned previously, the hypochlorite is detrimental to subsequent processing operations, or it may cause excessive corrosion of the processing equipment.

The upper temperature limit will be determined by the boiling point of the caustic solutions employed. This may be as high as 217 F. for a 10% solution. Naturally, these temperatures are for boiling points at atmospheric pressure. Preferably the temperature of the caustic scouring solutions, will be within the range of 180 to 200 F.

The sodium hydroxide concentration of the caustic scouring solutions is not critical and we have found that our invention is applicable to any caustic soda concen tration normally encountered in the caustic scouring of cotton goods. The advantages of the chlorine treatment of the solutions are best obtained if the caustic soda concentration is at least 2% and generally not more than 10%. These are limits which are normally employed for the causticscouring of the cloth and do not limit the benefits of the chlorine treatment.

The chlorine treatment would be beneficial to caustic soda solutions Outside of this concentration range but the 2 to 10% range represents the concentrations normally encountered. If less than 2% sodium hydroxide is used in the caustic solution, there is insufficient removal of the impurities in the cloth to warrant the treatment. If a concentration in excess of 10% sodium hydroxide is used, there is danger of degrading the cellulose by the formation of oxycellulose. Cellulose which has been converted to oxycellulose by caustic soda treatment will lose its strength and fall apart. Cotton cloth which is scoured with sodium hydroxide solutions at concentrations of less than 2% does not have sufficient of the impurities removed, so that thereafter the cotton goods cannot be satisfactorily bleached or dyed.

In one aspect of our invention the chlorine gas may be introduced to what is known in the trade as a boilotf machine. In this apparatus cotton cloth is subjected to the action of caustic soda solution containing 5% to 6% sodium hydroxide at a temperature ranging from F. to boiling. By means of suitable machinery, the cotton cloth is unwound from the shaft on which it is rolled, the cloth then passing into and out of the caustic solution, after which it is again rolled on a shaft. Each roll thus progresses along the machine and is generally subjected to five immersions during the process. The total time for a given roll of cloth containing about 1500 yards to progress through the boil-off machine is about three hours. Naturally, because of the rolling and unrolling operation, the cloth is not in the caustic solution for this entire time. The caustic solution is contained in a long tank running the length of the machine. The volume of the solution amounts to about 2400 gal- Ions in the average situation. The cloth is given a water rinse as it leaves the machine.

The chlorine gas is conveniently introduced into the caustic solution in a diliused state by methods which are well known in the art. For example, a ceramic or polyvinylidene fluoride diffusing disc may be employed which breaks up the chlorine gas into many fine bubbles. Another way of accomplishing diffusion of the chlorine gas in the boil-off tank is to introduce the chlorine gas by means of a pipe 'as a solid stream and thereafter diffuse the gas by a high speed turbine. Other means will will be obvious, such as introducing the chlorine gas into the turbine of a pump which may be recirculating the caustic scouring solution.

Generally the introduction of the chlorine is controlled by means of a flow meter whereby the rate of the chlorine introduction may be observed. Periodically the chlorine solution may be analyzed to determine that there is no hypochlorite present. This may conveniently be done by the use of starch iodide paper or by taking a portion of a solution, adding it to potassium iodide solution containing starch indicator. In the latter case the presence of the iodine color will indicate that there is hypochlorite in the caustic scouring solution. In the case of the starch iodide test paper, the presence of dark color on the test paper will indicate the presence of hypochlorite ion.

The best mode of practicing our invention will be apparent from a consideration of the following examples.

EXAMPLE 1 This example shows the prior art operation of the caustic boil-off machine. In a boil-oif machine containing about 2400 gallons of 5-6% sodium hydroxide the temperature was maintained at about 200 F. Cotton gray goods were introduced at a rate of approximately 1250 lbs. per hour. No chlorine was introduced. The solution gradually acquired a dark color and after about six hours the solution was so dirty, as noted b the smudge deposits on the cloth, which could not be removed by a water wash, that the entire tank had to be discarded.

EXAMPLE 2 After the boil-off machine in Example 1 had been re-= plenished with fresh 5% sodium hydroxide solution, the temperature was raised to 200 F. and the introduction of the cloth at a rate of 1250 lbs. per hour was begun. Simultaneously with the starting of the cloth through the boil-01f machine, chlorine was introduced through piping and dispersing apparatus at a rate of 25 lbs. per hour. It was found that it was possible to continue the throughput of the cotton gray goods at a rate of 1250 lbs. per hour for 72 hours before it was necessary to shut down and discard the caustic liquors.

Examination of the cloth coming from Example 2 wherein chlorine had been used indicated that it appeared to be whiter than cloth coming from the boil-off machine when chlorine was not used, as in Example 1. However, when samples of these'cloths were similarly washed with soap and soda ash solutions, the cloth from the machine from Example 2 usingchlorine was not as white as cloth from the machine which did not employ chlorine. This is evidence that the chlorine was not accomplishing any bleaching of the cloth. The fact that the cloth treated with chlorine appeared whiter when it first came from the boil-off machine is attributed to the fact that the adhering alkaline solution contained fewer impurities than the alkali solution which had not been treated with chlorine.

To determine whether a difference existed between the quantity of impurities in the alkali solutions from the chlorinated caustic liquor in Example 2 and non-chlorinated liquor in Example 1, aliquots of each solution were acidified, oxidized with excess standard potassium permanganate by boiling and any remaining potassium permanganate determined by addition of potassium iodide and titration with standard sodium thiosulfate to the starch end-point. The consumption of permanganate per unit of solution indicated the content of organic substances. The results are as follows:

(a) Machine without chlorine addition, solution used 14 hours, 39.2 ml. 0.1 N KMnO per ml. solution.

(b) Machine using chlorine addition, solution used 45 hours, 43.6 ml. 0.1 N KMnO per ml. solution. I

It is evident that the content of organic substances in the solution from each machine was substantially the same, even though the solution to which chlorine had been added was used over three times as long as the solution without chlorine addition. For operation of the boiloff machines in these-examples for identical lengths of time, the permanganate number of the caustic solution in Example 2 will be approximately one-third of that in Example 1.

Frequent discard of the caustic solution and preparation of fresh solution is costly in time and ingredients, and can the substantially reduced by the introduction of chloline into the caustic solution. Furthermore, discharge of used caustic solutions into sewers and streams .-results in pollution problems because of the high content of oxidizable material, which can be harmful to plant and animal life, even if the alkali is neutralized before discharge. As shown by the permanganate test, oxidizable material remaining in the chlorinated alkali solution would be reduced for any given period of time, in which the total discharge of spent liquor would also be reduced.

Samples of the cloth emerging from Examples 1 and 2 were washed with water and then dyed. It was found that cloth from the chlorinated scouring solution of Example 2 dyed much more uniformly and that there were about 33% less variations in color as were noticed in the dyed cloth which had been treated as in Example 1. The reason for this is not apparent since the chlorinated scour= ing liquor did not produce a whiter fabric. However, it is believed that the lesser quantity of impurities in the chlo-= rinated alkali per unit of time results in less redeposition of impurities on the fabric and hence fewer difficultly dyed areas.

EXAMPLE 3 Cotton gray goods were passed into a caustic satuator containing sodium hydroxide at 3-4% concentration. The cloth was introduced at a rate of 200 yards per minute and the saturator held about 200 gallons of liquor. The temperature was maintained at F. After removal from the saturator the cotton goods were steamed in a J- box for 40-60 minutes, water rinsed and then bleached with hydrogen peroxide.

EXAMPLE 4 Using the same cloth and equipment as in Example 3, the temperature in the saturator was raised to F. and chlorine was introduced into the machine simultaneously with the introduction of the cloth at a rate equivalent to 0.5% of the weight of the dry cloth passing through the machine. After about an hour of operation the liquors were checked for available chlorine, but no available chlorine was observed. The goods leaving the saturator were passed into a J-box and steamed for about an hour, and then moved through a hydrogen peroxide saturator, followed by heating in a J box, rinsing and drying. Cloth emerging from the saturator in Example 4 used 20% less hydrogen peroxide than the goods emerging from Example 3 when bleached to the same degree of whiteness.

EXAMPLE 5 Cotton drill goods were introduced to an open width saturator containing 5% sodium hydroxide solution. The cloth was introduced to the saturator at a rate of 3000 lbs. per hour. Chlorine was diffused into the bottom of the saturator by the use of a polyvinylidene fluoride disperser at a rate of 10.0 lbs. per hour. The temperature was maintained at 180 F. The solution which was squeezed from the cloth by the last set of rollers as the cloth emerged from the saturator was analyzed at hourly intervals for available chloride by titration of a portion of the liquid. After 13 hours operation no active or available chlorine was detected on the liquors squeezed from the cloth as the cloth emerged from the saturator. Following emersion from the saturator, the cloth was steamed open width for approximately 40 minutes and then washed and peroxide bleached. The caustic solution remained clear and without appreciable color. Satisfactory cleaning of the cotton drill goods was obtained and the material bleached satisfactorily.

EXAMPLE 6 Using the same equipment and the same cloth and chlorine feed as in Example 5, the saturator was operated at a temperature of 130 F. With operation of the sodium hydroxide solution at 130 F. the water expressed from the cloth leaving the saturator, upon being analyzed for available chlorine, averaged about 0.3 to 0.4% available chlorine. This level of available chlorine in the goods leaving the saturator is objectionable both to subsequent processes and because of severe corrosion of the metal equipment which transports the cloth. After an hour and five minutes of operation at this temperature and with no significant decrease in the available chlorine of the liquor accompanying the emergence of the cloth from the saturator, the machine was shut down.

While the invention has been described with reference to specific examples, it is of course understood that the invention is not intended to be limited thereby since many modifications may be made in the process within the skill of the art, and it is not intended that the invention be so limited except insofar as appears from the accompanying claims.

We claim:

1. In a process wherein cotton gray goods are continuously scoured with caustic soda solution, the improve-= ment which comprises diffusing chlorine gas into said caustic soda solution at a rate to maintain the concentration within the range of 0.1% to 2% chlorine by weight based on the dry weight of the cotton gray goods being continuously introduced into said caustic soda solution and continuously removed therefrom while maintaining the said caustic soda solution at a concentration within the range of 2 to 10% sodium hydroxide by weight and the temperature of the said caustic soda solution within the range of 160 F. to boiling.

2. The process of claim 1 as applied to a boil-01f machine in which the temperature range is maintained between 180 F. and boiling.

References Cited UNITED STATES PATENTS Rogers 8-108 Lindsay 8-109 Anderson et al 8-108 Partlow 8-108 X Riley et a1. 8-108 X J. TRAVIS BROWN, Acting Primary Examiner.

I. P. BRAMMER, Assistant Examiner.