Nitrocellulose


GENERAL DESCRIPTION OF OPERATION

The manual has been reformatted for easy access current technology.  Instead of a straightforward book it now uses a series of links. Most links below will take you to the section called “Theory”.  Additional links will access sketches, photos, or sub-sections.

This manual is designed specifically for use by Indiana Ordnance Works. Layout diagrams of the nitrocellulose facilities at this location are presented in Fig. 1 (A, B and C lines) and Fig. 2 (D, E and F lines). In addition, a sche­matic flow sheet is presented in Fig. 3.

The manufacture of military nitrocellulose for smoke­less powder consists essentially of three general operations, (1) nitration, (2) purification and (3) finishing. Through­out this manual, these operations will be discussed systemati­cally under the sub-headings listed below:

NITRATION

I.     History

II.    General Description of Operations

III.    Cellulose Preparation

IV.   Acid Supply

V.   Dipping

VI.  Wringing and Drowning

VII. Spent Acid Filtration

VIII. Fume Acid Recovery

   PURIFICATION

IX. Stabilization

X.  Pulping

XI. Poaching

   FINISHING

XII.Screening

XIII Blending

XIV Final Wringing

XV.Slurry Tank House

XVI Sample Drying

The chemical specifications for the various grades and types of military nitrocellulose which may be prepared by the above sequence of operations are listed in Section XVII, Page 406. However, at Indiana Ordnance Works, the finished product is usually blended nitrocellulose (Grade C – Type I) which is delivered to the powder areas for use in the manufac­ture of smokeless powder. It consists of about 65% High Grade (13.35% min. nitrogen) and 35% Pyro (12.60% 10% nitrogen) with an average nitrogen content of about 13.15%.

A brief description of each process in the manu­facture of nitrocellulose follows.

Manpower requirements listed will vary according to production schedules.

An understanding of the nature of both cotton linters and wood cellulose is dependent upon a knowledge of carbohydrate chemistry. Cellulose, a polymer or long chain molecule of glucose, acquires its chemically useful properties from the hydroxyl groupswhich are a part of its molecular structure. These alcoholic groups react with acids to form products called esters, such as the nitric acid ester of cellulose, nitrocellulose.

The fundamental building unit within the cellu­lose molecule is glucose. The glucose units, not at the ends of long chain cellulose molecules, have three, free, hydroxyl groups which may react with nitric acid during nitration. The terminal glucose units have four, free, reactive, hydroxyl groups.

The degree of esterification, measured by the nitrogen content of the nitrocellulose after nitration, depends upon how many hydroxyl groups, per glucose unit, in the cellulose molecule, react. A degree of esterifi­cation of two, theoretically, corresponds to the reaction of two hydroxyl groups per glucose unit.

The theoretical determination of the nitrogen con­tent of nitrocellulose, with a degree of esterification of 2, is 11.1%; with a degree of esterification of 3, it is 14.1%; and with a degree of esterification of 4, it is 15.6%. At the Indiana Ordnance Works, Pyro N/C has a nitrogen content of about 12.6% and High Grade, 13.45%. Thus, theoretically, the Pyro nitrocellulose made at I.O.W. may correspond to nitrocellulose composed of equal amounts of the second and third degree esters, while the High Grade may correspond to nitrocellulose composed of thirds of the second, third and fourth degree esters.

This points up the essential random character of the nitration of cellulose (esterification) which cannot be controlled to yield a nitrocellulose that is completely and ideally homogeneous. Conditions can be governed to yield a product with a desired nitrogen content since the nitrogen content represents the average nitrogen, corresponding to a nitrocellulose which is a composite of nitric esters of two, three and four degrees of esterification.

Since specifications on the properties of the N/C used for granulation are less tolerant than those upon the Pyro and the High Grade, cellulose is first nitrated to Pyro and High Grade. Calculated proportions of the Pyro and High Grade, depending upon their physical and chemical properties, are then blended to yield an N/C with desired characteristics.

I. Nitration

A. Cellulose Preparation

Cellulose preparation serves two purposes. First, the  cellulose is placed in a suitable physical form for contact with mixed acid, and second, the moisture content of the cel­lulose is reduced to the lowest practical value.

Both cotton linters and woodpulp are used as the basic raw materials for the manufacture of nitrocellulose at Indiana Ordnance Works. Cotton linters are received in compressed bales about 22″ wide by 18″ high by 48″long, and weighing about 150 pounds. The bales of cotton are transported to the feed end of the linters dryer as needed stripped of heavy wrapping paper and baling wire, broken into batts 2″ to 3″ in thickness, and placed on a conveyor which feeds a picker roll. The picker roll reduces the batts to a fluffy mass and dis­charges this material to the dryer feed conveyor. The dryer feed conveyor drops a uniformly thick layer of cotton onto an endless metal screen conveyor which carries the cotton through the dryer tunnel. Circulating hot air, maintained at about 95º C. reduces the moisture content of the cotton to between 0.5 and 2.5%. At the discharge end of the dryer, the cotton is carefully weighed into tared containers, The containers are placed on a chain conveyor, and transported to the dipping floor of the nitrating house.

Wood pulp is received in rolls about36″ in diameter, consisting of a continuous sheet about 30″ wide by 0.045″ thick, and weighing about 600 pounds. The rolls of wood pulp are transported to the feed end of the wood pulp dryer as needed, stripped of paper wrapping, end discs and plugs. A steel mandrel is fastened in the core of the hoist. Usually three sheets are dried simultaneously, each making three com­plete passes from one end of the dryer to the other. Circu­lating hot air, maintained at about 165º C. reduces the mois­ture content of the woodpulp sheet to less than 2.5%. On leaving the dryer, the three pulp sheets pass under a common idler roll and are fed as a unit to a Hercules shredder by means of a power driven feed roll. The shredder shreds the pulp sheet to a fluffy mass of particles about 3/4″ long by 45″ wide by 0.045″ thick. The shredded particles are blown through a discharge chute to a pneumatic conveying system which discharges to a cyclone collecting bin. Shredded wood-pulp is raked from the collecting bin and carefully weighed into tared containers, each holding one dipping pot charge of cellulose. H.G. containers on linters hold two charges., The containers are placed on a chain conveyor, and trans­ported to the dipping floor of the nitrating house.

B. Acid Supply

Acid Supply has two functions: first, nitrating acids of compositions corresponding to predetermined stan­dards are prepared for the dipping operation, and secondly, the mixed acid is delivered to the dipping pots at the op­timum physical condition for nitrating.

Nitrating acids for the manufacture of nitrocellulose at Indiana Ordnance Works are prepared from spent nitrating acids and fortifying acids. Fortifying acids include sulfuric acids, 66º Baume and/or Oleum, F-83, an acid high in nitric and low in sulfuric, and weak nitric acid (45-48%) from the fume recovery system. Sulfuric acid and F-83 acid are received from the Acid Manufacturing Area; spent nitra­ting acids and cyclone acid are recovered from the Nitration Operation as described in subsequent sections.

To make a mix, a desired amount of spent nitrating acid is pumped from the spent acid storage tanks into scale tanks. After this spent acid has been agitated, a sample is taken and sent to the laboratory for analysis. From this analysis, the amounts of fortifying acids to be added are determined by the laboratory. The exact amounts by weight of fortifying acids are then taken and sent to the laboratory for analysis. If the analysis of this acid is within speci­fications, it is approved by the laboratory for use in ni­tration. The “mix” is then pumped to a mixed acid lag storage tank. If the “mix” is not within specifications, further fortification is made, as outlined above, until an acid con­forming to specification is obtained.

Mixed acid of the proper composition is pumped from the lag storage tanks to an agitator equipped acid warming tank, where the temperature is adjusted to prescribed speci­fications.

When the desired temperature has been reached and the acid is ready for use, it is pumped continuously through the header supplying the measuring tanks for the dipping operation. Excess acid flows through a return header to the warming tanks.

Four types of mixed acid are produced for use at Indiana Ordnance Works: High Grade Linters Mix (used in the production of high grade nitrocotton), Pyro Linters Mix for pyro nitrocotton, High Grade Woodpulp Mix for high grade nitropulp, and Pyro Woodpulp Mix for pyro nitropulp. These mixes differ only in the relative amounts of nitric and sulfuric acids that are found in each. The procedures used for each type of mixed acid are identical except as to the amounts of acid used and circulation temperatures.

C. Dipping

Dipping is the immersion of purified and dried cotton linters or woodpulp in mixed acid with agitation for a speci­fied period of time. The purpose of dipping is the complete and uniform nitration of cellulose to two grades of nitrocellulose, “pyro” and “high grade.” The manner in which the dipping operation is conducted largely determines the working properties of the resulting nitrocellulose.

Mixed acid of the specified composition and tempera­ture is continually circulated between the dipping floor and warming tanks. Fiber cans containing the dipping pot charges of cellulose are delivered continuously on a monorail con­veyor. A full container is removed from the conveyor, and placed on a rack adjoining a dipping pot charging port. A volume of mixed acid corresponding to exactly one dipping pot charge is drawn into a measuring tank. The high hat cover on the charging port is removed and the flow by gravity of the mixed acid is started to the dipping pot. Meanwhile, high speed agitation is started and a full charge of cellulose is rapidly forked through the high hat opening into the dipping pot. The high hat cover is then replaced. The acid inlet valve to the dipping pot is subsequently closed. After a period of digestion on high speed, agitation is reduced to low speed. When the total period of digestion has been com­pleted, the contents of the dipping pot are discharged by gravity into a centrifuge where the nitrocellulose is separated from most of the spent nitrating acid. In the meantime, the empty container is placed on the conveyor for refilling.

The operating procedure is the same for the manu­facture of both grades of nitrocellulose. The difference in nitrogen content is effected by the differences in acid compositions, temperatures, cellulose charge weights and immersion times for the types of nitrocellulose.

The two grades of nitrocellulose are kept separate until the blending operation.

D. Wringing and Drowning

The purpose of wringing is the separation by centrifugal action of the spent nitrating acid from nitro­cellulose. The primary purpose of drowning is the dilu­tion with water of the spent acid retained in the wrung nitrocellulose. Dilution of the retained acid reduces and decelerates chemical reactions of the nitrocellulose and the acid, such as hydrolysis and oxidation. A secondary purpose of drowning is to put the wrung N/C in a convenient form for transfer to the stabilization operation.

The wringing and the drowning are carried out as follows: An empty wringer is allowed to accelerate on low speed until a speed of 180-300 rpm is reached. Charging is then started by opening the discharge valve between the dipping pot and wringer. As soon as the dippingpot is empty, the discharge valve is closed. The loaded wringer is then placed on high speed for the remainder of the wring­ing time. This speed allows uniform basket loading and tends to force only a minimum of nitrocellulose into or through the perforations in the basket.

After the wringing time is completed, the control switch is turned off, the brake applied, and the wringer stopped. An operator dons a face mask and rubber gloves, picks up a fork, and steps on the treadle board to start the water flow through the immersion basin. The wringer lids are set aside, the valve plate is raised, and the flow of water to the immersion basin is checked by looking into the immersion basin and listening for the sound of running water. The charge is then forked out of the wringer basket into the immersion basin. The wringer is turned on low speed for approximately 5 seconds after which the wringer basket is scraped to remove small pieces of nitrocellulose. The valve plate is then lowered, the lids replaced, and the treadle board catch released.

The nitrocellulose which has been forked out of the wringer drops into the immersion basin, where it is drowned and broken into small pieces by the action of a water sparger. The nitrocellulose – water slurry flows by gravity through a stainless steel trough to a large wooden tub in the basement of the building. From this tub the nitrocellulose water slurry is pumped to the stabilization operation.

E. Spent Acid Filtration

Acid recovered by wringing always contains en­trained particles of nitrocellulose. This nitrocellulose, if allowed to remain during storage, would increase the nitrosyl content of the acid at the expense of using nitric and sulfuric acids. In addition, the probability of acid tank fires would be increased. Therefore, all spent ni­trating acid is filtered as soon as it leaves the wringers.

The spent acid flows by gravity from the nitration wringers and enters the filter through the top. The filter contains a perforated filter basket which retains the en­trained N/C particles and allows the acid to flow through it. This spent acid is pumped to spent acid storage tanks in the Acid Mix Tank Farm for reuse. The pressure forcing the acid through the accumulated cake of nitrocellulose comes entirely from the pressure head of acid in the filter. When the resistance of the nitrocellulose cake becomes so great that a full pressure head of acid no longer produces an ade­quate flow of acid from the filter, the flow of acid from the wringer is diverted to another filter.

After a spent acid filter has become filled with nitrocellulose, it is drained to remove as much acid as possible, but enough acid remains with the nitrocellulose to make its recovery by displacement economical. Water is floated on the nitrocellulose – acid heel in a filter. Under the influence of this head, the entrained acid is forced from the filter. As very little intermixing of acid and water occurs, recovery at high acid concentration Is possible. As the acid reaches the bottom of the filter, it is pumped from there into the pyro filter that is re­ceiving spent acid from the nitrating process.

As the water – acid interface moves through the ni­trocellulose, denitration of the nitrocellulose occurs. How­ever, the amount of nitrocellulose retained by a filter is too small for the change in nitrogen content to be noticed elsewhere.

The nitrocellulose contained in the filter is trans­formed into a slurry by the addition of water through a high pressure sparger. It is then pumped to the drowning tubs in the nitration house from which it is subsequently sent to the boiling tub house.

F. Acid Fume Recovery

An exhaust system is installed on dipping pots, wringers, and spent acid filters to protect operating per­sonnel and equipment from acid fumes. Once acid fumes have been picked up by the air entering the exhaust system, the concentration must be substantially reduced prior to re­leasing the contaminated air to the atmosphere. This is the primary purpose of fume acid recovery. A secondary purpose is to remove the acid from the air at such a concentration that it may be economically reused.

The current operating practice at Indiana Ordnance Works, of a two cyclone fume recovery system (connected in series) is to spray a portion of the strong first cyclone acid into the main fume duct and the weak second cyclone acid to the exhaust duct fan and duct feeding the second cyclone.  As additional liquid volume is needed, steam condensate from the cellulose dryers is added to the second cyclone weak acid. Steam condensate is used to prevent chlorides from entering the system.  About 95% of the total acid recovered is collected prior to and at the first cyclone.  The remaining 5% is collected at the second cyclone.  Of the 95% recovered at the first cyclone, about 45% is removed in the main fume duct.

Nitric acid from various points along the main fume duct and from the first cyclone is collected in a hold-up tank at a concentration of 45% – 48% HNO3.  The acid which is not used as spray in the main duct is pumped to the Mixed Acid Tank Farm for use in fortification.

II. Purification

A. Stabilization

The purpose of this operation is to eliminate the impurities and unstable particles that have been formed during the nitrating process. This is accomplished in large wooden tubs known as Boiling Tubs.

Nitrocellulose boiling tubs at Indiana Ordnance Works  are filled by one of two procedures: the hot water process or the cold water process. In both processes, the filling pro­cedure is identical with the exception that water used in the hot water process is preheated to 850 C in an external wooden tank elevated 40′ above the ground. The actual fill­ing process is as follows: The nitrocellulose slurry is pumped from the drowning tank in the nitration house to the tub being filled in the boiling tub house. The water in the nitrocellulose slurry is continually drained as the tub is filled. When the nitrocellulose reaches a predetermined level, the nitrocellulose slurry is diverted to another boil­ing tub. Water, hot or cold, depending on the process being used, is then added to completely cover the drained nitro­cellulose. The entire contents of the tub are heated by steam to 850 C, and the water is then drained from the tub. This lowers the level of the nitrocellulose in the tub and more nitrocellulose slurry is pumped to the tub to refill it.

The tub is then filled with water. Steam is sub­sequently admitted to the tub. When the contents of the tub reach a temperature of 850 C, the Thermo-Compressor System is brought into use. This system enables the nitro­cellulose slurry to be more economically heated with steam by effecting a greater circulation of the steam in the tub. This increased circulation thus requires a smaller quantity Of steam to maintain the required boiling temperature.

The time of boiling is started as soon as the re­cording thermometer on the tub reaches 950 C. provided the temperature of the tub surpasses 980 C within an hour. As soon as the temperature reaches 980 C, the steam flow to the tub is regulated to maintain a temperature of 98-100º C.

The acid boil for High Grade Nitrocellulose is sixty hours. During this time acidity tests are taken at the end of 2, 10, 20, 30, 40, 50 and 60 hours of boiling. If during these acidity checks the acidity is higher than 0.40% (cal­culated as sulfuric acid), the tub is allowed to drain suf­ficiently to reduce the acidity within a range of 0.28 to 0.40%. The tub is then refilled with hot water to the predetermined level and boiling is continued. If the acidity is below .28%, a proportionate amount of mixed acid is pumped from the nitration house.

At the end of the sixty hour boil the steam is shut off and the tub is drained. It is then filled with hot water and put on boil for five hours. The above procedure is re­peated for a second five hour boil. After the second five hour boil, the tub is drained and filled with cold water and allowed to stand until it is sent to slurry tube* The sta­bilized nitrocellulose is subsequently pumped to the Pulping House.

The same filling and boiling procedure as outlined above is followed for pyro nitrocellulose with the exception of the initial boil, which is thirty hours with acidity tests taken after the 2, 10, 20 and 30 hour boils. Pyro nitro­cellulose also requires two five hour boils with a change of water between each boil.

B. Pulping

The purpose of pulping is to reduce the size of the nitrocellulose particles. This enhances the leaching of the undesirable nitric esters from the nitrocellulose in the poaching operation. Pulping also facilitates the blending of pyro and high-grade nitrocellulose in the blending process. Further, pulping increases the ether alcohol colloiding pro­perties of the nitrocellulose. Thus the colloiding process in First Stage Powder is benefited.

Pulping is the subdividing of the nitrocellulose in a number of Jordan engines. In general, this requires that a charge of high-grade nitrocotton pass through three Jordans (3 passes) while pyro nitrocotton receives two Jordan passes. Both grades of nitropulp normally receive two Jordan passes.

A slurry of stabilized nitrocellulose is pumped from the boiling tub house to the pulping house, where it is dis­charged on a Memphis Dewaterer for removal of excess water. The dewatered slurry passes into an initial slurry tank where it is agitated with sufficient sodium carbonate to give a slightly alkaline reaction to phenolphthalein. The slurry is then pumped to an overhead stuff tank above the first Jordan and flows by gravity through the first Jordan to a dewaterer from which it is pumped to the overhead stuff tank above the second Jordan. This operation is repeated until the nitrocellulose has passed through the necessary number of Jordans. The nitrocellulose flows from the last Jordan into a catch tank where it is diluted with “white water” from the savealls and then pumped to the Poaching House.

All water leaving the pulping operation and sub­sequent operations passes to large concrete circular basins (savealls) where particles of entrained nitrocellulose are settled out. Periodically, a saveall is drained, and the nitrocellulose is slurried with high pressure water and pumped to poaching for a complete poaching treatment and reuse.

C. Poaching

The purpose of the poaching treatment is to neu­tralize any acid liberated from nitrocellulose by pulping and to remove any soluble impurities from the nitrocellulose.

Nitrocellulose water slurry from the pulping house is pumped to a cypress tub which has been prepared for filling by proper adjustment of all valves and lines. Water is con­tinuously decanted and pumped to a saveall as the tub is fill­ing. When about 70% of the volume is filled with settled nitro­cellulose, the tub is completely charged. The decanter line is lifted, flow of slurry into the tub stopped, and agitation started.

After sufficient soda ash solution to give an al­kaline reaction to phenolphthalein has been added the tub is filled with hot water, and heated to above 95½º C by the injection of steam through a Venturi sparger. The temperature is maintained at 95½0 C for 4 hours. The steam is then shut off, the agitator stopped, and the tub contents allowed to settle one half hour. Water is then decanted from the tub to the 30%mark and the tub is refilled with hot water and put back on boil. This boiling procedure is repeated for one two-hour and two-one-hour boils.

When the last one hour boil is completed, the tub is decanted to the 30%mark, filled with cold water, and agitated for thirty minutes. The agitator is stopped and the and the slurry allowed to settle for one hour. The tub is again decanted to the 30% mark and refilled with cold water. This washing procedure is repeated until the speci­fied number of cold water washes have been given the tub charge. The tub is filled to the 115″ mark with cold water the seventh time. Agitation is resumed and after thorough mixing, samples for stability tests and final weight deter­mination are taken. From every tenth charge an additional sample is also taken for solubility in ether, alcohol, and acetone.

At Indiana Ordnance Works, in order to increase the production capacity the poacher houses are equipped with a number of Internal Screen Tubs for cold water washing. Whenever possible, charges are transferred at the end of the poaching boils to an Internal Screen Tub. After the transfer to the Internal Screen Tubs, the charge is drained to about half the tub volume. The tub is then refilled with cold water, and the agitator is started. The charge is agitated for 15 minutes and immediately drained to about one half its volume. The above process is repeated for the specified number of cold water washes. Speci­fied samples are taken after second and fourth tub refills.

After the poaching treatment has been completed, the charge is agitated in the tub until it is to be used in the screening operation.

III. Finishing

A.   Screening

The purpose of the screening operation is to separate from the nitrocellulose, any foreign material or portions of the product that have not been properly pulped in the Pulping House.

After final treatment in the poacher tub, nitro­cellulose, in slurry form, is discharged to either one or both vibrating Packer Screens. Nitrocellulose of the de­sired size passes through slots in the screen. This nitro­cellulose flows as a slurry into the top of a settling tank. Any foreign matter that is heavier than nitrocellulose sinks to the bottom of the tank. The nitrocellulose slurry flows into a dewaterer where its consistency is reduced. The slurry from the dewaterer is continuously pumped to the Blending House. Any nitrocellulose that fails to pass through the Packer Screen is recovered and returned to the Pulping House to be repulped.

B. Blending

The purpose of the blending operation is the mechanical mixing of slurries of high grade and pyro nitrocellulose to ob­tain the desired ether-alcohol solubility and nitrogen content required for the manufacture of smokeless powder. The time of agitation is controlled to obtain uniform mixing whereas the consistency is controlled for uniform operation during subse­quent wringing.

Nitrocellulose is pumped directly from the packer screen to a blender. The relative amounts of high grade and pyro nitrocellulose pumped to a blender are calculated in order that the final nitrogen content of the blend is 13.15± .05%. A blender is continuously decanted as it is being filled. When a blender has been filled, the consistency of the charge is adjusted to a predetermined level.

After the consistency has been adjusted, the blend is agitated for a predetermined period of time and sampled. The sample is sent to the laboratory for analysis. If the analysis shows that the blend has the desired chemical speci­fications, the nitrocellulose is ready for the final wringing operation. However, the failure of the blend to meet the desired nitrogen content requires that the blend be recalculated. This is necessary so that additional high grade and/or pyro nitrocellulose can be added to the blend to bring the nitrogen level within specifications. After the addition of the recalculated amounts of nitro­cellulose, the blend is agitated once more and again sampled. The above procedure is followed until the de­sired nitrogen content has been achieved. The approved blend is continually agitated as it is pumped to the final wringers through a recirculation line.

C. Final Wringing

The purpose of final wringing is to reduce the moisture content of the nitrocellulose to within speci­fied limits. Moisture limits are necessary (a) to re- duce the danger of nitrocellulose fires and (b) to pre­pare the nitrocellulose for complete dehydration.

The general principles of acid wringing may be applied to final wringing. During the wringing of a nitro­cellulose blend, nitrocellulose slurry is continuously being circulated between the given blending tank and the final wringers through a recirculating line. The slurry is discharged to individual wringers from this line. A wringer is first placed on low speed for a short period of time. The slurry valve is then opened and the wringer charged. When the wringer has been charged, the flow of slurry to the wringer is stopped and the wringer is placed on high speed. Upon completion of the total wringing time, the wringer is stopped and nitrocellulose is discharged from the wringer directly into a nitrocellulose car below. When a car is filled, it is sampled for moisture content and pushed into lag storage where it remains awaiting dehydration. The water removed during wringing is pumped to an empty blender where entrained nitrocellulose is settled out and used as a heel for another blend. The excess water that is decanted off flows to a high grade saveall tank.

The samples that are taken from the nitrocellulose cars after final wringing are sent to the Sample Dryer in the Nitrocellulose Area. The Sample Dryer also receives the following samples from various locations in the Nitro­cellulose Area:

1.    All solubility samples

2.    Final weight samples

3.    Nitrogen samples

4.    All stability samples

The desired tests on the above samples are either made on the nitrocellulose at the Sample Dryer or these samples are sent to the laboratories of the Technical Department for analysis.