Saturday, August 6, 2016

Black Powder XI - Pressing

In our last post, we saw how the ingredients of black powder were ground down and mixed together. The next step of the process is to press the mixture down into cakes. We will study the pressing process in today's post. This is how the process was done in the nineteenth century.

The purpose of the pressing process is to make cakes of uniform density. This way, when the cakes are broken down into grains later, this ensures that the grains are of uniform density as well, which means that the rate of combustion will also be uniform between different batches of gunpowder. This is very important for accuracy because the gunpowder should fire with the same power, even if the powder is taken from different barrels.

After the ingredients of the powder are mixed together, it must be broken down to a meal powder before it is put into a press. This breaking down process was done in two ways: really large pieces were hit with wooden mallets to break them into smaller pieces and then the pieces were put into a "breaking-down" machine.

A typical "breaking-down" machine. Click on the image to enlarge. Public domain image.

The above image shows a "breaking-down" machine made by Hick, Hargreaves & Co., which was used by the Royal Gunpowder Factory at Waltham Abbey, England. The machine has two pairs of rollers (A and B) made of gun-metal. The upper pair of rollers are grooved and placed directly above the lower pair, which are smooth. The rollers revolve towards each other. The rollers are placed on sliding bearings connected by counter-weights (C). These exert a pressure of about 56 lbs. between the rollers. If any hard substance should get in between the rollers, they slide apart once the pressure exceeds 56 lbs. The rollers are fed from a hopper (D) by means of an endless canvas band with cross strips of leather sewn onto it at intervals of 4 inches. The endless band revolves around two tightening rollers (E), one of which is at the bottom of the hopper D and the other is at the top of the rollers B. The tension of this band can be adjusted by the screw F. The band transports the cake from the hopper to a point where it falls onto the first part of rollers and is broken down. The broken cake then falls onto a second pair of rollers, where it is crushed into a fine flour (meal powder) and then falls into wooden boxes underneath, from which it is transferred into a magazine ready for pressing. Any pieces that are too large to pass through the rollers fall down an incline G and slide into the box H at the bottom, where they can be collected and manually broken down with mallets and fed back into the machine. 

The next step is to press the powder into cakes of uniform density, using a pressing machine. There were three kinds of presses used: screw-presses, roller-presses and hydraulic presses.

Screw presses are simply where one plate is fixed and the other is attached to a screw. The screw is turned and the plates move closer to each other and press anything in between them. 

A screw press. Click on the image to enlarge. Public domain image.

Screw presses were invented by the Romans around the first century A.D. and were used in olive oil and wine production. Incidentally, Gutenberg also used a screw press when printing his Bible in the 15th century. While they were used in gunpowder production in the early days, they went out of use by the nineteenth century, because they could not handle large quantities of powder.

Roller presses were first introduced in France. Originally, they consisted of only two rollers, but later a third one was added. The basic concept can be understood with a simple diagram.

One roller is fixed about its axis and the other one is movable and can be adjusted to apply a given pressure. The material is placed between the rollers where it is compressed to a given density.

A Krupp roller press. Click on the image to enlarge. Public domain image.

The above image is a roller press made by F. Krupp, Grusonwerk of Buckau. It has three rollers carried in cast-iron side frames. The lower roller (C) is made of cast-iron and drives the middle roller (B) which rests on it. B is made of paper, while the top roller (A) is made of chilled cast-iron and rests on B. Roller A is the pressure-roller. An endless band (D) carried by the three rolls passes under the hopper E. The rollers A and B can move vertically in the side frames. The pressure applied by roller A can be adjusted by a weighted lever placed under the floor and acting on the shaft carrying A. The weights can be adjusted to apply any pressure up to 5 tons. As the band passes through the hopper, it carries a stream of powder through the rollers and carries it out to the far side in the form of cake. As the cake comes out of the rollers, its edges are trimmed by adjustable knives F. A worker ensures that the hopper is filled with moistened powder. The cake coming out of the rolls breaks off under its own weight and falls into a box placed below. The materials that are trimmed by the knives F are broken down with wooden mallets and returned back to the hopper. 

The most common presses used in the nineteenth century were hydraulic presses, mainly because these could be built to handle large volumes of powder.

A Hydraulic Press. Click on the image to enlarge. Public domain image.

The above image shows a press for the production of powder cake. A box (K) made of oak was used for pressing the powder. Three sides of the box were hung on strong brass hinges and would be turned down for filling, and then secured by suitable fastenings to form a strong box for pressing. To charge it, the box was removed from te press and brass or copper plates put in, held in place at the proper distance apart by brass distance-strips. The box was placed on its side so that the plates were vertical and about 800 lbs. of powder was rammed in between them with wooden rods. The distance-strips were then removed and the side (i.e. the top in the charging position) was firmly secured. It was then transferred back to the press table and aligned to the wooden pressing block. A scale painted on the wooden block served for measuring how much pressure was being applied by noting how far the block entered the pressing-box. This method of measuring the pressure was found to be more useful than reading a pressure gauge on the hydraulic cylinder because the resistance of the meal powder to compression depends on the amount of moisture it contains and the humidity of the air in the room. By using the scale, these variables could be properly adjusted by using different amounts of compression depending upon the time of year and the relative humidity of the atmosphere.  

However, presses of this kind went out of fashion later in the nineteenth century, because they were too dangerous. The cake often stuck firmly to the sides of box after pressing, and it was necessary to loosen it by hitting it with heavy mallets. The brass bound box was also heavy and difficult to manipulate and a better design was found to make cakes without using a box. The construction of an improved hydraulic press is shown below.

An improved hydraulic press. Click on the image to enlarge. Public domain image.

The head and bed plate of this press are made of cast-iron or steel. A truck running on rails receives the layers of powder and plates. In charging the press, first a plate is put on the carriage, then a wooden frame is placed on top and powder put into the frame and the top smoothed off using a flat lath and then a second plate is put on top. The frame is then lifted higher up and more powder is piled on and then another plate is put on top and so on, until a column consisting of layers of powder and pressing-plates is built up. The carriage is then run on the rails under the table of the press and the pressure is applied. At one time, the plates were made of brass, but these were later replaced by ebonite plates because ebonite plates do not bend out of shape as easily as brass, retain a flat surface better and also have sufficient elasticity to transmit the pressure evenly through the powder, even if the plates are not horizontal. 

An arrangement of valves and levers ensure that the press's accumulator is stopped when the maximum desired pressure is reached. 

A pressure between 375 to 450 pounds per square inch is applied, according to the fineness of the powder, the amount of moisture it contains, the humidity of the atmosphere etc. The compression process lasts about 30 to 40 minutes. The work is done slowly and the pressure is eased off and re-applied several times, in order to get greater density without applying excessive pressures. As it happens, the edges of the cake are always less compressed than the middle, because the powder layer can fall away around the edges. Therefore, the sides of the cakes were cut off at about 1inch thickness on each edge and the center sections were used for further processing. 

One problem about using ebonite plates in between is that they become easily electrified. In fact, the layers of ebonite and powder form an electric pile. Therefore, the presses were provided with an earth connection to prevent accumulation of static electricity. 

As soon as the pressing was finished, the water is let out and the ram is released, at which point the carriage is run out on to the rails, and then column of cakes is taken apart as the plates easily separate from each other.

Using this process made it possible to produce multiple batches of cakes, all at the same uniform density. The ideal average density for cakes produced by this process was somewhere between 1.7 and 1.8. The pressed cakes were then taken to the next stage of the process, granulation, which we will study in our next post.

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