Autoclaves have operating pressures between 10,000 psi and 40,000 psi, depending upon the chemistry of the process used. Because of these high pressures, the material used to make the autoclave is heat and pressure treated to provide a very dense matrix. One step in the process (of manufacturing the autoclave) is to take the whole autoclave and put it into an even larger autoclave (vessel) where the temperature is raised to around 3500 decrees C and the outer autoclave is pressurized with something like 60 atmospheres of argon. This step is done to force any voids, bubbles, or defects out of the structure.
The wall of a small
commercial autoclave would be around 3" thick. The cap to the autoclave
resembles the breech plug of a naval cannon. There is usually a very
special sealing arrangement required to withstand the high pressures developed
in an operating autoclave.
When the vessel
is put into service, the manufacturer performs a 'conditioning' run which allows
a crystalline coating to form on the inside of the vessel. This coating is
stable and prevents interaction between the vessel and the chemical solution
used in the vessel.
Natural quartz called
Lascus is mined from the earth and provides the raw material for the
process . The material is milled to a uniform size, approximately 1"
to 1.5" in diameter, cleaned, and etched. The solute used in the
autoclave is only partly water. Either a sodium hydroxide (NaOH) or sodium
carbonate (Na2CO3) solute is used. The quartz and the solute nearly fill the vessel
when it is closed. This is one of the reasons high pressures developed as water
is basically incompressible.
Heating of the
autoclave is accomplished by means of electric heaters fastened to the outside
of the vessel. There are heaters along the length of the vessel with a higher
density at the bottom. The temperature of both the bottom dissolving zone
and the top growing zone has to be carefully controlled. An internal baffle is
situated between the two zones. This baffle allows the bottom temperature
and the top temperature to be significantly different and to help each zone have
an isothermal characteristic.
The baffle also
restricts the convection flow and channels the convection flow into a desirable
geometry. Around the autoclave and the heaters there is an exterior
insulation package. This insulates the autoclave and helps isolate it from
external temperature changes. There is usually an air space between the
insulation package and the vessel. There are also vents at strategic
places to allow the air flow to be controlled. Changing these vents and
the power delivered to the heaters determine the thermodynamic condition of the
autoclave. The vents may be automatically controlled by the process
The vents may be automatically controlled by the process computer.
The bottom zone is heated and the quartz lascus will dissolve. The solution in the bottom of the autoclave becomes a saturated solution. This means that it has dissolved the maximum amount of quartz that can be held in solution at temperature. The top zone is held at a slightly lower temperature. Consequently, the solution in the bottom of the vessel will convect upward through the baffle into the upper region where it will cool. The solution now has more quartz in solution that it can hold at the lower temperature. This is called super saturation. The dissolved quartz starts to come out of solution and builds upon the crystal structure of the seed plates. The process continues until the process is stopped or the quartz supply in the bottom of the vessel is consumed.
At the beginning of a run, the top and bottom temperatures are reversed. This causes the seed plates to dissolve quartz from the surface of the plate. This is done to eliminate the mechanical damage on the surface of the seed plate that has been caused by machining the plate to dimension. Once this is accomplished, the temperatures are reversed and the process of growing a stone on the seed plate progresses.
The main parameters that are monitored in the process is the temperature at various points on the vessel and the pressure of the vessel. These parameters are used to control the growth process. Target values for these parameters are varied during the process because at startup the majority of the quartz material is at the bottom of the autoclave. The top only contains the seed material. At the end of the process, most of the quartz in the bottom has dissolved and re-crystallized on the stones in the upper growth zone. As you can imagine, moving this much material around greatly affects the hydrothermal flow conditions and the thermodynamic state of the vessel. For this reason, the control algorithms are designed to make continual changes during the process to compensate for these changing conditions. It is also necessary to maintain very tight control of these parameters over the growing cycle. A sudden fluctuation in temperature can cause the crystalline structure to vary creating defects (seed veils) in the finished stone.
While x-rays have been used in the laboratory to determine the end of the
growing cycle, normally they are not used in a production operation.
The growth process is well understood and is allowed to proceed under very
controlled conditions. Often a computer is used to run the autoclave
during the 30 to 120 day growth cycle. (Time is a function of the size and
orientation of the stones being grown.) It is very possible to predict
quite exactly the point at which the growth process needs to be terminated.
Usually the grown stones will have less than 1/4 of an inch between them at the
end of the cycle.
While x-rays have been used in the laboratory to determine the end of the growing cycle, normally they are not used in a production operation. The growth process is well understood and is allowed to proceed under very controlled conditions. Often a computer is used to run the autoclave during the 30 to 120 day growth cycle. (Time is a function of the size and orientation of the stones being grown.) It is very possible to predict quite exactly the point at which the growth process needs to be terminated. Usually the grown stones will have less than 1/4 of an inch between them at the end of the cycle.
These stones do not look like a natural quartz crystal because of the shape of the seed upon which the crystal is grown. In nature the crystal will start with a single point and over a vary long period of time the classic quartz crystal shape will emerge. The seed used in a commercial autoclave has been carefully shaped to maximize the amount of material in a finished bar. The growth is controlled and stopped at the optimal point in the process. This causes the shape to be somewhat rectangular. If the commercial stone were allowed to grow, the facets on the end of the stone would continue to lengthen. Eventually they would meet and the resulting stone would look more like it's natural counter part. It will never look the same because of the difference in starting with a small point sized seed vs. a rectangular plate.
Seed plates with different shapes and crystallographic orientation have been designed so that stones can be grown for different end product requirements.
Commercial autoclaves range from 10" in diameter x 10-15' long to 1 meter (> 3 ') and nearly 12 meters (over 32 ') high. Companies that grow quartz may have as few as 20 autoclaves up to several hundred vessels.
Two similar processes are the most widely used in the cultured quartz industry. They are known as the low pressure and high pressure processes.
The high pressure process typically uses 1.0M sodium hydroxide solution, a growing temperature of 380 °C, a temperature difference of 25 °C, and a pressure of 1000-1500 bars. While the capital costs for such equipment are much greater, growth rates of up to 1.0 mm/day (Z) can be achieved at these conditions.
The low pressure process used at Sawyer Research Products, typically uses a 0.6 to 0.8M sodium carbonate solution, a growing chamber temperature of 345 °C, a temperature difference of 10 °C, and a pressure of 700-1000 bars. Typical growth rates are 0.4 mm/day in the Z-direction. The chief advantage is the lower capital expense of autoclaves which operate at this pressure.
The anisotropic structure of quartz results in large differences in growth rate in different crystallographic directions. Growth rate in the Z-direction can be up to three times the growth rate in the X-direction. The orientation of the seeds used can affect not only the growth rate of the crystals but also the uptake of impurities.
Copyright © 2001 thru 2013 by Theodore Lind