Materials handling systems, concentrate controls

Features common to all CVD reactors include source evaporators with an associated gas handling system to control input gases and gas-phase precursor concentrations, a reactor cell with a susceptor heated by either radio frequency or infrared radiation, and an exhaust system to remove waste products (which may include a vacuum pump for low-pressure operations). Substrate temperatures can vary from less than 200 °C to temperatures in excess of 1000 °C, depending on the nature of the material layer and precursor used. Schematic diagrams of some simple CVD reactors are shown in Figure 4. 

Concentration Control. Sequestration, solubilization, and buffering depend on the concentration control feature of chelation. Traces of metal ions are almost universally present in Hquid systems, often arising from the materials of the handling equipment if not introduced by the process materials. Despite very low concentrations, some trace metals produce undesirable effects such as coloration or instabiHty. 

Most of the systems that have been installed concentrate on the materials handling as much as on control of the mixing process itself. Early systems were controlled using punched cards which gave instructions on weighing and addition of raw materials and on dump criteria. Basically, these systems reduced the chance of human error. More recent systems do all of the above, but also include a programmable controller. One important advantage of these is that they can provide a printed record of each batch. Details of the... 

The pneumatic classification system should be designed to handle ha2ardous dust (28). A ha2ardous dust is one which, when finely divided and suspended in air in the proper concentration, bums, produces violent explosions, or is sufficiently toxic to be injurious to personnel health (see Air pollution control methods Powders, handling). At the least, almost any dust can be irritating to personnel because of inhalation or skin or eye contact. Fully oxidi2ed and hydrated materials are generally considered safe. 

Batch TDUs are typically only used for smaller applications because their throughputs are often much lower than other thermal desorption systems. These systems do have the benefit of requiring less pretreatment of the contaminated media because feedstock consistency and particle size are not as crucial to control as they are in continuous feed units. Unlike continuous feed units, it is easy to vary the residence time and/or temperature between batches to handle different contaminant concentrations in the feedstock. Also, there is no material feeding equipment to clog with larger particles, although it is still not economical to treat extremely large particles in a batch process. However, particle sizes up to 15.2 cm have been treated effectively by batch units. ... 

Finding an economic method to continuously reuse 90 to 95 percent of an acid will (1) reduce incoming chemical consumption (for the factory), (2) reduce the cost to process parts (fewer rejects), (3) provide better quality control, and (4) reduce the chemicals (for example, caustic soda) required to neutralize the spent acid before discharge. Eliminating the need to handle spent acids will decrease the exposure of hazardous materials to employees and will reduce the wastewater treatment burden. This type of bath purification system may eliminate the need to dump selected concentrated wastes. 

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