Semi-continuously agitated systems

Most industrial precipitation units are simple in construction. The prime aim is usually to mix reacting fluids rapidly and to allow them to develop a precipitate of the desired physical and chemical characteristics. Batch operation is more often favoured for industrial precipitation than is the truly continuous mode, although the merits of semi-continuous operation deserve serious consideration, i.e. a continuous reactant mixing step from which the product stream passes to batch-operated agitated hold-up tanks to permit equilibration, phase transformation, ripening, etc. The hold-up tanks may be arranged in a sequenced battery or in a continuous cascade system (see section 9.1.2). 

The methods for both semi-batch and continuous reactors are generally restricted to the 02-aqueous system because of the limitations of the analytical technique for the direct measurement of liquid-phase concentration. Novel agitated vessels for measurement of transient absorption rates are described by Danckwerts and Kennedy (1954), Oishi et al. (1965) and Govindam and Quinn (1964). 

Additives are used in the developer solution to prevent foaming. The solution is filtered to remove resist particles and either replenished with fresh solution to maintain a consistent dissolved resist content and solution concentration or operated continuously for a certain amount of product and then replaced. Waste-developer solution is treated (aqueous and semi-aqueous) or distilled and reused (solvent). Rinsing is important in stopping the dissolution and, for aqueous photoresists, water with a high-mineral content often improves the resist image and the conductor yield. Tank systems can also be used with photoresists that have a wide-process latitude. Ultrasonic agitation is often used to aid in the dissolution. 

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