Dorr agitators

The Dorr agitator (Coulson and Richardson, loc. cit.) consolidates in one unit the principles of the thickener and the Pachuca tank. Resembling a rake-equipped thickener, it differs in that the rake is driven by a hollow shaft through which the solids-liquid suspension is lifted and circulated by an air stream. The rake moves the pulp to the center, where it can be entrained by the air stream. The unit may be operated batchwise or continuously. 

Dorr Agitator.—This is an ordinary tank in the center of which is a vertical shaft to the bottom of which are attached two arms with rakes, so that when the shaft is revolved they will rake any solids which settle out towards the center. This shaft in the center is hollow and has openings at the bottom and top and it is made into an air lift by a compressed-air pipe leading in at the bottom. To the top of the shaft are attached two perforated launders which revolve with the shaft and any material coming up through the center of the air lift flows into them and will be distributed over the entire surface of the solution in the tank. 

If we place in a Dorr agitator a solid suspended in water the solids will of course tend to sink to the bottom as the arms revolve at such a low rate that there is practically no agitation by them. In sinking they pass through the solution and thus the film of concentrated solution is continually washed away. When they reach the bottom the rakes take them to the center of the tank and deliver them to the air lift which shoots them up to the launders where they are distributed over the surface of the solution and allowed to percolate through the liquid or solution again. Here... 

Pachuca.—This type of agitator, similar to the Dorr in. what it attempts to accomplish, has in recent years been almost completely replaced by other types. It consists of a very high tank with a conical bottom. Reaching from the top of the tank to the center of this cone is a pipe to be used as an air lift as in the Dorr, the idea being that as the particles sink they strike this conical bottom and slide... 

Continuous Counter-current Decantation.—This system is being quite generally used in the chemical world to replace the older intermittent agitation and decantation washing systems as well as more modern filter installations. It has as its basis the application of the Dorr thickener to remove suspended solids contin-... 

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