Chrome reduction.
The rate at which the reaction
takes place was examined in order to optimise the time taken for reaction completion within the limits of pH which may arise within the system. To optimise this parameter plant design was based on centralised collection of strong acids, high chrome liquor and general swills. The latter constituted the bulk volume and formed the base load flow. Since the production of strong acid liquors and high chrome liquors were on a batch basis these were kept separats and metered into the swill flow to ensure as near consistent conditions as regards pH and chrome content as possible.
SO2 requirements were approximately 17 lb/hour and considerations were given to injection of liquid S02 аs opposed to the vaporisation and gas injection systems. Mechanically there is no problem associated with handling liquid gases but injectors, in a wet environment with SO2, are subject, to chemical attack. Plastic injectors were used oh the theory they were inert, but absorption of SO2 by the plastic created problems. Stainless steel injectors of low cost were finally installed.
LIME NEUTRALISATION.
The lime requirement for neutralisation of acids and precipitation of insolubles could maximise at around 180 lb of Ca(0H)2 per hour. To nautralise effectively lime was used as a slurry of approximately 10 per cent solids by weight and a special completely automatic mixing system was devisel which, by a system of control devices, ensured that lime was consistently mixed for continuous application to the acid stream.
LIQUID-SOLIDS SEPARATION.
After neutralisation with lime the liquid becomes a slurry and the plant at this stage was designed for the flow of 11 455 gal/hr containing in suspension: chromium hydroxide - 18,4 lb/hr calcium phosphate - 12,9 lb/hr calcium floride - 18,5 lb/hr
calcium sulphate - 99,6 lb/hr ferric hydroxide - 8,9 lb/hr together with residuals from lime which did not pass into solution. At this solids content the obvious first stage choice of primary solids separation was a thickener, a circular settling tank with a tank bottom scraping device and suitable sludge extraction pumps.
Solids settle to the bottom of the tank by gravity and water overflows via a peripheral weir from the tank. Under the conditions of precipitation the precipitated solids are of very fine particle size and as such don't settle freely within economically designed tank. Polyelectrolyte flocoulent was used to flocculate the solids completely so that they settled in a fully flocculated state at a sufficiently rapid rate to give good thickener overflow clarity. For possible re-use as general swill water this overflow is passed over sand filters for polishing. The thickened sludge produced by settlement is removed from the thickener tanks by the sludge pumps. It is then passed to a rotary vacuum filter where the liquid phase is extracted by vacuum means. The residual cake produced or the filter medium is removed by flow in the form of a paste suitable for disposal. The moisture content of the cake is in the range 75-80% .
CONTROL. Two basic control parameters were requirements of this plant. One was that the final effluent from the plant would be at pH in the range 7-10 and the other was that no hexavalent chromium would pass out of the circuit. Consideration was given to supply special system utilising large underground tanks already installed. This system was introduced in two sections in series, one to ensure that all chrome was effectively reduced and the other to ensure that the pH was within the limits.
AUTOMATION. The overall basic requirement of this plant was that it should, where possible, be of a automatic nature with no special labour fully deployed to its operation.