Objective
To demonstrate the overall thermal improvement to an alumina digestion process with the introduction of an injection heater utilising regenerative steam for pre-heating. The feed slurry has high scaling properties and is very abrasive.
The technique involves the use of regenerative steam to heat the slurry in a specially developed injection heater constructed from hardened, ductile steel and stainless steel with "Spray Stelliting" for enhanced wear resistance.
The overall thermal input to the plant will be reduced by 7,902 TOE/Annum; also, 144,452 tonnes of treated water should be saved annually.
With present systems the Bauxite slurry is pumped into the digester relatively cold. Due to the high scaling and abrasive nature of the standard method of pre-heating using in-line injection has not been successful. The standard injection system attempted consisted of a series of holes around the circumference of the slurry flow pipe. Erosion and blockages gave serious problems resulting in poor mixing and heat transfer.
The new injection system has been previously used to heat water and slurries in the chemical and paper pulp industries, however, these applications are at low temperatures, below 120 deg. C. To cater for the higher temperatures required for this application a special injection heater has been designed using hardened, ductile steel (hardened to 450 Brinell) for the main body. The stem and nozzle will be machined from stainless steel with "spray stelliting" for enhanced wear resistance. The regenerative steam enters at the top of the heater body through a 300 mm diameter inlet port and turns 90 deg. C into the variable nozzle section. This section is adjustable by moving the centre stem towards the tapered nozzle. The slurry enters at the bottom, down stream of the nozzle, into a chamber containing a centrally mounted combining tube for mixing.
The steam from the nozzle is forced down the combining tube with the slurry entering around the circumference. The position of the combining tube is also adjustable to vary the entry area of the slurry.
The design incorporates a series of injection heaters ; Stage 1 raising the temperature of the slurry to 95 deg. C and Stage 2 to 145 deg. C.
The heat necessary to elevate 400 tonnes/hr of the Bauxite slurry from 90 deg. C to 145 deg. C is 12 million KCal/hr. Based on steam at 9.3 bar.a. the required steam flow from Flash Tank 6 is 22 tonnes/hr.
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Programme(s)
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Coordinator
CO LIMERICK
Ireland
The total costs incurred by this organisation to participate in the project, including direct and indirect costs. This amount is a subset of the overall project budget.