LIME SLAKING SYSTEMS
LIME SLAKING
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Lime slaking is the process of converting quicklime to hydrated lime (CaO + H20 -> Ca(OH)2) by mixing with water. Lime slaking is an exothermic process which generates heat and creates steam. Slaked or hydrated lime is often used in mining and process industries for raising pH or neutralising acidic solutions.
Hydrated lime can also be purchased in dry powder form, allowing it to be simply mixed into solution without slaking on site. The cost per tonne of hydrated lime is typically higher than quicklime and, as such, it is often only economical at lower usage rates. Conversely, with high usage, it is often more cost effective to install a lime slaking system, despite the higher upfront capital cost.
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BHT lime slaker systems are designed and manufactured to ensure long life and reliable service in the harshest of operating environments. All components are designed to allow easy access for cleaning and maintenance, with minimal operator involvement required between service intervals.
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Typical features and benefits include:
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A tailored solution, engineered to suit customer specific site requirements,
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Compliance to customer engineering and manufacturing specifications,
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A consultative design process utilising the latest industry standards and guidelines for process control, Australian Standards and BHT proprietary designs,
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Provision of 3D models to allow convenient integration into overall plant layouts, ensuring accurate interface and tie-in,
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Operator ergonomic considerations when handling the dry quicklime and equipment for maintenance.
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Option for full turn key solution
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Quicklime Slaking/ Grinding - Benefits and Features
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Lime slaking systems comprise multiple components, each specifically designed to form part of the lime slaking system. Correct design ensures the reliable production of a MOL slurry at the specified rate and with the required concentration of solids.
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Typical components of a slaking lime plant include:
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Dry quicklime receival and storage; bulk bag breaker or silo with dust collector and level indication,
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Heavy duty screw feeders or rotary valves to feed the dry quicklime
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Suitably sized vibrating conveyors
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Stainless steel vortex feed bowl with flow control valves and instruments.
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Roller mounted grinding ball mill or detention type slaker
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Robust rubber lined slurry pumps, including transfer and ring main dosing pumps
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Hydro-cyclone or separation tanks on closed circuit grinding systems
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Suitably sized wet scrubber for steam extraction
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Mix tank, with agitator and level sensor,
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Agitated MOL Storage Tank
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Access platforms
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Control system with integration into plant DCS
Lime Slaking Plant Design Focus
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Process Control
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With lime slaking, the two key inputs to be controlled are water and quicklime addition. These ingredients must be added in the correct ratio to meet the required system production rate and also ensure optimum conditions for slaking of the quicklime.
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The quicklime feed rate is typically volumetrically controlled using a screw feeder or rotary valve, while a flow meter and modulating flow control valve will maintain the required water flow rate.
It is important to ensure that water and quicklime are added in the correct ratio as:
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Too much water can 'drown' the reaction, preventing the slurry from reaching the optimum temperature for slaking. This will slow the slaking reaction and can lead to unslaked lime presenting to the storage tank.
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Too little water will result in insufficient water to slake the quicklime and potentially causing build-up in feed pipes and chutes as the mixture becomes too thick and cohesive to flow through the system.
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Following slaking, water is then added for dilution to achieve the required final solids content of the slurry.
Ball Mill
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The ball mill has two primary functions within the quicklime slaking system; firstly the reduction of quicklime / hydrated lime into smaller particles critical to the correct reaction with process, and secondly provide a rotating vessel to allow majority for the chemical reaction to occur. The two processes enhance each others functions with the grinding increase the particle surface area for better chemical reaction while the chemical reaction breaks down the particles to small sizes.
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Ball mills supplied by BHT are typically roller mounted mills varying from 0.5m to 2.2 meters diameter. If diameters larger than this are required, trunnion pinion mills can be supplied from 3rd party vendors as part of the package but this increases the pricing greatly and increases delivery period, hence it maybe more effective to supply multiple stream system with smaller mills and allowing redundancy. Dependant on the size of mill and throughput required, BHT supply mills with rollers manufactured from rubber tyres or polyurethane rollers.
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The lifters and liners set of the mill are manufactured from specialised rubber suited to the chemical situation and the temperature produced within. These lifters are held in position using forged T-bolts through the shell of the ball mill.
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Typically ball mills are supplied with automatic greasing system to ensure that the bearings are constantly purged and free of any contaminates.
Hydrocyclone
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The hydrocyclone is responsible for classifying the product discharged from the ball mill to ensure that the particle requested for the process is meet. Oversized material is rejected and funneled back to the inlet of he ball mill for regrinding, the undersize is discharged to the storage tank for use. The cyclone allows the mill size to be reduced by limiting the amount of grinding needed on already correct size particles.
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The typical request particle size, p80, for MOL of is usually 75 microns but finer can be achieved. Do note that decreasing the microns will increase the mill size exponentially.