On-line Submersible Clarity Analyser for clarifiers and settlers in alumina refineries
Alumina industry thickening and filtration technologies are evolving. The advent of large filtration capacity and new flocculants are changing the solid-liquid separation process.
PLA’s OSCA on-line submersible clarity meter / turbidity meter / suspended solids meter is installed to protect precipitation and existing security filter processes.
OSCA is a ruggedized automatic online submersible clarity analyser that provides real time measurement of fine suspended solids in high temperature and high scale liquor streams in clarifier and settler overflows.
There are many clarity / turbidity meters in the market that can handle high temperature. There are many clarity / turbidity meters that can handle scale, but there are none that can handle both high temperature and scale until now.
FEATURES AND BENEFITS
- Provides stable and repeatable readings at very low suspended solids (2 mgpl).
- Aids in the reduction of overflow solids events that contribute to performance downturns in security filtration by providing the operator with real time clarity readings to monitor the process.
- Contributes to the optimisation of filter aid addition and filter maintenance
- Features a rugged design for high process temperatures (130 deg C)
- Equipped with an automatic sensor scrubbing and water washing system that ensures the optical instrument is cleaned frequently so no scale build up and consequently drift in the measurement is experienced.
- Uses drift compensating ratio metric measurement technique
- Has no electronics inside the sensor itself
- OSCA clarity readings combined with SmartDiver interface and mud bed height measurements provide the operator with critical parameters to achieve the optimum thickener operation and control.
The main application for this instrument is in the Bayer Process used to refine bauxite ore into alumina. Specifically, at the decanter or clarifier where the quality of the overflow is critical to the alumina precipitation process and final quality of the product.
Sodium aluminate solution overflowing the primary clarifier still contains typically 50 to 200 milligrams of suspended solids per litre. Further separation of the pregnant liquor from the bauxite residue is performed utilising a series of security filters. The suspended solids in the liquor should be sufficiently low (<10 mpl) to provide an alumina product from the precipitation step which meets all of the industry standards.
OSCA clarity meter provides invaluable data to the operator that can be used to optimize the clarifier and settler operation, filter aid addition and filter maintenance.
With recent technology developments in design of high rate thickeners and synthetic flocculants, it is possible to achieve suspended solid levels in the clarifier over flow low enough to provide the opportunity to by-pass (partially or completely) the security filtration area. In this scenario, accurate and reliable measurement of suspended solids from instruments like OSCA will be critical.
OSCA was developed as part of the PLA “We’re Ready” ethos. We know that process enhancements such as security bypass and new innovative ultra-high clarity flocculants are in the early stages of implementation and we know that instruments will be required to keep these new processes honest.
In a standard alumina process, where the plant is co-dosing or using coagulants, there may also be an advantage in knowing the overflow clarity as the flocculant can be trimmed or the coagulant bought in and out based on the clarity reading from OSCA.
In addition, OSCA can be configured with any sensor technology and used to acquire measurements in the harshest environments minimizing the time the sensor is exposed to that environment. For example, a probe to measure pH can be deployed with OSCA in a slurry launder or in the last washer for neutralization control where scale rates are extreme and inline instruments won’t last. Simply fit whatever probe you want to use on the OSCA platform
HOW DOES OSCA WORK?
OSCA’s sensor is lowered into the tank by a pneumatic ram mounted on a frame. The mechanism will bring the sensor back up for a clean. Proximity sensors in the washing/scrubbing system will detect the presence of the sensor which will trigger a washing sequence started off timers through the PLC. An industrial bottle brush, with the aid of water, will scrub the optical instrument clean as it travels up and down. This reduces the requirement for manual soaks.
OSCA features an optical instrument that uses 90 deg light back scatter. The light is passed down through a fibre optic cable to the sensor and back so there is no on-board electronics making the system more reliable. Both transmitted light and scattered light at 90° are measured and mathematically combined using a ratio metric algorithm to calculate the clarity of the medium in NTU or mgpl.
For more information contact our technical team at firstname.lastname@example.org or call us on +61 3 9786 1711