Introduction
The McLanahan Lites-Out™ Flat Bottom Classifier is a variation of a hindered settling classifier. It was developed, to separate low specific gravity (SG) material from higher specific gravity material. The lighter and smaller particles report to the overflow launder, while the coarser and heavier material discharges through a valve(s) on the bottom.
The lack of dewatering cone on the discharge is where the Flat Bottom Classifier gets its name. The flat-bottomed nature of the classifier allows the feed height of the plant to be lower to the ground, taking up less room. Some areas have height restrictions on industrial installations, and the Lites Out™ can provide an avenue to meet those limitations. Also, the lower head height decreases the pumping horsepower needed and structural requirements.
How the Flat Bottom Classifier differs from other hindered settling classifiers
There are a number ways the Lites Out™ operates differently from many hindered settling classifiers.
On/off underflow valve
The first is the on/off action of the underflow valve. The on/off nature handles sudden fluctuations in feed throughput, especially when the unit is underfed. Many hindered settling classifiers use a modulating valve, which may have a hard time maintaining consistent control in upset conditions. The on/off setup causes a pulse-type discharge to induce a jigging action in the teeter bed. This approach helps dilute the bed and frees the coarser organics, like lignite, through the fluidized bed.
Level sensor
Secondly, the dart valves or plungers are controlled by a sensor, which monitors the overflow of the unit. The sensor performs an indirect measure of the specific gravity of the overflow. By looking at the overflow, the valves can be opened in response to an increase in slurry specific gravity caused by the presence of any higher specific gravity material.
The more traditional approach uses a gauge to measure teeter bed height. This allows the teeter bed height to be used as process control point, while the Flat Bottom Classifier maintains a deeper bed depth to promote separation by specific gravity.
Diffusion plate and plenum chamber
Finally, instead of teeter bars, the Lites Out™ injects water through a diffusion plate from a plenum chamber. The water pressure is generally more equalized between the different teeter nozzles on the diffusion plate. The amount of teeter nozzles is also significantly increased. This can create a particularly stable teeter bed. The outlet ports extend down through the plenum chamber, and for the large units, there are multiple ports to prevent rat-holing.
Applications for Flat Bottom Classifiers
A hindered settling classifier provides the sharpest separation of all wet gravity separation equipment, and the Flat Bottom Classifier takes a slightly different approach. While it works on a same principle as all hindered settling classifiers, the differences described above fit well in several applications.
Organics removal
The application in which Lites-Out™ classifiers have the highest utilization is organics removal, such as in soils washing or lignite removal.
The significant difference in specific gravity between the sand particles and the organics allows the separation of coarse, light particles into the overflow with the finer sand (equivalent weight particulates).
For example, when trying to remove lignite from a C33 concrete sand, the lignite may have a specific gravity between 1.4 and 1.7, while the typical specific gravity for concrete sand is 2.65 or 2.7. The difference between the two minerals can be as great as 1.3.
If contaminated by lignite, the C33 sand is not accepted for use in several markets, as the presence of lignite in concrete will create voids that reduce its strength. Different regions use similar a standard, which is sometimes refer to a color test. The test gauges the visible amount of lignite in the sand.The use of the Flat Bottom Classifier has had a major impact on the market value of lignite-affected sand in regions where lignite is common.
The teeter water carries fine sand and the lignite up and over the weirs, while the cleaned sand is discharged though the bottom of the unit. The overflow can be run over a sieve bend (static, curved screen) to separate larger organics and retain the finer sands in the undersize.
In most cases, removing the larger organics is enough of an improvement to meet specifications, but a second stage treating the sieve bend underflow can further remove the lighter organic material from the sand. The second stage can be approached a number of ways, including the use of a second Flat Bottom Classifier. Since the size gradation is much narrower in this stream, the separation is more definitive.
Mineral separation
While the example of removing lignite from sand was reviewed in detail, any time there is a significant difference in specific gravity between minerals, a Lites-Out™ Classifier can perform a separation. The rule of thumb is if there is at least a 0.7 difference between the minerals, an effective separation can occur using a Flat Bottom Classifier. The greater the difference between the minerals, the cleaner the separation.
In the mineral sands industry, Flat Bottom Classifiers are used in conjunction with spirals to separate the sand from the higher specific gravity mineral sands. The higher density or coarser solids, which discharge from the underflow, can be gravity fed to the next process stage, or sent directly to Dewatering Screen(s) for final dewatering and conveying to stockpiles.
When dealing with some iron ore deposits, a Flat Bottom Classifier can be used to upgrade the iron ore by sending the gangue material to the overflow and substantially improving the concentration of the iron ore for further processing.
A Flat Bottom Classifier is part of a group of equipment specifically designed for separating materials with different specific gravity. In many applications, they work hand in hand with a variety of jigs and spirals. Because each of them attack the same the process in their own way, when put together, they can achieve improved results. Whether alone or combined with other equipment, a Flat Bottom Classifier can fill a needed role in a wet application.
