In-Plant Testing of CrossFlow Separator in Coal Industry Essay Example

ports through the bottom of the separator.

As with any processing equipment, there are inherent inefficiencies associated with this design. The key operating variables that were identified as problematic with traditional hydraulic separators included: (i) turbulent feed distribution which can result in unwanted misplaced particles, (ii) limited throughput capacity due to the detrimental impact of feed water on separator performance, (iii) introduction of dead zones within the fluidization chamber caused by frequent blockage/plugging of the lateral pipes located in the base of the separation zone containing the elutriation water and (iv) maintenance of the blocked elutriation water pipes. To overcome these problems, an industry driven research program was initiated to develop a new family of innovative high-efficiency hydraulic separators that can be readily implemented in the commercial sector, called the CrossFlow Separator and HydroFloat Separator. Figure 1.1 is a schematic drawing comparing the traditional hydraulics separator with the new CrossFlow separator.

Advantages of the CrossFlow separator over traditional units Existing hydraulic separators utilize a feed injection system which discharges through a downcomer approximately one-third of the way into the main separation chamber. The pipe discharge is usually equipped with a dispersion plate to laterally deflect the feed slurry, but this approach creates turbulence within the separator that is detrimental to both the quiescent flow of the unit and the overall separation process. The additional water added to the system at the injection point causes a secondary interface of fluidized solids to form within the separator. The CrossFlow separator minimizes this discontinuity by introducing the feed stream across the top of the separator. A transition box and a baffle plate are used to reduce

the feed velocity and optimize the tangential feed introduction into the top of the separator.

In addition, the water that is injected with the feed solids must also report to the overflow launder. As a result, the rise velocity of the water is substantially increased at the feed injection point. The throughput capacity of existing hydraulic separators is limited by this introduction of water through the feed distribution pipe in the separation chamber and the excessive elutriation water added to the system. As previously mentioned, part of this problem was alleviated through the tangential feed distribution designed for the CrossFlow separator. A redesign of the elutriation water distribution, through use of a slotted plate at the base of the separation chamber, has minimized the amount of water used by allowing the water to better disperse through the separator. Larger diameter holes spread farther apart (6 inches versus 0.5 inches) allows for the water to be introduced into the chamber, and the baffle plate disperses the water throughout the chamber. This ultimately reduces the amount of overall elutriation water required, and increasing the throughput capacity of the separator.

The improved distribution of elutriation water also minimizes dead zones within the separation chamber that were often caused by plugging of the small diameter holes in the lateral pipes at the bottom of the separation chamber. By increasing the diameter of the holes and adding the baffle plate to fully distribute the water, separation efficiency has increased due to full utilization of the separation chamber. The increase in separation efficiency and throughput capacity reduces the operating demands in terms of power, water and maintenance when reported

on a per ton of concentrate basis when compared to traditional hydraulic separators.

Inefficiencies of the CrossFlow While the CrossFlow separator is a significant improvement over conventional hydraulic separators, the unit does have a few limitations. One of the significant limitations is that the unit requires a narrow particle size distribution for effective separation. Previous testing has proven that efficient concentration can only be achieved if the particles are in the size range of 200 mesh to several millimeters. The particle size ratio needs to be less than a 4:1 (top size to bottom size).

The other limitation of the CrossFlow separator is it requires a moderately large difference in particle densities. The separator often accumulates low density coarse particles at the top of the teeter bed, which are too light to penetrate the bed, but at the same time, too heavy to be carried by the rising water into the overflow. As a result, misplacement of low-density, coarse particles to the high-density underflow can occur. This inefficiency can be partially corrected by increasing the elutriation water, to try to carry the low density coarse particles into the overflow; however this can sometimes cause the fine, high-density particles to also report to the overflow instead of penetrating the teeter bed.

Project Justification While improvements in technology have assisted the U.S. mining industry in reducing its overall energy consumption, the industry still struggles to be as efficient as possible due to the current economic climate. It is difficult for mining companies to justify huge capital investment in energy efficient technology. However the incentive still exists to development cost effective equipment that will not

only reduce costs, but improve efficiencies as well. This is due to the fact that each ton of saleable ore or coal that is recovered through an improvement in plant efficiency adds the full market price of that ton of material to the company revenue. Otherwise, the full market value is lost to waste. For a typical coal preparation plant, a one percentage point improvement in plant efficiency is roughly equivalent to a 20 percent improvement in profitability for the overall mine. As a result, these optimization strategies become very attractive for industry representatives.

The implementation of the CrossFlow hydraulic separator will significantly reduce energy consumption and improve efficiency in the coal industry. When compared to conventional technology, the CrossFlow separator processing more material (as high as 40% solids) and operates at lower pressures (atmospheric versus 20 psig) for sizing the fine coal streams. These differences reduce the pumping requirements and minimize wear. For a typical unit, the overall savings is estimated to be 5.8 BTU per year per unit based on 3.5 million tons per year of raw coal feed to a typical preparation plant. In addition to reduction in pumping costs, the reduction in water consumption and reagent dosage associated with the higher percent solids will continue to reduce costs when compared to conventional units. Overall maintenance costs per ton of product will also be reduced.

The improved efficiency of the CrossFlow unit yields a sharper cut point, which ultimately produces additional clean coal for the same amount of raw coal processed by minimizing (i) the amount of coarse low density coal that is lost to fines and (ii) the amount

of high density slimes that report to the clean coal product. As a result, coal reserves will be better utilized, productivity will be increased, and waste requirements will be reduced. These factors will allow operations to be more profitable and more competitive in domestic and international markets.

The technology is also expected to have a significant impact on the heavy mineral sands industry. The mineral sands industry currently suffers from the use of low-efficiency operations that require many stages of recleaning to achieve the required market grade. The process is considered to be very energy intensive with high operating costs. Fortunately, through the development of the CrossFlow separator, it is projected the industry can improve metallurgical efficiency tremendously during the pre-concentration step, which in turn would substantially lower the tonnage of ore that must be reprocessed in subsequent polishing stages. This would ultimately make the process more profitable by increasing performance and reducing operating costs (i.e., electrical power, diesel fuel, process water, etc.).

Hydraulic Classifiers There are three main characteristics that distinguish a hydraulic classifier from other classifiers. First, discharge of the oversize material from the device depends upon its gravitational flow properties and not mechanical means such as a screw or rake. Coarse particles settle at a rate faster than the upward current of the elutriation water, and exit the unit through a valve or spigot at the base of the unit. The second distinctive characteristic of a hydraulic classifier is the unit is not fed under pressure; the primary source of classification is based on differential particle settling rates to segregate particles according to shape, size, and/or density. And finally, hydraulic

classifiers utilize at least one, and sometimes both, of the following two mechanisms:

Hindered Settling - An oversized particle settles against upward flowing fluid; the greater the density of the fluid, the larger the particle that will remain suspended (or teetered) in the fluid. Hindered settling is a function of particle size, density and concentration, liquid density and viscosity as well as the charge density. ii) Elutriation - An undersize particle is lifted by an upward flowing stream of water; the greater the upward velocity, the larger the particle that will be lifted. (NC State, No. 92-24-P)

Hydraulic classifiers are frequently used in the minerals processing industry to classify fine particle according to size and when the feed size distribution is within acceptable limits, these units can be used for the concentration of particles based on differences in density. Over the years various units have been developed and can be primarily categorized by the method in which the coarse material is discharged from the separation zone of the unit (Heiskanen, 1993). The two main operational categories are: (i) classifiers that operate with free and/or hindered settling that have virtually no control of the underflow (or coarse fraction) discharge and (ii) classifiers that do attempt to control the underflow discharge causing the formation of a teeter bed. Classifiers that do not attempt to control the underflow discharge can be further subdivided into mechanical and non-mechanical categories.

Mechanical Hydraulic Classifiers The Hukki Cone Classifier is a mechanical classifier invented by R.T. Hukki in 1967 and consists of a cylindrical tank where feed is introduced into the tank on a slowly rotating distribution disk, which

causes a slight centrifugal action to it. The bottom of the tank is conical shape where water sprays are used as elutriation water. Coarse material is discharged through a pinch valve in the bottom of the cone. The key to this unit is in the conical section; where a ring of vertical, radial vanes are located to allow the pulp to laminarly rise upwards. The unit was originally designed to treat low quality sands, but is not used in practice today.

The Sogreah Lavodune Classifier is another mechanical classifier that consists of a cylindrical tank and a cone. Lower density counter-current classification is enhanced by laminar flow in this unit. A downcomer introduces feed material into the unit approximately one third of the distance from the top of the unit. The volume of the unit is restricted in the cone section where classification takes place in high suspension densities. The fine material rises over the overflow of the unit. A plunger in the base of the unit is used to regulate the discharge rate through the bottom of the unit. As with the Hukki cone, this unit is not used in industry today.

Non-Mechanical Hydraulic Classifiers Linatex classifiers have been in the industry for several years, in a variety of applications. The Linatex S Classifier is the company’s version of a non-mechanical dense flow hydraulic classifier. The pulp is fed by a downcomer into the column where it comes in contact with a deflector plate that causes the flow to turn radially outwards and upwards. The ratio of water between underflow and feed streams controls the upward current at the deflector plate and

thus the cut size (Heiskanen, 1993). The unit is very inefficient for sharp separations as it inherently bypasses a large volume of material. It is best utilized for slimes removal.

The Krebs C-H Whirlsizer is another type of non-mechanical dense flow hydraulic classifier. It uses a controlled water addition to a gently swirling pulp to clean the coarse fraction from fines (Heiskanen, 1993). The upper part of the unit is cylindrical in shape, with the lower unit forming a cone as in many of the other units described thus far. The lowermost section of the cylinder contains an internal cone that forces coarse particles into the narrow gap between the wall and the cone. Elutriation water is added below this from small holes, moving the pulp in a swirling action. While no teeter bed is formed, classification takes place by means of hindered settling, allowing the coarse material to settle past the internal cone and the fines to overflow through the top of the unit. It is designed for sand classification and targets the non-spherical materials such as vermiculite, mica and kyanite (Heiskanen, 1993).

Fluidized Bed Hydraulic Classifiers A simplified diagram of a fluidized bed hydraulic classifier is shown in Figure 1.2. The traditional design of a fluidized bed hydraulic classifier consists of an open top vessel into which elutriation water is introduced through a series of distribution pipes evenly spaced across the base of the device. During operation, feed solids are injected into the upper section of the separator and are permitted to settle. The elutriation fluid in a fluidized bed supports the weight of the particles within the bed by flowing

between the particles. The small interstices within the bed create high interstitial liquid velocities that resist the penetration of the slow settling particles. As a result, small particles accumulate in the upper section of the separator and are eventually carried over the top of the device into a collection launder. Large particles, which settle at a rate faster than the upward current of rising water, eventually pass through the fluidized bed and are discharged out one or more restricted ports through the bottom of the separator.

One of the first hydraulic classifiers to utilize a teeter bed was the Stokes unit which was developed to sort the feed to gravity concentrators. Each teeter chamber is provided at its bottom with a supply of water under constant head which is used for maintaining a teetering condition in the solids that find their way down against the interstitial rising flow of water (Wills, 1992). Each chamber is fitted with its own pressure sensor that monitors the conditions in the chamber and automatically adjusts the discharge to maintain a balanced pressure caused by the teeter bed. A valve at the base of each compartment can be hydraulically or electrically operated to adjust the height of the teeter-bed. As the bed level increases, the pressure will also increase and the valve will open, when the pressure decreases, the valve will close, keeping a constant level and therefore, density within the separator.

A more recent hydraulic classifier utilizing the teeter bed is the Linatex Hydrosizer. The Linatex Hydrosizer is a non-mechanical, hindered-settling classifier that maintains a fluidized teeter bed, but does not have the same elutriation water distribution

or feed distribution as the CrossFlow separator. The pulp is fed into a central feed column where it comes in contact with a deflector plate that causes the flow to turn radially outwards and upwards. Extensive testing of a pilot-scale unit at a North Carolina phosphate plant was conducted in the early 1990’s to attrition scrub and deslime flotation feed with promising results. Additional testing has been conducted at other mineral industries including mineral sands and aggregates. The Linatex Hydrosizer was marketed for sizing applications range from 28-mesh to 100-mesh, with some preliminary testing on finer material (NC State, No. 92-24-P).

Phoenix Process Equipment has developed another type of fluidized bed hydraulic classifier called the Hydrosort. This separator and classifier is currently utilized in the aggregate industry, as well as some others, for separating light, harmful contaminants, such as lignite and wood, in sand washing, and for fractional sand classifications (Phoenix Process Equipment, 2003). The Hydrosort incorporates a fluidized bed created by an upward current of water flow to classify product or separate impurities in the same fashion as the Linatex Hydrosizer. Phoenix Equipment emphasizes the units clog free classifier bottom, which distributes the upward water flow equally over the separating area. Unlike in the CrossFlow where feed enters the unit tangentially, both the Phoenix Hydrosort and the Linatex Hydrosizer have a feed distribution pipe that entered the top of the unit and discharges feed into the separation chamber.

The Floatex fluidized-bed classifier (or Floatex Density Separator) is the most recent hydraulic separator designed, utilizing a teeter bed which is formed by solids settling against an upward current of elutriation water. Coarse material

settles through the teeter-bed, while finer particles report to the overflow of the unit. A differential pressure cell and discharge valve controls the bed level in the unit. This efficient unit sees very little fines bypassed to the underflow and as a result, the unit produces a relatively clean underflow. Prior to the development of the CrossFlow separator, the Floatex separator was considered to be the most advanced commercial separator for hydraulic particle classification for material whose size was between what would be considered optimal for either screens (coarse) or hydrocyclones (fine).

Hindered Settling Hindered settling is a key operating parameter in all of the aforementioned hydraulic classifiers. Hindered settling considers the interaction of other particles in classification systems either on a particle-particle level or from the behavior of the particle assemblies. The interactions between two particles may be due to particles settling close to each other or to the wake effect of a larger particle on the settling of a smaller particle (Heiskanen, 1993). According to Littler (1986), the hindered settling phenomenon begins to take place at approximately 20% solids by mass.

The cohesive force between two particles settling very close to one another is great enough for the particles to fall together and be treated as a single particle of greater size and lower density. A wake effect is caused when a larger particle captures a smaller particle in its wake as it is settling and as a result, the smaller particle falls at a velocity much higher than its free settling velocity. In a teeter bed however, the high solids concentration increases the likelihood of particle collision, and these particles

lose some of their settling velocity in these collisions. The fine particles, therefore, have a higher likelihood of being driven to the overflow launder by the upward current of elutriation water. And as a result, hindered settling is more efficient than free settling classification due to the decrease in fines entrained in the underflow.

An analysis of the behavior of particle assemblies can be categorized into two parts. Particle assemblies settling may occupy the whole fluid or they may be considered as clusters of particles which only fill a fractional volume of the fluid (Heiskanen, 1993). When the assemblies occupy the entire fluid they may be treated as a uniform pulp where the interactions are between the individual particles. As clusters, the particles are analyzed as large particles of reduced density and rigidity. The probability of this occurring increases with narrower particle size ranges, and is magnified in gravitational classification where high solids contents are present.

From an analysis standpoint, hydraulic classifiers are characterized by two factors: (i) the size separation and (ii) the sharpness of the separation. For theoretical analyses it is convenient to define separation size as that of particles which settle just fast enough on the average, to be totally collected in the underflow (Weiss, 1985). Slight variations in settling rates will occur between particles of the same size and density due to differences in shape and turbulence in the separator. The sharpness of the separation defines how the particles segregate into the product and the tails streams.

Under ideal conditions, a classifier should partition particles coarser than the cut size d50 into the coarse stream and finer particles

into the overflow (Heiskanen, 1993). The efficiency of this cut is based on the amount of misplaced particles in both streams.