Column Flotation Parameter Testing
The batch column flotation results presented here were previously published. These data, however, were important in establishing the conditions for optimum continuous column flotation of the Fish Creek fluorite ore. Batch column flotation tests were conducted on column length, feed injection location, tailing recirculation, froth depth, wash water additions, and particle size fractions.
Since the inception of flotation columns in the early 1960’s, column length has been a concern to commercial mineral processing plants anticipating installation and operation of flotation columns. A column flotation cell is free from violent agitation. Feed and tailings slurry flow rates and particle settling rates affect the retention time of the particles in the column.
The collection zone has its upper boundary at the feed injection port and extends downward to the base of the column. This zone must have sufficient length to provide adequate retention time for the settling particles to attach to the rising bubbles. Column length design theory is based on this concept. Additional column length must be included for the upper three column zones as prescribed by the particular mineral system needs. Most work backing this theory has been performed on copper-molybdenum separations.
The effect of column length variations on the fluorite ore was studied by shortening the column, while maintaining a constant ratio of each column zone within physical limitations of the equipment and observing grade and recovery fluctuations. As the column flotation cell was shortened, recoveries of fluorite decreased (fig. B-1). Retention time was calculated for plug flow conditions based on the collection zone volume and tailings flow rate. Fluorite recovery decreased because particle retention time was not sufficient as the collection zone was shortened by decreasing the column length.
Fluorite grades increased with decreasing column length because only the particles with sufficient hydrophobicity to achieve bubble attachment were reported to the concentrate stream. As the column was shortened, the particle retention time decreased causing smaller fractions of the more liberated fluorite to be collected, while also reducing the amount of gangue that was either entrained or collected to the froth (fig. B-2).
Feed Injection Location
Tests were conducted to determine the effect of the vertical location of the feed slurry injection port on fluorite grade and recovery.
Fluorite recovery gradually decreased as the feed injection port was moved closer to the base of the column (fig. B-3). In essence, feed injection location is directly linked to particle retention time in the collection zone of the column. As the feed injection location was moved towards the base of the column, the length of the collection zone decreased, reducing the particle retention time, and decreased fluorite recovery resulted (fig. B-4).
Fluorite concentrate grades increased as feed injection location approached the base of the column. Lowering the vertical position of the feed slurry injection port is equivalent to shortening the length of the collection zone. This accounts for the similar response of fluorite recovery and grade to variations in either column length or feed slurry injection location. The increased fluorite grade observed as feed injection location moved towards the base of the column were due to the added concentration effects of the upper three column zones. Unlike the column length variation tests, the upper two zones remained at constant length, while the pulp phase cleaning zone increased in length as the feed injection location approached the column base.
Fluorite grade and recovery variations were investigated as a portion of the tailings stream was recirculated at different rates during column flotation. The tailings recirculation flow rates were converted to superficial pulp velocities based on plug flow conditions in the collection zone volume. The resultant trend is given in figure B-5.
Fluorite recovery suffered and the grade was enhanced as the recirculation velocity was increased. Since short circuiting of some of the feed slurry, caused by axial mixing, increases as recirculation velocity increases, these trends were expected. Furthermore, trending of data from this investigation versus superficial liquid velocity showed no correlation, therefore, the reduced recoveries and increased grades were concluded to result from the decrease in the particle retention time distribution. Under these conditions, only those particles with the strongest adsorption energies have sufficient time to attach to the rising bubbles.
These results show that the collection zone must be lengthened to provide sufficient particle retention time as the particle retention time distribution is broadened and to compensate for increase in pulp mixing.
A series of tests were conducted on the fluorite ore to determine the effect of froth depth on mineral grades and recoveries. Froth depth had a discernible effect on fluorite concentrate grades; as froth depth increased, fluorite grades increased (fig. B-6). Fluorite upgrading occurred in the froth phase cleaning and pulp-froth interfacial zones. The froth phase is more efficient than the pulp phase in upgrading the concentrate. At greater froth depths, more fluorite cleaning took place and higher grade concentrates were obtained.
Fluorite recovery data were scattered. No adequate trend could relate the recovery data with greater accuracy than a line with a slope of nearly zero. Although the source of these fluctuations could not be identified, no direct connection with froth depth variations could be made. It was concluded that froth depth had no primary correlation with fluorite recovery.
Since froth depth enhanced fluorite grades without hindering recoveries, froth depth should be maintained at as great a depth as possible, while maintaining sufficient col¬umn length for the collection zones to provide the necessary particle retention time to maintain mineral recoveries.
Wash Water Additions
The principal reason for using wash water in column flotation systems is to increase the grade of the recovered concentrate by displacing entrained hydrophilic (gangue) particles that report to the froth phase. Wash water additions have also been observed to aid in stabilizing the froth bed. Tests were performed to determine the effect of changes in wash water addition rates on column
flotation grades and recoveries of the Fish Creek fluorite ore. Wash water was introduced through a spray nozzle located 1 in above the top of the column. The addition rates were normalized by dividing by the volumetric feed slurry flow rates to the column.
Wash water additions affected fluorite grade and recovery during column flotation in a complex manner (fig. B-7). Increasing wash water additions from 0 to 6 pet of the volumetric feed slurry flow rate increased fluorite grade, but decreased recovery. Increasing wash water additions from 6 pct to approximately 35 pct improved fluorite recovery and decreased fluorite grade. Above 35 pct wash water addition, fluorite grade again increased, while recovery remained approximately constant.
The complex effect of wash water additions on fluorite flotation in the column may be attributed to the twofold nature of column wash water in removing gangue material reporting to the froth, while fluidizing the froth bed to prevent mineral overloading. As detailed in figure B-7, the optimum wash water addition rate was 6 pct. However, wash water flow rates in excess of 40 pct produced grades and recoveries that approached those at 6 pct. These additions may not be feasible due to increased water consumption, system dilution, and downstream materials handling problems.
Particle Size Fractions
To quantify the separation efficiency of column flotation compared with conventional flotation, particle size distribution analyses were performed on products obtained from each method. The study was conducted by extracting a portion of the conditioned column feed slurry and sending it to a conventional batch flotation process. The column and conventional flotation products were sized using Tyler 48-, 65-, 80-, 100-, 150-, 200-, 270-, 325-, and 400-mesh screens.
Column rougher flotation produced substantially higher grade concentrates than did conventional flotation of the fluorite ore (fig. B-8). Column flotation fluorite grades were greater than those of conventional flotation for all size fractions.
Conventional flotation recoveries of fluorite were slightly higher than those of column flotation, except particles between 65 and 100 mesh (fig. B-9). Although conventional flotation provides a small increase in fluorite recovery for most particle size fractions, column flotation still holds the advantage because conventional fluorite recoveries would fall well below column fluorite recoveries in cleaning stages that would be necessary to meet the fluorite grades achieved in column flotation.
Typical Laboratory Flotation Column
A 5.5-m (18-ft) by 6.1-cm (2.5-in) diam plexiglass flotation column with 0.64-cm (0.25-in) walls was used for testwork (Fig. 1). The column was composed of two 0.3-m (1-ft) and eight 0.6-m (2-ft) flanged sections, with 2.9-cm (1.125-in) diam ports located at 0.3 m (1 ft) intervals along its length. Solenoid-actuated sample valves with stainless steel spring loaded plungers were designed and constructed. Sampling valves were fitted into the 2.9-cm (1.125-in) diam ports located at 0.3~m (1-ft) intervals along the column length. These valves permitted rapid, consistent extraction of a representative pulp or froth sample. A concentric cylindrical overflow weir was mounted on the top section of the column. A conical bottom with a 1.3-cm (0.5-in) diam port was attached to the bottom section of the column.
The versatile column design allowed parameters such as column length, feed injection, tailings removal and recirculation, air and wash water injection, and sampling locations to be readily varied. The clear plexiglass construction allowed visual monitoring; consequently, response to flotation parameter changes could be observed immediately.
The conditioned feed material was pumped through flexible tubing from the conditioner to a port located 3.4 m (11 ft) from the column base using a peristaltic pump. Mineralized froth was continuously removed from the top of the column while tailings were pumped from the column base to filtration.
The fine bubble generation system was used to aerate the flotation column. The bubble generator was a 15.2-cm (6-in) tall by 5.1-cm (2-in) diam clear cylindrical plexiglass chamber with 2.5-cm 1-in) thick walls (Fig. 2). The generator had a removable plexiglass top attached to the generator body with an O-ring seal. A spacer supported a fritted glass disk in the chamber to prevent short circuiting of air. The remaining chamber volume was filled with 1-mm (0.04-in) diam glass beads to improve air-water contact. A section of 28-mesh screen was placed over each orifice to prevent loss of glass beads. House air (110 psig) was regulated to 60 psig and introduced through the side port of the bubble generator. Water, pressurized to 60 psig by a turbine blade pump, was introduced through the top bubble generator orifice. Air and water were mixed in the contact chamber; the pressurized mixture exited the chamber through the bottom port and was injected into the column through an aluminum tip with a 1-mm (0.0f-in) diam orifice. The generator design allowed for bubble size control from less than 0.1- to over 3-mm average bubble diameter by adjusting air and water flow rates and by adding Dowfroth 100 frother. Previous investigation showed coarse bubbles (3- to 5-mm diam) produced the best results on the Fish Creek fluorite ore, and they were therefore used for all parameter testing. An air flow rate of 4,500 cm³/min and a water flow rate of 800 mL/min were used to generate the 3- to 5-mm-diam bubbles for this testwork.