4.1. Desliming
The mass and oxide balance for the optimized desliming conditions for Sample 1 are shown in
Table 2. The feed of the first desliming stage contained 8% solids by weight and produced an underflow with 16% solids by weight and an overflow with 2–3% solids by weight, which was discharged as final tailings. The mass and P
2O
5 recoveries of the underflow were 70% and 78%, respectively. The P
2O
5 grade increased from 13.3% at the feed to 14.3% in the underflow. The P
2O
5 in the overflow was 10.1%. The underflow of the first stage fed the second stage, which was carried out in the same hydrocyclone applied in the first stage with an apex finder of 5.5 mm and the operation pressure was regulated to 3 kgf/cm
2. The solids content in the overflow of the second stage was 4–5%, the
d50 was 5 μm, and the P
2O
5 grade was 11% and was discharged with the final tailings together with the overflow produced at the first stage. The solids content in the underflow of the second stage was 35–36%, the d
50 was 18 μm, and the P
2O
5 grade was 16.4%. The mass and P
2O
5 recoveries at the second stage were very similar to the first stage, reaching 70% and 77%. The underflow obtained at the second stage fed the conditioning circuit and then the flotation circuit. Considering both stages, the overall mass recovery was around 50% and the P
2O
5 recovery was 60%. Regarding the SiO
2 and Fe
2O
3 contaminants, it was observed that the iron impurities tended to decrease after desliming, as opposed to what was observed for the SiO
2 content, which tended to increase after this process.
The results of the desliming stage for Sample 2 are shown in
Table 3. The P
2O
5 content at the flotation feed (underflow of second stage) was quite similar to that obtained for Sample 1, assaying around 16%. Regarding the SiO
2 and Fe
2O
3 impurities, a reduction of the iron bearing minerals content in the flotation feed was observed when compared to the feed sample, as observed in Sample 1. The SiO
2 content in the flotation feed was almost the same as that in the feed process sample.
4.2. Flotation Studies
This topic presents the results and discussion of the apatite flotation studies considering rougher/cleaner flotation in 6”, 4”, and 2” columns. As mentioned before, one circuit consisted of the application of a 6” column for the rougher stage and a 4” column for the cleaner. The second circuit applied the 4” column for rougher flotation and a 2” column for the cleaner. The influence of collector and depressant dosages, wash water superficial velocity, pH, and air superficial velocity were evaluated on flotation separation parameters. The flotation recovery was calculated as a function of the hydrocyclone underflow (flotation feed).
Figure 3 shows the curve of the P
2O
5 grade versus the recovery, considering apatite rougher/cleaner configuration for both circuits evaluated. As shown in the figure, the P
2O
5 recovery ranged from 80% to 25% with a P
2O
5 grade variation from 22% to almost 38%. In general terms, the flotation performance (grade and recovery) in the circuit with 6” and 4” columns was better compared to the circuit with 4” and 2” columns, especially for the P
2O
5 grade between 30% and 34%.
Figure 4a shows the comparative results for the P
2O
5 grade/recovery curve considering the rougher flotation in the 6” and 4” columns and for cleaner flotation considering the 4” and 2” columns (
Figure 4b). The results for the circuit applied in the 6” and 4” columns were obtained with the flotation of Samples 1 and 2 and considering the Sample 1 flotation for the circuit with the 4” and 2” diameter columns. The performance (grade and recovery) of rougher flotation in the 6” diameter column was superior to the results obtained for the 4” diameter column. As it can be observed, the P
2O
5 recovery ranged from 90% to 40%, with a P
2O
5 grade variation from 17% to almost 34%. Furthermore, a linear relation between the P
2O
5 grade and losses was verified for the rougher tailings.
A fundamental difference between both columns is the fact that the bubble generator of the 6” column is a cavitation tube, while the bubble generator of the 4” column is a porous tube. Hydrodynamic cavitation to generate bubbles has been use to enhanced the flotation of fine and ultrafine particles [
17,
18,
19]. Tao et al. [
17] evaluated the effect of picobubble injection produced by the hydrodynamic cavitation principle in association with the conventional sized bubbles produced by a static mixer on the flotation response of fine coal particles. The results indicated that picobubbles significantly enhanced the coal flotation process with higher recovery and lower product ash. The flotation recovery increased by 10–30% depending on the process operating conditions. Zhou et al. [
18] incorporated a cavitation tube in the feed line to a conventional flotation cell for the flotation of fine silica and ZnS precipitates (<5 µm). The results showed that there is a substantial increase in fine silica recovery for a given flotation period when using the cavitation tube (without added air). Additionally, a 40% increase in rate constant was obtained using a cavitation tube (1.3 mm nozzle diameter), even though the overall aeration was less (2.15 L/min compared to 3 L/min without the tube). This increase in the flotation rate constant again suggests that small bubbles generated by cavitation in the feed stream played a role in enhancing flotation kinetics.
As for cleaner flotation, it could be observed that the results obtained in the flotation with the 2” column showed a higher distribution compared to the results obtained for the 4” column. This can be explained by the fact that cleaner flotation in the 2” column had more process variables tested, focused on optimization of this stage, including changes in the superficial air velocity (Jg) and wash water (Jw), besides the reagents dosage variation. For the flotation in the 4” column as a cleaner, only the effect of the collector, depressant dosage, and pH were evaluated for Jg and the Jw kept fixed at 0.51 cm/s and 0.20 cm/s, respectively. The P2O5 losses on the cleaner flotation with the 4” column ranged from 2% to a maximum of 12%, considering the P2O5 grade variation from 9% to 25%. On the other hand, it could be observed that the cleaner flotation in the 2” column showed a much higher variation, especially for the P2O5 losses, which ranged from 10% to values up to 24% for the similar P2O5 content in the sink fraction.
The relation between the P
2O
5 grade in the final apatite concentrate and the content of Fe
2O
3 and SiO
2 impurities for both circuits evaluated are shown in
Figure 5. The variation of the Fe
2O
3 content in the final apatite concentrate is very similar for both circuits and it is independent of the sample. The SiO
2 content has the same trend observed for the iron impurities in Sample 1 for both circuits, whereas the SiO
2 content in the final concentration of Sample 2 is significantly lower than that obtained for Sample 1. These results indicate that contamination of the apatite concentrate strongly depends on the ore characteristics that feed the plant and it is less influenced by the scale tested. As shown before, the SiO
2 grade in Sample 2 is much lower when compared with Sample 1, unlike the iron content, which is quite similar in both samples.
A summary of the best results achieved using rougher/cleaner configuration for the circuit with the 4” and 2” columns is shown in
Table 4 for the flotation studies with Sample 1. Considering the average results for the three tests, mass recovery was around 23% and P
2O
5 recovery and grade were 54% and 35.2%, respectively. The SiO
2 and Fe
2O
3 impurities levels were 3.0% and 5.2%. Reagent consumptions were 117 g/t to the collector, around 2700 g/t to the depressant, and 410 g/t to NaOH and the pulp pH was fixed at 9.7.
Table 5 shows the effect of pulp pH variation, for the values 9.7 and 10.8, on the flotation performance for the experiments using the 6” and 4” columns in rougher/cleaner configuration in experiments with Sample 1. The collector dosage was 130 g/t and the depressant was 2900 g/t. It can be observed that a reduction of the pulp pH from 10.8 to 9.7 decreases the P
2O
5 content in the concentrate, going from 33.4% to 31.4%. The reason for the dilution of the concentrate is exclusively caused by the increase of the SiO
2 from 5.3% to values around 10%, since the iron content is virtually the same. The increase in mass recovery, from 34.6% to 36.9% with the decrease of the pH value, is due the increase in the SiO
2 content once the P
2O
5 recovery is almost the same for both pH values evaluated, reaching 72%.
Flotation results for Sample 2 in the experiments carried, out with the 6” and 4” columns, are shown in
Table 6. The average collector dosage for the four tests was around 80 g/t and the depressant was 2600 g/t at the pulp pH of 9.7. The average mass and P
2O
5 recovery were 27% and 51% for a P
2O
5 grade of 32.8%. Compared to the results obtained from Sample 1, the SiO
2 content in the final concentrate was much lower in the experiments with Sample 2, reaching an average grade of 1.0%. On the other hand, the iron content is very similar for both samples. The lower SiO
2 content reached in the concentrate of Sample 2 can be explained by the lower grade in the slime feed sample that was analyzed at 12.5%, compared with the 19.7% of Sample 1.
Regarding the reagent consumption (collector and depressant), it was observed that the average collector dosage is similar to the levels applied at the industrial scale in Brazilian plants. On the other hand, the depressant dosage is much higher, reaching values up to 2500 g/t. Usually, the depressant dosage does not exceed 1200–1300 g/t. Matiolo et al [
15] evaluated the depressant dosage on apatite flotation from slimes. The results indicated that both parameters (P
2O
5 grade and recovery) improve when the depressant dosage increases from 1230 g/t to values reaching higher than 1700 g/t. It was also found that to control the iron impurities in the final flotation concentrate, the depressant dosage must be up 2200 g/t to a maximum of 3000 g/t.
A similar approach to that evaluated in this study of recovering the valuable fine phosphate particles (<45 μm) from their slimes through the application of the column flotation technique was tested by Abdel-Khalek [
20]. Tests were performed using oleic acid as a collector for the phosphate minerals and sodium silicate as a depressant for their associated gangues. The main operating parameters affecting the performance of column flotation were investigated. The results indicate that the best operating conditions for column flotation of phosphate slimes are as follows: A superficial gas velocity of 0.84 cm/s, a frother concentration of 0.1 kg/ton, a column height of 230.5 cm, and a superficial water velocity of 2.2 cm/s. Under these conditions, a product assaying 25.3% P
2O
5 and 14.64% I.R. (insoluble residue), with a P
2O
5 recovery of about 51.52%, is obtained from a feed containing 18.26% P
2O
5 and 24.03% I.R. Such grades and recoveries are not obtained by applying the conventional froth flotation technique, even after cleaning the rougher concentrate. Recovery of more than 50% of phosphate from disposed slimes will improve the economic viability of the beneficiation process for phosphate ores. It will also help to solve the environmental problems associated with the disposal of these slimes.