The 2017 technical report describes a three-phase mineral processing facility designed to treat saprolite and fresh carbonatite ore to produce phosphate concentrate and agricultural lime.
Report context
Dated 2017, the Três Estradas Phosphate Project technical report presents the proposed mineral processing facilities designed in three phases. Phase 1 treats saprolite ore during the first years of operation. Phase 2 treats fresh carbonatite ore during the remaining years of the mine life. Phase 3 recovers, dewaters, and sells aglime disposed during carbonatite treatment.
Processing route
Phase 1 – Saprolite
During Phase 1, run-of-mine (ROM) material is transported by 30 t trucks from the mine to the beneficiation plant. ROM is dumped into a hopper and passed through an apron feeder to a toothed roll crusher (model MMD 500 or similar, mobile system over crawlers). The crusher product, sized less than 25 mm, is transported by conveyor belt to a stockpile. Crushed ore is reclaimed from the stockpile and transported to a hopper by a front-end loader, then passes through a belt feeder onto a conveyor belt leading to a primary 12.5' x 16' rod mill with 900 kW drive. The rod mill discharge is directed to a pump box where dilution water and a solution of NaOH are added before the slurry is pumped to the conditioning stage.
The flotation circuit for Phase 1 includes a conditioning tank, low intensity magnetic separation (LIMS), rougher and cleaner 1 direct column flotation stages, and high intensity magnetic separation (WHIMS) of the cleaner 1 concentrate. Classification by hydrocyclones of the WHIMS tailings removes the magnetic material and stores it in a bay. Slurry treated with a collector and a depressant (corn starch) enters conditioning tanks prior to magnetic separation. After conditioning, the slurry is pumped into a slurry distributor feeding the LIMS stage. The non-magnetic fraction undergoes flotation to concentrate phosphate content. The magnetic fraction goes through dewatering to recover process water.
Flotation is a two-stage process: rougher stage followed by cleaner stage. The non-magnetic fraction from LIMS is directed to a pump box, where frother (butyl glycol ether) is added before entering two rougher flotation columns (5 m diameter, 10 m height). Rougher concentrate is pumped to two cleaner stage flotation columns (5.0 m diameter, 10 m height). Rougher tailings flow to a thickener feed box feeding a tailings thickener. Cleaner concentrate is pumped to a high intensity magnetic separation stage.
The final flotation concentrate (cleaner concentrate) feeds a high intensity magnetic separator producing a magnetic fraction (tailings) and a non-magnetic product (concentrate). The non-magnetic fraction flows to a thickener feed box feeding the concentrate thickener.
Phase 2 – Carbonatite
With the exhaustion of saprolite ore reserves, the comminution circuit is modified and complemented. A primary jaw crusher and secondary cone crusher operating in open circuit are installed. A new conical surge pile with reclaim system using vibratory feeders replaces the saprolite surge pile circuit. An additional rod mill and two ball mills are installed, operating in two parallel lines (one rod mill and one ball mill each line).
ROM is transported by 70 t trucks to the beneficiation plant and dumped into a hopper with a fixed grizzly. A mobile rock breaker breaks rocks larger than 750 mm. Ore is reclaimed by an apron feeder to a vibrating grizzly (3 m x 5.6 m) with 100 mm openings. Oversize material (-750 mm +100 mm) feeds the primary crusher (C140 Metso or similar). Grizzly undersize together with primary crusher discharge is collected by conveyor belt to secondary crushing. Material from primary crushing is transported to a secondary crusher feed silo, reclaimed by belt feeder to an 8' x 24' banana vibratory screen, classified at 25 mm with oversize feeding the secondary cone crusher (HP 400 Metso or similar). Screen undersize together with secondary crushing product is transported to the crushed ROM stockpile.
Crushed ore is reclaimed from the stockpile using vibratory feeders and transported by conveyor belt to the rod milling feed silo. Material feeds by belt feeders into two primary 12.5' x 16' rod mills equipped with 900 kW drives operating in open circuit. The primary grinding circuit also includes a secondary grinding stage with two ball mills operating in closed circuit. Primary grinding product is discharged in pump boxes and pumped to classification hydrocyclones batteries. Cyclone underflow feeds the two 12' x 18' ball mills with 1,100 kW drives. Secondary grinding product is collected in a pump box and pumped to conditioning tanks before feeding the LIMS.
In Phase 2, additional column flotation stages are installed: cleaner 2 and cleaner 3 flotation columns with respective slurry pumping systems. The non-magnetic fraction from LIMS, after conditioning, is pumped into a slurry distributor where frother is added before entering six rougher flotation columns (5 m diameter, 10 m height). Rougher concentrate is pumped to four cleaner 1 flotation columns (4.5 m diameter, 9 m height). Cleaner 1 concentrate is pumped to cleaner 2 stage comprising two 4.5 m diameter and 9 m height flotation columns. Cleaner 2 concentrate is pumped to cleaner 3 stage feed. Cleaner 3 stage comprises two 4 m diameter and 8 m height flotation columns. Cleaner 3 concentrate is pumped to a HIMS stage.
The carbonatite mineralization contains approximately 40% CaO or approximately 71.4% CaCO₃, which remains in the phosphate flotation tailings.
Thickening
Both concentrate and tailings thickeners are designed to attend both phases 1 and 2. Sizing criteria is based on dewatering test work developed by Pocock.
For Phase 1, dewatering of phosrock flotation consists of thickening, filtration, and drying to reduce moisture to 2%. The final flotation concentrate enters a 14 m diameter concentrate thickener. Thickener underflow, containing slurry with 70% solids in weight, is pumped to the filtering stage. Tailings from rougher and cleaning stages of flotation are directed into a thickener feed box where flocculant is added before feeding a 32.5 m diameter tailings thickener. Thickened underflow is pumped to the tailings dam.
For Phase 2, a portion of the thickened tailings (aglime, 1,000,000 t/y) is filtered to reduce moisture to 5% – 10% and the excess is pumped to the aglime dam for storage. Aglime from the tailings thickener underflow is pumped into a surge tank with an agitator, with two slurry pump systems: one to feed the aglime filter presses and the other to pump the excess to the storage dam.
Filtering
In Phase 1, phosrock thickener underflow is pumped to a filter feed surge tank with an agitator, then pumped into the filter press. This equipment produces a concentrate filter cake of low moisture content (8%) discharged to a concentrate stockpile. Filtrate is collected and reused in the plant.
In Phase 2, concentrate filtering is the same as Phase 1. A portion of the flotation tailings (aglime) is filtered after thickening to reduce final moisture to 5-10%. The thickened aglime slurry from a surge tank with agitator is pumped to feed two filter presses (2 m x 2 m with 65 plates each). This equipment produces a tailings filter cake of low moisture content discharged to an aglime stockpile, part in a covered area and part in an open area.
Phase 3 – Aglime Recovery
Operation consists in reclaiming the aglime from the tailings dam as slurry and dewatering it in the existing aglime filtering installation at an annual production of 1,000,000 t. Aglime is reclaimed from the dam by a dredging system and the slurry pumped to the aglime thickener. Thickener underflow is delivered to the aglime filter feed tank. Filter cake is stored at the storage shed and filtrate returned to the thickener.
Phosrock Drying
The drying process reduces moisture to a final content of about 2%. The phosrock dryer requires a hot gas generation system fueled by coal, comprising a fluidized bed hot gas generator (HGG). The HGG receives prepared coal and generates hot gas at suitable temperature for the phosrock drying system. The HGG also generates ash and gypsum waste dispatched by truck to an adequate waste disposal area. Coal preparation uses limestone to promote sulfur abatement and maintain gaseous effluents below SOx discharge limits.
Reagents
Flotation reagents include collector (Flotinor 9904 for Phase 1, Flotinor 7654 for Phase 2), frother (butyl glycol ether), corn starch (depressant), and sodium hydroxide for pH adjustment and starch gelatinization. Flocculant is used for thickening. Receiving and preparation/dilution systems for all flotation reagents are designed to attend both phases 1 and 2.
Process Design Basis
The process design is based on the metallurgical testing programs presented in Chapter 13 of the report. The most favourable results for phosphate recovery and concentrate production utilized column flotation technology to treat the whole material, without fines removal (minus 20 μm, or "slimes" fraction). The volume of the fine fraction is very significant and the phosphate grade of the fines is similar to the coarse fraction. Potential losses in removing the fines range from 20% to 45% in the saprolite and 40% to 50% in the fresh carbonatite.
Test work for comminution, flotation and liquid-solid separation has been completed; test results are detailed in Chapter 13.
Key reported parameters
| Parameter | Unit | Phase 1 (Saprolite) | Phase 2 (Carbonatite) | Phase 3 (Aglime Recovery) |
|---|---|---|---|---|
| Average P₂O₅ Feed Grade | % | 8.5% | 3.7% | – |
| Annual ROM Feed Rate | t/y | 1,348,488 | 3,183,024 | – |
| P₂O₅ Concentrate Production (dry basis) | t/y | 300,000 | 300,000 | – |
| Aglime Production (dry basis) | t/y | – | 1,000,000 | 1,000,000 |
| Operational Efficiency (grinding/concentration) | % | 85% | 90% | – |
| Effective Hours Per Year | h/y | 7,446 | 7,884 | – |
| ROM Feed Rate (dry basis) | t/h | 181 | 404 | – |
| P₂O₅ Concentrate/Phosrock (dry basis) | t/h | 40.3 | 38.0 | – |
| Aglime Production (dry basis) | t/h | – | 127 | – |
| P₂O₅ Recovery | % | 81.4% | 75.3% | – |
| P₂O₅ Yield | % | 22.2% | 9.4% | – |
| P₂O₅ Grade – Concentrate (after magnetic separation) | % | 32.7% (Saprolite Phosrock testwork) | 30.1% (Carbonatite Phosrock testwork) | – |
| Phosrock Filter Cake Moisture | % | 8% | 8% | – |
| Aglime Filter Cake Moisture | % | – | below 12% | – |
| Dried Concentrate Final Moisture | % | 2% | 2% | – |
| LIMS Equipment | – | Drum type, 2500 Gauss | Drum type, 2500 Gauss | – |
| HIMS Equipment | – | SRW-050PR, 14,000 Gauss | SRW-050PR, 14,000 Gauss | – |
| Concentrate Thickener Diameter | m | 14 | 14 | – |
| Tailings Thickener Diameter | m | 32.5 | 32.5 | – |
Project website: https://aguiaresources.com.au/projects/organic-phosphate-project/
Note: P₂O₅ concentrate grade values are derived from pilot column flotation testing results (ERIEZ, 2017). Saprolite phosrock before magnetic separation shows 33.3% P₂O₅. Carbonatite phosrock before magnetic separation (cleaner 3 flotation concentrate) shows 31.1% P₂O₅.
Technical qualifications
The process design is based on metallurgical testing programs presented in this report on Chapter 13. The most favourable results for phosphate recovery and concentrate production utilized column flotation technology to treat the whole material, without fines removal. Test work for comminution, flotation and liquid-solid separation has been completed; test results are detailed in Chapter 13. Following the pilot testing, a bench scale survey was carried out by Eriez to identify a cost-effective collector. This test campaign results have identified alternative collectors for saprolite and carbonatite ores. Based on the tests results, some premises were established to simulate the flotation performance in the industrial operation using the alternative collector and to establish the design criteria. However, the expected performance and criteria must be confirmed. A pilot column flotation testing program with the selected reagents used in the last mechanical cells testing is recommended to allow confirmation of the concentrate grades, recoveries and collector consumptions.
Source: Três Estradas Phosphate Project NI 43-101 Technical Report, 2017, Sections 17 (Recovery Methods, Process Design Criteria, Products Characteristics, Process Plant Description, Processing Plant Description).
