Crushing, Grinding & Classification

1. Beneficiation Processes

Crushing, Grinding & Classification

1.1 Process Flow

Stage 1: Run-of-mine ore (18-22% P₂O₅) is coarsely crushed to -150mm by a jaw crusher with no beneficiation conducted during the process. The crushed product is transported by a belt conveyor equipped with iron removal and dust suppression devices. The purpose of this stage is to initially crush the mined large ore lumps (≤500mm) to a particle size suitable for subsequent secondary and fine crushing equipment (-150mm). The core of this stage is the reduction of physical size; there is almost no beneficiation or grade change, serving merely as preparation for subsequent operations.

Stage 2: The coarsely crushed product enters a closed circuit of two-stage cone crushing and screening, producing qualified crushed ore of -12mm. In this process, part of the +25mm low-grade gangue (ore grade<6%

Stage 3: The qualified crushed ore is ground in a closed circuit of ball milling and classification to a fineness where 55-60% of the particles are -0.074mm, yielding overflow slurry with 21-25% P₂O₅ and MgO ≤1.0%. Grinding further dissociates minerals, providing ore with qualified particle size and grade for subsequent flotation. The purpose of this stage is to grind the finely crushed qualified product to the particle size required for monomer dissociation of minerals and ensure qualified particle size through classification. This stage is a key step for sufficient mineral dissociation, providing slurry with qualified particle size, concentration and grade for the subsequent flotation operation that sorts minerals based on surface property differences.

Figure 1: Phosphate Ore Crushing and Screening Flow

Not all phosphate ores need to go through all three stages. Whether a phosphate ore requires the complete three-stage process of "coarse crushing - secondary crushing - grinding" depends on the required feed particle size and ore properties:

• Single-stage process: For soft phosphate ores with particle size ≤300mm, 24-30% P₂O₅ and little argillaceous encapsulation, crush the ore to 50mm, then remove mud with a drum washer. Screening to 0-10mm can directly produce commercial powdered ore.

• Two-stage process: For phosphate ores with 18-22% P₂O₅, embedded particle size of 0.2-0.5mm, high SiO₂ and low MgO, after crushing to 12mm, tailing rejection by heavy medium cyclone or spiral concentrator can produce concentrate with 26-28% grade without ball milling.

• Three-stage process: Phosphate ores with P₂O₅

  55% of the particles are -0.074mm to produce concentrate with grade over 30%. Therefore, the complete process of crushing → cone crushing → ball milling must be carried out.

1.2 Required Equipment and Parameters

The phosphate ore crushing, grinding and classification process is equipped with the following equipment: jaw crusher (PV510×850) for coarse crushing, disc feeder (D-1800) for feeding, cone crusher (SH90C) for secondary crushing, double-deck circular vibrating screen (YK1860) for screening, and return ore belt conveyor (B650) for transporting returned ore; the grinding section adopts an ore ball mill (MQY4060); classification is completed by a high-efficiency spiral classifier (FG-20). Meanwhile, a slurry pump (6/4D-AH) can assist in material transportation.

Figure 2: Equipment and Parameters for Phosphate Ore Crushing

1.3 Three Wastes Treatment

Wastewater is mainly generated from washing processes, workshop and equipment flushing water, and domestic sewage, containing mainly acids and heavy metals. Neutralization and precipitation technologies are adopted; nitrate production wastewater is reused after treatment. Waste gas is generated from drying and crushing processes, containing SO₂ and dust, and is treated by desulfurization towers and activated carbon adsorption; SO₂ produced from phosphate ore acidification is treated to recover sulfur resources. Solid waste is generated from phosphate ore crushing and ball milling processes, including gypsum slag and tailings slag, which are disposed by stockpiling and brick making; gypsum slag can be used for brick making, and tailings are used for mine backfilling or fertilizer production after treatment.

Figure 3: Three Wastes Generated from Phosphate Ore Crushing, Grinding and Classification

1.4 Energy Consumption

• Crushing equipment energy consumption: For the initial crushing of mined phosphate ore, the standard power consumption of a jaw crusher is usually 10-15 kWh per ton of ore crushed. Other types of crushing equipment such as cone crushers have different energy consumption standards according to their specifications and performance, generally fluctuating within a reasonable range.

• Grinding and classification energy consumption: The standard power consumption of grinding equipment such as ball mills is usually 20-30 kWh per ton of phosphate ore ground. New grinding technologies and equipment are constantly emerging, aiming to reduce the energy consumption of this stage.

2. Beneficiation Processes

2.1 Scrubbing and Desliming

The scrubbing device mainly includes ore washer scrubbing, cyclone classification desliming and screening secondary desliming processes. The specific process flow is shown in the figure:

Ore with a grade of 18%-28% after particle size classification is distributed to a trough washer by a belt conveyor, with the first classification based on a 3.5±0.5mm mesh. The fine ore of <4mm is recovered as acid-process ore, while the overflow of >4mm enters the head of the ore washer, is transported by a belt to subsequent screening, crushing and secondary desliming processes, and finally produces yellow phosphorus ore (coarse particle size) and acid-process ore (fine particle size) with a grade of 28%.

Figure 4: Phosphate Ore Scrubbing and Desliming Process Flow

The core equipment for the scrubbing and desliming stage includes 9 sets in total: trough washer (2200×8400) for high-intensity scrubbing, linear screen (2SUL1.5×3.0AT) for wet screening classification, three-stage cyclone group (Φ350×4, Φ200×7, Φ125×15) for step-by-step desliming, 1# and 2# dewatering screens for dewatering to obtain concentrate, thickener plus 6 sets of filter presses for treating ore slime water and recycling water for circulation.

Figure 5: Main Equipment for Scrubbing Stage

All three wastes generated in the phosphate ore scrubbing and desliming stage are recycled or disposed of on-site in compliance with environmental requirements: scrubbing wastewater is reused after precipitation with no process wastewater discharged; tailings are sent to a dry stockpile, and leachate is also reused after precipitation; unorganized dust is controlled by water sprinkling for dust suppression; equipment noise is mitigated by shock absorption and noise elimination measures; solid waste and domestic garbage are stockpiled separately or sent to a garbage treatment station.

Figure 6: Three Wastes Discharge of Scrubbing and Desliming Method

2.2 Heavy Medium Beneficiation

To avoid the decrease in separation efficiency and increase in cost caused by fine particles, only the coarse fraction of low-grade collophane enters heavy medium separation after crushing and screening. After screening, the oversize material (+8mm) enters a pressureless three-product heavy medium cyclone for separation. In the first stage of separation (the ore feeding point), the separation density is generally controlled at about 2.80g/cm³, and the rejected tailings are low-density shale; the second-stage separation system realizes stepless online adjustment, concentrating the heavy medium again to form a separation density of 2.90g/cm³, separating apatite bands from dolomite bands with a slight density difference of 2.80~2.90g/cm³. The heavy medium concentrate and tailings are demediated by a demediation screen to become final concentrate and tailings; the tailings are screened again to obtain ore sand with about 20% P₂O₅, which is combined with the undersize material of the original ore for product recovery.

Figure 7: Heavy Medium Beneficiation Process Flow

Figure 8: Ore and Medium Properties

The core of the phosphate ore heavy medium beneficiation stage is to separate apatite from dolomite within a narrow density window of 2.80-2.92g/cm³. The key equipment is a pressureless three-product heavy medium cyclone group (Φ600-Φ750mm, 60-120t/h), supported by -0.045mm magnetite powder weighting agent, demediation curved screen, 180mT double-drum magnetic separator, FB-2300 nuclear density meter automatic density control system, as well as qualified/dilute medium pumps and pre-screening crushing circuit, ensuring concentrate P₂O₅>28.4%, tailings ≤10%, comprehensive recovery rate over 83%, and medium consumption<0.5kg>

Figure 9: Equipment and Parameters for Heavy Medium Beneficiation

- Wastewater: Derived from medium circulation fluid and demediation flushing water, recycled after step-by-step neutralization for heavy metal removal and precipitation-biochemical degradation of organic matter (circulation rate >90%), meeting discharge standards;
- Solid Waste: Including beneficiation underflow and filter press residue, the medium is recovered by magnetic separation (recovery rate >98%), tailings are stockpiled or resourcefully utilized (e.g., water glass production, backfilling) after thickening and filter pressing (moisture content<12%);
- Waste Gas (Dust): Originating from crushing and screening processes, controlled by bag dust removal + wet spray dust suppression and closed pipeline system, with dust emission ≤10mg/m³, complying with environmental requirements.

Figure 10: Three Wastes of Heavy Medium Beneficiation

2.3 Flotation

Flotation is the most widely used technology in phosphate ore beneficiation. Flotation processes are divided into direct flotation, reverse flotation and two-stage flotation (direct-reverse flotation, reverse-direct flotation and double reverse flotation).

• Direct Flotation: Phosphate ore direct flotation is carried out under alkaline conditions (pH 9-10), mostly using depressants to inhibit silicate gangue minerals, thereby separating to obtain the target mineral phosphate concentrate with a relatively simple process flow.

• Single Reverse Flotation: Single reverse flotation usually requires adjusting the slurry pH to 4-6; after adding phosphate mineral depressants, anionic collectors are used to float carbonate gangue minerals to reduce the Mg content of phosphate concentrate. This process is simple and low-cost with mature industrial production technology, and is mostly suitable for the flotation of calcareous phosphate ores.

• Two-stage Flotation: Two-stage flotation adopts different reagents in different media to remove silicon and reduce magnesium, with common processes including direct-reverse flotation, double reverse flotation and reverse-direct flotation. The direct-reverse flotation process is more complex than direct flotation and single reverse flotation, with more types and larger dosage of flotation reagents, resulting in higher beneficiation costs; however, it has strong adaptability and is conducive to the recovery of low-grade ores. Reverse-direct flotation is often used for the beneficiation of silicon-calcium mixed collophane.

Figure 11: Direct and Reverse Flotation Process Flows

Figure 12: Three Double Flotation Process Flows

Figure 13: Comparison of Five Flotation Types

The main equipment (flotation machine, thickener, filter press) is basically the same for the five processes, but the number of flotation stages in the process flow determines the number of flotation machines required and the complexity of the pH adjustment circuit and reagent preparation system. Direct-reverse, reverse-direct and double reverse flotation require two to three stages of flotation, with corresponding additional capital investment of about 30-50% in pipelines, pump stations and control systems.

Figure 14: Equipment and Parameters for Five Flotation Methods

As the flotation process progresses from "direct → reverse → direct-reverse/reverse-direct → double reverse", the amount of three wastes and treatment complexity increase step by step: direct flotation only produces alkaline wastewater and low-silicon tailings with the simplest treatment; reverse flotation requires neutralization and acid removal towers due to acid usage; direct-reverse/reverse-direct flotation produces two-stage alkaline and acidic wastewater and two types of tailings, requiring a double neutralization system with a treatment load 1.5-2 times that of direct flotation; double reverse flotation involves acid addition twice in succession, generating the largest amount of waste gas that requires multi-stage acid removal; the wastewater has the highest chemical oxygen demand and suspended solids; the total amount of three types of tailings is the largest but the MgO and SiO₂ content of the slag has been greatly reduced, which is more conducive to subsequent safe landfilling or filling, resulting in the highest overall environmental protection investment.

Figure 15: Three Wastes of Five Flotation Types

The increase in energy consumption mainly comes from longer flotation operation time, additional reagent addition, and secondary grinding required in some processes. Therefore, the more complex the process, the higher the power consumption per unit output.

Figure 16: Energy Consumption of Different Flotation Processes

2.4 Comparison of Beneficiation Processes

• Scrubbing and Desliming: Treats argillaceous weathered phosphate ores through scrubbing, desliming and screening classification, with simple process, open-air production and no reagent pollution, but low phosphate concentrate grade and recovery rate.

• Heavy Medium Beneficiation: Uses heavy medium cyclones as separation equipment and magnetite as weighting agent, adopting the process of ore washing and desliming + one roughing and one cleaning, with low pollution and good beneficiation indicators for coarse-grained phosphate ores, but not suitable for fine-grained embedded phosphate ores.

• Flotation: Includes various processes such as direct flotation and reverse flotation, being the main method for phosphate ore beneficiation, suitable for complex and refractory phosphate ores with good product indicators.

Figure 17: Application Scope, Advantages and Disadvantages of Three Beneficiation Processes

2.5 Performance Indicators

• Phosphate ore flotation product: P₂O₅ (dry basis) 29%-35% (up to 36%-38% for direct-reverse and other processes), MgO removal rate 85% (0.6%-1.0%), SiO₂ can be reduced to ≤0.5%, particle size 5-50mm, moisture content 10%-12%.

• Raw material for yellow phosphorus (electric furnace method): Requires P₂O₅≥30%, MgO≤2%, SiO₂/CaO ≤0.2 (SiO₂ usually ≤5%), F≤0.5%, etc., with the same particle size as flotation products.

• Raw material for wet-process phosphoric acid: Requires P₂O₅≥28% (high grade can reduce acid consumption), MgO≤2%, SiO₂<5%, etc..>

Figure 18: Performance Indicators of Beneficiated Phosphate Ore