Direct Flotation, Reverse Flotation and Combined Processes for Phosphate Ore

As a vital non-metallic mineral resource for obtaining phosphorus, phosphate ore is widely used in industry, agriculture and medicine. The main beneficiation processes for phosphate ore include heavy-medium separation, magnetic-electrostatic separation, scrubbing and desliming, roasting-digestion, flotation, photoelectric separation, chemical leaching and combined beneficiation flowsheets. Flotation remains the most widely used and effective method for the separation of phosphate ore in industrial applications.

The selection of a flotation process depends largely on the composition of the phosphate ore and the varying concentrations of its components. Selecting an appropriate flotation process for different types of ore is a key factor in achieving good flotation results. The following are some commonly used processes:

a) Phosphate ore direct flotation process

This process is generally applied to the beneficiation of low-grade phosphate ores in which phosphate minerals and gangue minerals are finely disseminated, including siliceous, silico-calcareous and calco-magnesian phosphate ores. However, this process is only suitable for ores with low Mg content, and must be used in conjunction with strongly suppressive inhibitors to produce a product that meets industrial production requirements.

b) Reverse flotation process for phosphate ore

This process is frequently employed to separate colloidal phosphate ore from dolomite. The principle involves using fatty acid-based collectors to perform reverse flotation on phosphate ore under weakly acidic conditions, thereby floating the dolomite gangue and concentrating the majority of the phosphate concentrate in the cell. The removal rate of dolomite generally reaches 70% to 80%. However, this process is less effective at separating silicate minerals, and satisfactory flotation performance can only be achieved when combined with high-quality inhibitors and collectors that possess moderate collecting power and good selectivity.

c) Combined positive and reverse flotation process for phosphate ore

For silico-calcic-magnesian gel phosphate ores with high silicate, calcium and magnesium content, neither positive nor reverse flotation alone can produce a phosphate concentrate that meets industrial requirements. To overcome the shortcomings of single-stage positive or reverse flotation processes, a combined positive-reverse flotation process is employed to treat this type of silico-calcic-magnesian gel phosphate ore.

The principle of this process is to first carry out direct flotation in a weakly alkaline medium to remove silicate minerals and reduce the silicon content in the ore. The pH of the slurry is then adjusted to a weakly acidic level for reverse flotation to recover magnesium-bearing minerals and reduce the magnesium content in the ore, ultimately yielding a phosphate concentrate that meets industrial requirements. Compared to other flotation processes, the positive-reverse flotation process results in the final concentrate being enriched within the flotation cells, and the concentrate particle size is relatively coarse, which is highly advantageous for subsequent dewatering.

Positive-reverse flotation is currently a relatively mature process for phosphate ore beneficiation, involving first positive flotation to remove silicon, followed by reverse flotation to remove magnesium. This process is highly adaptable.

d) Double reverse flotation process for phosphate ore

The double reverse flotation process involves two reverse flotation operations. The first reverse flotation removes carbonate minerals such as dolomite from the ore, whilst the second removes silicate minerals such as quartz, thereby concentrating the phosphate concentrate within the flotation cell.

A key feature of this process is that it achieves relatively favourable flotation indices even under ambient temperature conditions and with coarser particle sizes. It is subject to fewer operational constraints, has lower environmental requirements, and eliminates the need to adjust the pulp pH. This effectively resolves the issue of adjusting pulp pH in combined direct-reverse flotation processes, thereby reducing flotation costs to a certain extent.