Graded flotation process solves the challenge of separating complex lead-zinc ores

For lead-zinc sulphide co-occurring ores characterised by uneven grain size distribution and high levels of fine zinc impurities, traditional unified grinding and flotation processes often result in zinc impurity levels in the lead concentrate exceeding permissible limits, whilst making it difficult to achieve satisfactory recovery rates for both metals. Consequently, the graded flotation process has become the mainstream solution for such ores. The entire process is divided into three main parts: the lead flotation circuit, the classification and enrichment unit, and the zinc flotation circuit. The entire process adopts the core principle of prioritising lead flotation followed by zinc recovery from the tailings, combined with a differentiated reagent system to achieve efficient separation of lead and zinc.

The raw ore first enters the grinding system, where the target fineness is controlled to ensure thorough liberation of galena, sphalerite and gangue minerals; the ground slurry is then fed into the lead roughing stage. During the roughing stage, lime is used to adjust the pulp to a high-alkali environment; this is combined with composite inhibitors and ethylthio-nitrogen selective collectors and frothers to prioritise the flotation of lead minerals, producing lead rough concentrate. The roughing tailings enter the lead scavenging operation to recover any unflotated lead metal, whilst the scavenging middlings are returned to the roughing stage for cyclic separation.

The lead rough concentrate is fed into the classification unit, where it is divided into two main fractions—coarse and fine—based on particle size for separate concentration. The +0.037 mm coarse lead rough concentrate undergoes conventional multi-stage concentration, with the continuous addition of zinc inhibitors to gradually reduce the zinc content in the concentrate; The -0.037 mm fine-grained sphalerite, which is difficult to suppress using conventional lime, undergoes separate multi-stage enrichment using strong alkali to raise the pulp pH, combined with a composite inhibitor. Differentiated reagent conditions are employed to prevent the flotation of fine-grained zinc; the two streams of enriched concentrate are combined to produce a qualified lead concentrate, whilst the middlings generated during enrichment are returned to the corresponding enrichment stages for recycling based on particle size.

Following lead recovery, the flotation tailings are transferred to the zinc separation process. Copper sulphate is first added to reactivate the suppressed sphalerite, followed by adjustment of the pulp alkalinity to an appropriate level and the addition of a zinc-specific collector to carry out zinc roughing; The roughing foam undergoes multiple stages of scavenging to enhance the grade of the zinc concentrate, whilst the roughing and scavenging tailings are combined and discharged as final tailings; the intermediate product from the zinc circuit is returned in reverse to the roughing stage for repeated separation.

The entire process is based on the principle of differentiated flotation by particle size, matching specific reagents and separation conditions to the flotation characteristics of coarse and fine-grained minerals, thereby addressing industry challenges such as the difficulty in suppressing fine sphalerite and the severe intergrowth of lead and zinc. Intermediate products are circulated in a closed-loop system throughout the process, minimising metal loss whilst simultaneously recovering valuable metals such as silver associated with the ore. The process balances the grades of both lead and zinc concentrates with the overall metal recovery rate, and is well-suited to complex lead-zinc sulphide ores with uneven distribution of coarse and fine particles. Compared to traditional uniform flotation schemes for all particle sizes, it achieves significantly improved separation results and offers stable, practical application value.