1. A wet gravity beneficiation process for asbestos-type minerals, characterized in that:

(i) it is applicable to the beneficiation of various types of asbestos ores, including chrysotile asbestos (white asbestos), brucite asbestos, fibrous sepiolite asbestos, amphibole asbestos, and the like;

(ii) the process comprises: soaking and penetrating asbestos ores with water as a medium, followed by wet crushing, grinding, fiberizing, and opening, then separating various grades of asbestos fibers, needle‑like fibers, gangue, and mineral slime (asbestos dust) using various wet gravity concentration methods, followed by a fine splitting operation for full loosening, and finally dewatering and desliming by squeeze‑filtration to obtain high‑performance asbestos fiber products;

(iii) the process conditions are: process water used in the beneficiation plant has a pH of 6.5–7.5, wherein the pulp pH for crushing and grinding liberation is 7–9, the high‑concentration asbestos pulp pH is 7–10, and the return water pH is 7–8; the solid‑to‑liquid ratio (by weight) for coarse crushing and fiberizing is 1:0.1–0.5; for medium crushing and fiberizing/opening is 1:0.5–3; for fine crushing and fiberizing/opening is 1:2–5; for grinding and opening is 1:2–5; for roughing is 1:10–40; for cleaning is 1:10–50; for scavenging is 1:8–15; the feed concentration for the fine splitting operation is a solid‑to‑liquid ratio of 1:6–18 (all ratios are by weight); the dewatering and desliming of the cleaned fibers adopt a squeeze‑filtration method, and the dewatered product has a moisture content of 16–25% (by weight), and a –200 mesh dust/slime content of 4–36% (by weight of moisture, controlled according to the quality requirements for different grades of asbestos fiber products);

(iv) the method of the present patent is also applicable to the secondary processing of commercial asbestos fibers produced by dry processes from mines (beneficiation plants) or raw lump asbestos as raw materials in asbestos product factories;

(v) the method of the present patent is also applicable to the wet recovery of short asbestos fibers from dry‑process asbestos tailings.

2. The process according to claim 1, wherein the water used in the operations of wet crushing, grinding, opening, wet separation, and wet fine splitting of fibers can be fresh water (pH = 6.5–7.5) or circulated return water (pH = 7–10).

3. The process according to claim 1, characterized in that alkaline inorganic compounds may be added to the water used in the operations of wet crushing, grinding, fiberizing, opening, wet separation, and wet fine splitting, with an alkalinity adjustable within a pH range of 7–14, and an optimum operating pH of 7–10.

4. The process according to claim 1, characterized in that the inorganic alkaline compounds for adjusting the pH of the process water are selected from one or more of the following, used singly or in combination: sodium hydroxide, limewater (Ca(OH)₂), sodium carbonate, sodium bicarbonate, sodium silicate, sodium hexametaphosphate, and sodium tripolyphosphate, with the dosage controlled to maintain the operating pH at 7–14, preferably 8–10.

5. The process according to claim 1, characterized in that the water consumption for each stage of crushing, grinding, fiberizing, and opening is increased stepwise as the specific surface area of the ore increases, wherein the solid‑to‑liquid ratio (by weight) is 1:0.1–0.5 for coarse crushing/fiberizing, 1:0.5–3 for medium crushing, 1:2–5 for fine crushing, and 1:0.5–6 for grinding.

6. The process according to claim 1, characterized in that the water consumption for roughing and cleaning operations is increased stepwise according to the required degree of washing, wherein the solid‑to‑liquid ratio (by weight) is 1:10–40 for roughing, 1:10–50 for cleaning, and 1:8–15 for scavenging.

7. The process according to claim 1, characterized in that the pulp concentration for the fine splitting operation is 5–16% (solid weight concentration), and the number of circulation cycles for the fine splitting is 1–4 times (pump‑refiner circulation) or 10–30 minutes (tank beating circulation).

8. The process according to claim 1, characterized in that the dewatering and desliming of the cleaned asbestos fiber are completed in the same operation, and the equipment used in this operation is a squeeze‑filter machine having both scooping and filtering functions, the technical features of which are:

(i) pulp feeding by a side‑flow pulp feeding method;

(ii) completing the first‑stage dewatering and desliming by a rotary screen scooping method;

(iii) completing the second‑stage dewatering and desliming by squeezing between a press roll and the rotary screen;

(iv) transferring the pulp from the rotary screen onto a felt, and completing the third‑stage dewatering by vacuum suction filtration during felt movement;

(v) conveying the fiber‑loaded felt to a breast roll section, and completing the fourth‑stage dewatering by squeezing with a finished‑product take‑up drum;

(vi) transferring the final dewatered fibers onto the finished‑product take‑up drum and scraping off the product with a scraper, the product having a moisture content of 16–25% (by weight).

9. The process according to claim 8, characterized in that the technical features of the squeeze‑filter dewatering and desliming equipment are:

(i) feed pulp concentration by the flow‑feeding method is 0.5–4.0% (solid weight concentration);

(ii) the rotary screen has a diameter of φ500–1200 mm, an outer screen mesh of 80–200 mesh, and 1–5 screens per machine; the center distance between the press roll and the rotary screen is 80–160 mm; the felt width is 700–1200 mm (length depending on the number of screens); the linear speed of the felt and rotary screen is 20–50 m/min; the vacuum degree is 100–420 mmHg; the breast roll diameter is φ300–450 mm.

10. The process according to claim 8, characterized in that the squeeze‑filtration process and the squeeze‑filter machine are equally applicable to dewatering and desliming processes in the beneficiation of various metal ores, non‑metal ores, and coal, with the applicable scope and features being: feed particle size of –2.0–0 mm (preferably –0.5–0 mm), pulp solid concentration of 0.5–6% (preferably 0.5–3%), removed slime particle size of –200 mesh or –325 mesh; the final dewatered and deslimed product has a moisture content of 6–25%, with 15–25% for cohesive mineral products and 6–15% for non‑cohesive mineral products (by weight).

アスベスト系鉱石の湿式重力選鉱法

The present invention relates to a beneficiation process for asbestos-type minerals.

Asbestos-type minerals refer to a class of natural mineral fibers having a fibrous structure and spinnability. Examples include: chrysotile asbestos (white asbestos), amphibole asbestos (crocidolite, amosite), anthophyllite asbestos; brucite asbestos – sepiolite (fibrous sepiolite), and palygorskite, among others. These minerals, when formed in nature, are intermixed or cemented with their gangue minerals and occur as relatively dense ore aggregates. To be useful, they must be processed through crushing, grinding, liberation (fiberizing), opening, loosening, sorting, and impurity removal to separate the valuable asbestos minerals.

Both domestic and international traditional asbestos beneficiation processes have been dry processes. That is, the ore is first heated and dried, then subjected to dry crushing and stage grinding for liberation, followed by stage air classification (suction separation) and dry screening using air as the medium to produce different grades of marketable fiber. Common dry process equipment includes: rotary dryers, jaw crushers, impact crushers, vertical-shaft hammer crushers, edge runners, self-centering vibrating screens, plane shaking screens, plane gyratory screens, fiber suction drums, cone-angle screens, square plansifters, counter-current screens, fiber drop drums, dust collection chambers, etc.

However, dry separation of asbestos-type minerals has four fatal drawbacks:

1. During the entire dry crushing, fiberizing, opening and dry separation processes, a large amount of dust composed mainly of short asbestos fibers (asbestos dust) is generated. In severe cases, asbestos dust spreads not only within the plant but also around the plant area. This is a key cause of asbestosis.

If the concentration of asbestos dust in the air exceeds the safe allowable limit (2 to 0.5 f/ml, i.e., more than 2 or 0.5 fibers per milliliter), it poses a certain health hazard. When inhaled into the lungs, it gradually accumulates and causes asbestosis, affecting the body through dissolution, penetration, migration, and deposition; some fibers enter the esophagus, stomach, and intestines, while most are excreted. The main factor affecting human health is the physical (mechanical) irritation caused by needle‑like fibers (insufficiently liberated hard structural fibers), which can cause pathological changes in living cells. A small proportion (1–2%) of asbestosis patients may develop cancer due to the combined effects of other factors (e.g., smoking or simultaneous exposure to certain organic chemical raw materials).

Among various asbestos fibers, crocidolite is recognized as the most carcinogenic. In addition to its strong mechanical irritating effect, its high adsorption activity may also be a contributing factor. In contrast, chrysotile asbestos, which is now widely used, is much safer; brucite asbestos is more readily dissolved by weak acids in the human body, and to date no cancer cases have been reported for it. Nevertheless, the hazard of dust (asbestosis) remains an issue that cannot be ignored.

The present invention employs a fully wet separation process in an aqueous medium, which can fundamentally eliminate the hazard of asbestos dust.

2. In dry operations, the ore must be kept dry. During drying (at 200–400 °C), the ore and its asbestos fibers lose adsorbed water and temporarily lose part of their structural water, which reduces the strength of the fibers and makes them somewhat brittle. Consequently, during liberation and opening operations, the fibers are easily broken and shortened, generating new asbestos dust. This greatly reduces the proportion of medium‑ and long‑length fibers and the overall fiber recovery, and degrades quality and performance.

The present invention achieves liberation and separation in an aqueous medium. Water has penetrating and selective liberating effects, which can protect the original fiber quality and performance to the greatest extent, and significantly improve recovery.

3. Dry separation uses air as the medium. Due to air’s very low density, the separation efficiency, judged by gravity concentration theory, is very low. At the same time, sand grains entrained in the loosened fibers are difficult to remove, and the high adsorption capacity of asbestos fibers causes a large amount of dust to adhere to the fiber surfaces, leading to higher dust content in the product and poorer quality.

The present invention uses water as the medium. Water has a density of 1, providing much higher separation efficiency and readily washing away sandy impurities. Moreover, water has a good rinsing effect to remove dust adsorbed on fiber surfaces, so the product quality is clearly superior to that of dry processes.

4. To overcome the above drawbacks as much as possible, modern dry‑process beneficiation plants must adopt very complex flowsheets with multi‑stage crushing, grinding, fiberizing, and staged separation, together with elaborate dust collection and fiber recovery systems. This results in complicated processes, high investment, long construction periods, high processing and management costs, and high energy consumption. At the same time, a large amount of short fiber is still lost in the tailings.

The present invention adopts a water‑based gravity beneficiation method. Because of its high separation efficiency, large operating capacity, simple process flow, investment and costs can be greatly reduced, energy consumption is low, and short fibers in the tailings can be recovered to the maximum extent.

Chinese patent application (No. 97112703.4) proposes a “wet flotation modification process for asbestos”. This is different from the present invention. That patent uses flotation (froth flotation). The fundamental difference between flotation and gravity concentration is that flotation requires the addition of organic flotation reagents (one or several surfactants). That invention selects sodium dioctyl sulfosuccinate (i.e., rapid penetrant T), which has good foaming, penetrating and collecting abilities for fibers, or fatty alcohol polyoxyethylene ether surfactants, etc. Although this method can effectively float asbestos fibers on the water surface (froth flotation) for effective separation, it has several disadvantages: ① Rapid penetrant T is only effective for certain magnesium‑rich (Mg²⁺‑rich) types of chrysotile and lacks selective collecting ability for many other chrysotile types or other asbestos‑type minerals, while fatty alcohol polyoxyethylene ether has even poorer collecting and foaming properties; ② The addition of various surfactants contaminates the pulp with organic active agents, preventing reuse of return water and causing environmental pollution; ③ The accumulation of froth in the pulp worsens the performance of separation and auxiliary operations; ④ The addition of defoamers (e.g., organosilicon defoamers or aluminum sulfate, as used in patent 97112703.4) increases the defoaming ability in the return water, thereby partially deactivating the added surfactants; ⑤ The addition of surfactants and defoamers significantly raises costs; ⑥ The asbestos fibers that have adsorbed large amounts of surfactants and defoamers show a marked increase in water absorption and viscosity, making final concentrate dewatering very difficult, and that patent does not indicate what special dewatering method (or technology) should be used.

In summary, the present invention aims to overcome the various disadvantages of dry processes and provide a new technology that allows a completely wet process, free from dust and toxic contamination, starting from the raw ore, and achieving high recovery, high quality, low investment and low cost. Moreover, it is fundamentally different from the wet flotation method using surfactants.

The present invention is applicable not only to various asbestos ore beneficiation plants, but also to secondary processing operations (raw fiber workshops) in asbestos product plants that require further quality improvement (sand removal, dust removal and further fiber loosening) of commercial asbestos, thereby completely eliminating secondary pollution caused by re‑processing raw asbestos in such plants. It is also suitable for recovering short asbestos fibers from tailings discarded by dry‑process plants. In addition, this process provides good conditions for chemical or surface‑chemical modification of asbestos fibers (including modification for high‑performance utilization and safe utilization). The present invention is described in detail below with reference to Figures 1 and 2.

The asbestos‑bearing raw ore mined from the mine is first coarsely screened to remove large waste rock (gangue) and then sent to the beneficiation plant, where the wet beneficiation process begins.

1. The ore entering the plant (generally –400 to 0 mm in size, depending on ore properties and capacity) is immediately soaked or sprayed with water (Operation ①). Under the condition where water has penetrated into the gaps between fibers and gangue, wet crushing or grinding methods are used to liberate the fibers (Operations ②, ③, ⑤, and ⑥). The present patent uses wet crushing and liberation starting from the ore’s entry into the plant. This is not only different from dry asbestos processes, but also different from beneficiation methods for various metal ores, coal, or other non‑metallic minerals. In those plants, coarse, medium and fine crushing are generally also dry operations; only grinding and beneficiation steps may use wet processes. The wet crushing and grinding equipment can be conventional equipment that allows water addition, such as modified jaw crushers, impact crushers, hammer crushers, roll crushers, etc., or pebble mills (autogenous mills), ball mills, or rod mills.

The water consumption for crushing and grinding depends on the ore particle size (which determines specific surface area and water adsorption capacity) and is determined by the need for good anti‑sticking properties and flowability. Generally, water consumption (ore:water by weight) for coarse crushing is 1:0.1–0.5; for medium crushing 1:0.5–3; for fine crushing 1:2–5; and for grinding about 1:2–3. Autogenous (pebble) milling, which combines coarse, medium and fine crushing, may also be used, with a ratio of 1:2–5 being suitable. All ratios are by weight.

2. All process water can be ordinary fresh water (Operation ⑳) or return water from closed‑circuit recycling (Operation ④). Fresh make‑up water may be river water, well water, mountain spring water, etc., but seawater should not be used (because it contains high chloride ions, which are harmful to the corrosion protection requirements of asbestos products). Water hardness is not specially required (though soft water is better), and the pH should be neutral (pH = 6.5–7.5).
3. A small amount of alkaline inorganic compounds or inorganic salts may also be added to the process water (fresh water and return water), such as sodium hydroxide (NaOH), limewater (Ca(OH)₂), sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃), or phosphate‑based inorganic dispersants. The dosage is such that the pH of the aqueous medium becomes slightly alkaline (pH = 8–14, optimum pH = 8.0–9.0), but surfactants need not be used. The best point for adding the inorganic alkaline agent is to the high‑concentration slurry of the cleaned concentrate after wet cleaning and thickening (Operations ⑱⑩).

Using a slightly alkaline aqueous solution is more conducive to water penetration and selective liberation of asbestos fibers than neutral water, protecting medium‑ and long‑length fibers from damage during liberation. The mechanism is that alkaline water is rich in OH⁻ groups, which have better ability to penetrate between fiber layers, swell and split the fibers. Meanwhile, all asbestos fiber crystals are hydrous minerals rich in structural water (OH⁻ groups). However, due to differences in local metallogenic geological conditions, some deposits may have insufficient hydration during mineralization, resulting in deficient OH⁻ groups, incomplete crystallization, and insufficient fiber strength. Infiltration of OH⁻ groups helps improve the strength quality of the asbestos fibers.

4. After the above steps 1, 2, and 3, most of the valuable asbestos fibers in the ore have been opened (loosened from fiber bundles), and can then be subjected to staged wet separation (separation of fibers from gangue sand).

Wet separation is a gravity concentration process carried out in an aqueous medium at higher water content and lower pulp density. The purpose of low density is to ensure that the liberated fiber flocks have sufficient space for expansion and suspension. The solid concentration by weight is 1–6%, optimally 2–4%.

In gravity concentration, the ease of separating valuable minerals from gangue is mainly determined by two factors: ① the density difference between the valuable mineral and the gangue – the greater the difference, the easier the separation; ② the difference between the density of the separating medium and the densities of the mineral or gangue – the smaller the difference, the greater the suspending power of the medium for the material to be separated, and thus the easier the separation.

In asbestos ores, unopened asbestos (lumps) has a density of 2.50–2.54 g/cm³, and gangue 2.60–2.64 g/cm³, making it extremely difficult to separate. However, after opening, the asbestos fibers expand in volume and their apparent density drops (to about 0.3–1.1 g/cm³, depending on ore properties), making them easy to separate. At the same time, the density of the water medium is much higher than that of air, meaning that water has a significantly higher suspending power for loosened asbestos fibers than air. Thus, in water, asbestos becomes a very easy‑to‑separate mineral. This is the basic principle underlying the higher efficiency of wet separation over dry separation.

5. Methods suitable for roughing of asbestos fibers (sand and impurity removal) are various gravity concentrators in which the pulp flow can keep the fibers in suspension under mechanical, centrifugal or gravitational forces (Operations ④⑦⑧). Examples include: sluices, washing pans, hydrocyclones, centrifugal concentrators, or slow‑stirred thickening funnels, stirred tanks, etc. Fan‑shaped sluices and spiral concentrators may also be used. The pulp concentration for roughing is 2–10%, optimally 3–5% (solid weight concentration).
6. The method principle for cleaning of asbestos fibers (removal of fine sand and product classification) is the same as in item 5. However, because higher fine separation efficiency is required and the fibers must be separately classified by length into long fiber, medium‑long fiber, medium‑short fiber, short fiber, and needle‑like hard‑structure fiber, various film‑flow concentrating equipment are mainly used, such as multi‑layer or single‑layer shaking tables (sand shaking tables or slime shaking tables), multi‑layer tilting tables, belt sluices and fan‑shaped sluices. Small‑diameter (Φ25–Φ125) hydrocyclones can also be used. The pulp concentration for cleaning is 1–6%, optimally 2–3% (Operation ⑩①). To improve recovery, the tailings from roughing or cleaning can be scavenged using cleaning equipment (Operation ⑨).
7. The gangue minerals from roughing or cleaning can be further subjected to wet comminution mainly by attrition and liberation, to further recover short fibers and purify the tailings. For example, edge runners, disc mills, or pebble mills, ball/rod mills, hydropulpers, etc., can be used. The pulp after further fiberizing is separated according to items 5 and 6.
8. The cleaned asbestos concentrate needs further thickening (Operations ⑫⑬). Thickening can be done by gravity sedimentation or centrifugal sedimentation. Main thickening equipment includes: thickening funnels, thickeners, settling tanks (pools), or side‑pressure thickeners, etc. The overflow from thickening is recycled as return water. The underflow pulp concentration should reach 5–16%, optimally 8–12%, for further loosening and deep splitting of the fibers.
9. Theoretically, asbestos fibers can be split along the parallel direction to very fine diameters while still maintaining good strength and elasticity. For example, the diameter of a single chrysotile fiber is only about 0.002–0.003 μm, which is unmatched by any other man‑made mineral fiber (or man‑made organic fiber). Under the premise of maintaining a high aspect ratio, making the fiber diameter very fine increases the specific surface area, greatly enhancing its ability to adsorb inorganic or organic polymer binders when used. This is an important prerequisite for fully exploiting the composite reinforcement functions of asbestos in various composite materials.

Dry separation is very difficult to achieve deep splitting of fibers, whereas wet processes, especially at high pulp densities, readily achieve deep splitting while maintaining a high fiber aspect ratio. At the same time, this operation also provides theoretical and practical justification for the full utilization of short asbestos fibers.

10. The method for deep splitting (fiber deep processing) is as follows: under high‑density pulp conditions (6–18%, optimally 8–12%), the fibers are arranged and moved in a parallel direction, and undergo scrubbing and attrition between themselves and with the grinding media, thereby achieving opening and splitting. The equipment used includes: high‑consistency refiners, dense slurry pumps, disc refiners or edge runners, etc. High‑speed beaters or hydropulpers can also be used (Operation ①⑤). The indication that deep splitting has been achieved while maintaining a high fiber aspect ratio is that, under constant pulp density, the pulp consistency increases several times, and the suspended volume looseness and specific surface area increase significantly.

Taking chrysotile medium‑short fiber of grade 5‑50 as an example: before deep splitting, looseness is 120–150 ml/5g, specific surface area 60–90 dm²/g; after deep splitting, looseness is 200–300 ml/5g, specific surface area >150 dm²/g.

11. The asbestos fibers after cleaning or further deep splitting, which contain water and mineral slime (fiber dust), must be subjected to dewatering and desliming. Dewatering and removal of mineral slime (called “dust” in dry processes) are completed in the same operation. The equipment used is the “filter‑press‑type dewatering machine” (hereinafter “squeeze‑filter”) first invented in this patent. Because asbestos fibers are one of the best sealing materials (for water, oil or gas), and because in wet operations the fibers are highly water‑absorbent and swell, and contain fine mineral slime (dust), conventional dewatering methods for mineral materials cannot be implemented on an industrial scale (e.g., various filters, pressure filters, centrifugal dewatering machines, dewatering screens or thickeners). These conventional dewatering devices have very low dewatering efficiency for asbestos, the moisture content of the dewatered product often exceeds 200–300%, and continuous operation is difficult. This has been a key reason why wet asbestos beneficiation has been difficult to industrialize for a long time.

The dewatering and desliming technology of the squeeze‑filter method of the present invention completely solves this problem. The key to its success is that the water‑ and slime‑containing fiber pulp is always maintained in an extremely thin layer (Q = 0.10–0.5 mm) during the entire main dewatering process. At the same time, through a rotary screen with mesh apertures (80–200 mesh) selected to remove mineral slime (fiber dust) of the required particle size, it first screens and squeezes out large amounts of water and slime, and then vacuum‑sucks and filters the thin layer of pulp on a felt. Because the material layer on the felt never thickens, dewatering efficiency is high, continuous, and stable. By changing the feed pulp concentration, the felt running speed, and the vacuum level of the vacuum box, the production capacity and the quality of water and slime removal can be freely controlled.

12. The operation and equipment using the squeeze‑filter for dewatering and desliming are described below with reference to Figure 2:

The pulp to be dewatered and deslimed flows by gravity into the side‑flow feed trough (part 1), which feeds the pulp to the upper side of the rotary screen (feed concentration 0.5–4.0%, optimally 1.5–3.0% solid weight concentration). The pulp passes through the rotating screen drum (part 2), with drum diameter φ500–1200 mm, outer screen mesh 80–200 mesh, 1–5 drums per machine. After preliminary screening to remove slime and water, the pulp passes through the felt (part 5) and the press roll (part 3) for further squeezing dewatering and desliming. Then, the thickened cotton material is transferred onto the felt and conveyed forward as a thin layer (7) with a thickness of 0.10–0.4 mm via the turning roll (part 4). During conveying, it is further dewatered by suction through the vacuum filter box (part 8) (vacuum degree 100–420 mmHg), and then conveyed to the breast roll section (part 9), where it undergoes a fourth dewatering by squeezing between the breast roll (φ300–450 mm) and the finished product take‑up drum (part 10). Finally, the semi‑dry cotton material after dewatering and desliming is transferred onto the finished product take‑up drum and scraped off by a scraper (part 11) to obtain a final board‑like or flake product. The final product has a moisture content of 16–25% (solid weight %).

13. According to the content and methods described in items 11 and 12, the squeeze‑filter method and equipment of the present invention are also applicable to dewatering and desliming operations in wet beneficiation of other metallic or non‑metallic minerals and coal, other than asbestos wet processes.
14. Because the largest users of asbestos products (manufacturing plants) use wet processing (e.g., 70–75% for asbestos cement products, others for wet asbestos textiles and wet friction, sealing and thermal insulation materials), the dewatered asbestos can be sold as commercial asbestos. Users only need to soak it briefly in water or stir to fully disperse it. If the user requires a loosened, dry, water‑free asbestos product, it can be further processed by conventional drying and dry loosening methods. However, whether wet or dry products, users generally do not need to perform secondary processing.
15. The slime‑bearing wastewater discharged from the dewatering and desliming operation is sent to a sedimentation tank (Operation ⑰) (or thickener, settling cone). The overflow is reused as circulating water, and the underflow can be discharged periodically or further thickened for use as a comprehensive utilization raw material for tailings.
16. Because the patented technology enables closed‑circuit recycling of process water, the beneficiation plant requires very little fresh water make‑up. The main water losses are: ① water carried away by the asbestos product (about 20% of the product); ② water lost with the concentrated slime discharge (about 30–50% of the dry slime amount); ③ water freely evaporated to the air in open processes and auxiliary operations (about 1–4% of the total ore); ④ plant sanitary water (0.5–1% of the ore quantity). Therefore, any mine (beneficiation plant) that can provide fresh water in an amount of 20–40% of the raw ore throughput is suitable for implementing the wet asbestos beneficiation process.
17. According to items 1–16 of this patent specification, the present invention can achieve the following benefits:

① Improved recovery: Based on the raw ore asbestos grade (+0.3 mm fiber content) taken as 100% (using conventional dry‑method grade determination, with wet‑sieve standard inspection), the total recovery of the wet process (+0.30 mm) is >150–200%, whereas the same ore using dry process gives a total recovery (+0.30 mm) of 70–80%; among these, the recovery of medium‑ and long‑length fibers is 6–20% higher in the wet process than in the dry process.

② Improved product quality: According to the quality standard for machine‑selected asbestos (GB8071‑87), under the premise that the main fiber content (+1.18 mm) of the same grade products tested by wet method is equivalent to the grade, the +1.18 mm main fiber content of the wet‑process product is slightly higher than that of the dry‑process product, while the –0.075 mm dust content is reduced by 20–50%.

③ Investment comparison: Under the same raw ore grade and quality conditions, the investment for a wet‑process beneficiation plant is 20–50% less than that for a dry‑process plant.

④ Production cost comparison: Under the same raw ore grade and quality conditions, the wet beneficiation cost is 20–30% lower than the dry beneficiation cost.

The following are examples of the present invention:

Example 1: Wet gravity beneficiation process for chrysotile asbestos (white asbestos)

1. Raw ore feed grade: 6.2%, feed size: –350–0 mm, raw ore moisture: 4.2%.
2. Coarse crushing and fiberizing: jaw crusher (600×400), water spray rate (ore:water) 1:0.8, reduction ratio 1:3 (product size <150 mm), pulp pH 7–8, open circuit.
3. Medium crushing and fiberizing/opening: impact crusher (single rotor φ500×400), reduction ratio 1:8–10, product size <20 mm, medium‑long fibers basically liberated, pH 7–8, open circuit.
4. Fine crushing and grinding/fiberizing/opening: pebble mill (and edge runner) (φ1200×2500), closed circuit with a spiral classifier, filling ratio 35%, ore:water = 1:3.5, spiral classifier overflow supplemented with water (1:8), pH 7–8.
5. Roughing (and scavenging): washing pan φ2.5 m, ore:water = 1:8–10, overflow sent to cleaning, sand product to scavenging or returned to fine crushing; pH 7–7.5.
6. Cleaning operations:
   ① Hydrocyclone (φ125 mm, α20°, tangential inlet velocity 22 m/s);
   ② Sand shaking table (6‑S type) (4520×1825) treating cyclone overflow, separating medium‑long fiber, short fiber, fine short fiber, and needle‑like fiber, and discarding sand tailings;
   ③ Slime shaking table (small 450×900) treating cyclone underflow, recovering medium‑short fibers from tailings, discarding sand tailings.
7. Concentrate thickening: thickening cone φ2.5 m, α30°, agitator speed 3 rpm, add NaOH to raise pH to 9.0. Overflow returned to water tank, underflow concentration 9.5%, sent to deep processing (deep splitting/opening).
8. Deep processing of concentrate: dense slurry pump, single cycle twice; dense slurry pump + disc refiner combined cycle twice (pulp concentration 9.5%, pH 7.5–8.5).
9. Squeeze‑filter dewatering and desliming: three‑drum unit, felt specification 1092 type, feed pulp concentration 2.8%, vacuum degree 320 mmHg, felt linear speed 38 m/min, dewatered product moisture 18.3%. Product output (dry) 3.5 tonnes/shift·machine.
10. 20% of the wet product, as required by the user, was dried and opened using a chain‑plate dryer (heat transfer oil heating, 80–120 °C) and then a closed‑type air opener to obtain a dry product.
11. Process water was recycled in closed circuit; comprehensive return water concentration 0.06% (weight ratio) (pH 7–8); settled underflow slime was used for comprehensive utilization or partially discharged.
12. The concentrate weight and recovery in the above process meet the quantity and quality indicators shown in item 17① and ②.

Example 2: Wet gravity beneficiation process for fibrous brucite asbestos

1. Raw ore feed grade: 7.5%, particle size –300–0 mm, moisture 4.8%.
2. Coarse crushing and fiberizing same as Example 1, but alkali solution was added in advance to make pH = 8–8.5.
3. Medium crushing and fiberizing/opening same as Example 1, but water consumption increased to 1:3.2 (ore:water weight ratio); pH = 8–8.5.
4. Fine crushing and fiberizing used a roll crusher, reduction ratio 1:4, ore:water = 1:4; pH = 8.3.
5. The +1 mm fraction of roughing and cleaning tailings was reprocessed using a wet roller mill for fiberizing and re‑separation; ore:water = 1:2–3; –1 mm fraction was discarded.
6. Roughing used hydrocyclones (Φ250 mm, α35°, tangential inlet velocity 25 m/s) in two stages.
7. Cleaning used sand shaking tables, conditions same as Example 1.
8. Concentrate thickening used a thickener (Φ12 m, center‑drive type, pH again adjusted to 8.5); overflow water was reused.
9. Thickener underflow (concentration 8–10%) was sent for deep processing fine opening, using a slurry pump – disc refiner (high‑consistency refiner) circulating for opening twice.
10. Dewatering and desliming same as Example 1; return water and slime treatment also same as Example 1.

Example 3: Wet reprocessing of commercially produced dry‑process asbestos fiber

1. The commercial fiber bags upon arrival were opened and placed into a conical tank, which could be combined with a slurry pump or high‑density slurry pump to circulate 1–2 times.
2. Then a dense slurry pump + disc refiner combination was used to circulate twice for full opening and loosening.
3. A hydrocyclone (Φ250, α20°, inlet velocity 20–25 m/s) was used for one roughing.
4. The cyclone overflow was then cleaned once using a small‑diameter hydrocyclone (Φ125, α20°, inlet velocity 25 m/s).
5. If the fiber was required for asbestos textile products and high‑quality sealing materials, it could be further cleaned once on a shaking table as in Example 1 to remove needle‑like fibers and fine sand.
6. The cyclone underflow (tailings) could be discarded directly, or could be further processed on a slime (sand) shaking table to recover needle‑like fibers.
7. The refined fiber pulp after deep processing and cleaning was thickened as in Example 1; the underflow pulp concentration was 8–12%.
8. If the sand‑removed product also required thorough desliming, a side‑pressure thickener (dewatering) was used. After dewatering and desliming, the pulp concentration was 30–50% (solid weight %), which could be directly used in the wet processes of various product plants without further dewatering.

Example 4: Recovery of short fibers from asbestos‑bearing tailings from a dry‑process plant

1. Tailings (–20 mm) were excavated from the tailings dump (site) in the conventional manner and transported to the beneficiation plant using wet sluices and spiral conveyors.
2. A spiral classifier or washing pan was used to roughly recover free fibers and secondary free fibers newly generated due to storage weathering (ore:water = 1:5).
3. The spiral sand product was further liberated using a roll crusher and edge runner.
4. The re‑liberated sand containing fibers was sent to a large‑diameter (D = 500 mm or D = 300 mm) hydrocyclone for roughing (α = 30°, tangential inlet velocity 6–20 m/s).
5. The cyclone overflow was sent to a sand shaking table for primary cleaning, then to a slime shaking table for secondary cleaning.
6. The cyclone sand product was sent to a pebble mill for fine grinding and liberation at a grinding density of 28%, filling ratio 25%, with control of discharge particle size –0.5 mm > 80%.
7. The ground product was sent to a wet low‑intensity magnetic separator (drum or belt type) to remove magnetic minerals such as magnetite and chromite (accounting for 3–8%).
8. The non‑magnetic minerals were sent to a hydrocyclone for re‑separation and a slime shaking table for cleaning, yielding medium‑short, short, and –200 mesh ultra‑short asbestos fibers, along with a final tailings reject.