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Industrial wastewater, which is the main source of wastewater, presents greater challenges in treatment compared to domestic sewage; the treatment processes are more complex, resulting in severe water pollution. With the rapid development of industry, the types and quantities of wastewater are increasing sharply, leading to increasingly widespread and severe pollution of water bodies, which threatens human health and safety. This article will detail the treatment methods and principles for coal washing wastewater, one type of industrial wastewater, to serve as a reference for everyone. 1. Generation of coal washing wastewater: Wet coal processing requires a large amount of water. Taking jigging coal washing as an example, approximately 3–5 m3 of recycled water is needed for every 1 ton of raw coal processed, with additional fresh water also required. After the washing process, this water contains a large amount of fine particles; coal washing water containing suspended particles with a particle size of less than 1 mm is commonly referred to as slurry water, or coal washing wastewater. Coal washing wastewater can be divided into two categories: one is the wastewater generated during the washing of high-quality raw coal, and the other is the wastewater produced when washing raw coal with a shorter geological age and higher levels of ash and impurities. The particles in the first type of wastewater are larger in size and at lower concentrations, making it relatively easier to treat. This type of coal washing wastewater can be effectively treated using the physical method of concentration and sedimentation; the separated clean water can be reused for coal washing. The latter type of coal washing wastewater contains a large amount of coal slime particles; these particles are not only small in size and high in concentration, but also have a strong negative charge on their surfaces, resulting in a stable colloidal system. Therefore, this type of coal washing wastewater does not settle over time and is difficult to treat. The treatment of this type of coal washing wastewater generally employs coagulation and sedimentation methods at present. 2. Major pollutants in coal washing wastewater: The main pollutants in coal slurry wastewater include suspended solids, oils, and organic chemicals. 1. Suspended solids: Coal washing wastewater contains a large amount of suspended solids, which are a major type of pollutant. These pollutants in coal washing wastewater consist of fine mineral particles such as kaolin and coal dust. Since the organic matter in coal is mainly black and possesses special color-changing properties, it causes changes in the color of the water quality as well as poor light transmission. 2. Oil substances: The extensive use of flotation agents such as light diesel and kerosene, along with oil leaks from equipment and cleaning processes, result in oils remaining in coal. During coal cleaning, the washing water thus contains various amounts of oil substances. 3. Organic chemicals: During the coal washing process, slime water is typically subjected to a closed-loop treatment cycle that includes filtration, dewatering, concentration, and flotation. Various chemicals such as inhibitors, flocculants, and foaming agents are added during this process, and they remain in the coal washing wastewater, causing water pollution. Regarding the aforementioned coal slime water pollutants, suspended solids and pH in coal washing wastewater can be selected as the main pollutants along with corresponding control parameters for treatment. Coal washing water may contain elements such as manganese and iron; therefore, our country includes these two pollutants present in coal washing water within the specified limit values. III. Current Status of Wastewater Treatment in Coal Washing Plants at Home and Abroad. The goal of treating coal washing wastewater is to separate the sludge from the water; this means not only obtaining clean water that meets the standards required for coal washing and enabling a closed-loop circulation of such water, but also producing coal sludge with a low moisture content that is easy to dehydrate. With the growing environmental awareness and increasing environmental requirements in recent years, departments at all levels as well as coal washing enterprises now demand that coal washing wastewater must be subjected to closed-loop recycling. However, due to the difficulty in treating coal washing wastewater, over the years, energy and chemical industry experts as well as environmental protection experts from around the world have consistently regarded the treatment and reuse of such wastewater as a key focus in industrial wastewater management, conducting specialized research on this topic. 1. Current status of coal washing wastewater treatment abroad: At present, some major coal-producing countries around the world, such as Russia, the United States, Germany, the United Kingdom, Australia, Ukraine, South Africa, Poland, etc., have basically achieved zero discharge of coal washing wastewater, and the separated slime is also utilized effectively. The reason is that these **coal slime separation devices have excellent performance; the quality of the raw coal fed into them is generally good, which results in coal washing wastewater with a low concentration and low ash content. As a result, the treatment effect is good, and a closed-loop system can be basically achieved. However, there is little research on **technologies for treating high-concentration coal washing wastewater, and no highly developed experience exists in this area. Moreover, the coagulants used for treating coal washing wastewater are relatively limited, mainly organic polymer coagulants; due to their high cost, the expense of using them alone for treatment is also substantial. Therefore, the treatment of coal washing wastewater in China cannot rely on foreign experiences; instead, efficient and low-cost treatment technologies must be developed based on China’s actual conditions to meet the demands of the rapidly growing coal chemical industry. 2. Current status of coal washing wastewater treatment in China: Although China has carried out renovations to the coal washing wastewater treatment systems in coal processing plants in recent years, due to factors such as the large volume of raw coal processed and imperfect coal processing technologies, there are still many problems in the treatment of coal washing wastewater. Currently, the use of coagulants and flocculants to treat coal washing wastewater is a common treatment method in China. The agents employed include inorganic salt coagulants, organic polymer flocculants, and inorganic polymer flocculants. Common inorganic salt coagulants mainly include aluminum salts, iron salts, etc. Generally, the use of inorganic salt coagulants alone is insufficient to ensure effective treatment of coal washing wastewater. The commonly used organic polymer flocculants are mainly polymers or copolymers of polyacrylamide or its derivatives. In practical engineering, inorganic salt coagulants are often used in combination with organic polymer flocculants, or inorganic polymer flocculants such as polyferric sulfate, polyaluminum chloride, polysilicate sulfate, and polyaluminum ferric chloride are employed to treat coal washing wastewater. In order to examine the sedimentation effects of three different coagulants—lime, basic aluminum chloride, and polyaluminum ferric chloride—on the slime water in coal preparation plants, Li Ruiqin conducted comparative tests on the sedimentation of slime water using equal amounts of these three agents. The experimental results showed that under the same conditions, the slurry water with polyaluminum ferric chloride (PAFC) added had the fastest sedimentation rate and the lowest concentration in its supernatant. By using ferrous sulfate as a raw material and employing solid-phase chemical reactions, Bai Qingzi developed a new type of iron-based inorganic polymer water treatment agent. When this agent is used in combination with polyacrylamide (PAM) to treat coal washing wastewater, it achieves good particle aggregation, rapid sedimentation, and results in water of relatively high clarity, with a light transmittance of 83%; it can also be reused. The by-product, coal sludge, can be used to produce briquettes, thereby saving binders and additives. Li Yafeng and others conducted experimental studies on the treatment of coal washing wastewater using calcium carbide slag and polyacrylamide (PAM). Experimental results and practical engineering applications show that the combined use of calcium carbide slag and polyacrylamide yields a good coagulation effect on coal washing wastewater. When the dosage of polyacrylamide reaches 20 mL, the clear water separation rate is 40%, and the specific resistance of the sludge drops to 1.8×1010 m/kg. Moreover, all the parameters of the treated coal slime water meet the **discharge standards, as well as the water requirements of the coal washing process. The experiments also showed that polyacrylamide alone has no flocculating effect on coal washing wastewater, but it does exhibit a flocculating effect when used together with calcium carbide slag, indicating that calcium carbide slag plays a very important role in the coagulation of coal washing wastewater. Li Chunxiao and colleagues studied the use of a naturally modified plant cationic flocculant, prepared by graft copolymerization with an etherifying agent using plant wood powder as a raw material, to treat coal washing wastewater; this flocculant is easy to degrade and does not cause secondary pollution. Studies have shown that at room temperature, when pH is 6–8 and the addition amounts of this flocculant and polyalumina are 10 mg/L and 5 mg/L respectively, the removal rates of turbidity, CODCr, and solid suspended matter in coal washing wastewater can reach 96.8%, 69.3%, and 97.9% respectively. Liu Kefeng and others used the newly developed coagulants BH-1 and BH-2 to treat coal washing wastewater. The composition of the new coagulant BH-1 mainly consists of organic polymer flocculants, while BH-2 is primarily composed of inorganic coagulants. Experimental results show that the new composite coagulant, which is a mixture of organic and inorganic components, exhibits excellent performance in treating coal washing wastewater. Compared with other coagulants, it achieves faster flocculation and forms larger flocs. When added at a concentration of 19.3 mg/L, and in a ratio of approximately 1:6 between BH-1 and BH-2, its coagulation efficiency reaches its optimal level. When this new coagulant is used in coagulation tests, the sedimentation time is short; significant results are already observed after 15 minutes, with a turbidity removal rate of over 96.50%. Moreover, compared to similar coagulants, it is cheaper to use for treating coal washing wastewater, at around 0.19 yuan per m3, offering good economic benefits and making it suitable for large-scale application. The inorganic polymer iron-calcium-aluminum coagulant (PFCA) developed by Fu Jianzhong and others exhibits excellent performance in treating coal washing wastewater, and it also reduces the amount of PAM required. It can be seen that the biggest challenge in the treatment of coal washing wastewater at present is how to select appropriate treatment chemicals, so as to not only obtain recycled water for coal washing but also coal slurry with a high calorific value. IV. Conventional treatment methods for coal washing wastewater: In the 1960s and 1970s, many coal processing plants discharged the slime water directly into slime sedimentation tanks, with some of it being released outside the plant. In some cases, simple sedimentation is carried out to recover coal slurry, and the clarified water is then reused or discharged. Small coal slurry particles, clay particles, and the like in wastewater are difficult to settle. As a result, the coal slime particles become increasingly fine during the circulation process, which leads to an increase in the SS concentration of the circulating water. This severely affects or even disrupts the normal coal washing process; in such cases, it is necessary to discharge some of the wastewater with high concentrations or add large amounts of clean water for dilution. This also leads to an imbalance in the washing process, preventing the circulation of the cleaning water. As a result, it causes environmental pollution and results in the loss of coal sludge, leading to waste of resources. 1. Coagulation sedimentation method: This is the most commonly used method for treating coal washing wastewater at present, and it is a widely applied technique. In the actual treatment process, different flocculants should be used depending on the properties of the coal washing wastewater. The coagulation-sedimentation method shows excellent treatment efficiency for coal washing wastewater, with a high removal rate of SS. Compared with gravity concentration precipitation, it can effectively ensure the closed-loop circulation of washing water; even when wastewater is discharged, it generally meets the discharge standards, offering many advantages. However, the flocculants used in conventional coagulation sedimentation methods require large quantities, resulting in high chemical costs, and they also pose a certain degree of corrosion to water treatment equipment. The resulting flocs have low density and a high moisture content, which also hinders the progress of filtration and pressure filtration dehydration processes. The main characteristics of coal slime water are high concentration, fine particle size, high ash content, and the surfaces of most particles carrying a negative charge. The repulsive force between like-charged particles keeps these particles in a colloidal dispersed state in water; they are affected not only by gravity but also by Brownian motion in water. Furthermore, the interactions between solid particles at the interfaces in slime water complicate its properties; it exhibits not only characteristics of a suspension but also some properties of a colloid. It is precisely because these colloidal particles are charged that they prevent the caking together of coal slurry particles, and this prevents the coal washing wastewater from aggregating naturally. Therefore, in order to achieve the separation of coal washing waste cement from water, it is necessary to add an appropriate coagulant to disrupt the stability of the colloids and reduce the zeta potential. Coagulation sedimentation is a water treatment method in which, under the action of coagulants, colloids and fine suspended particles in wastewater are aggregated into flocs, which are then separated and removed. The coagulation and clarification method is widely used in water treatment; it can not only reduce sensory parameters of water quality such as turbidity and color, but also remove various toxic and harmful pollutants. 2. Gravity concentration and sedimentation method: In water treatment using the gravity concentration and sedimentation method, processes such as sedimentation tanks and concentration units are commonly employed. In the slurry water treatment processes of some coal washing plants belonging to Huaibei Mining Group, in some cases the slurry water enters the thickener via a skimming pit, with the solid content in the overflow being no more than 10 g/L; other systems use inclined tube sedimentation tanks, and after treatment the flow rate of the slurry water is 350 m3/h, with three square tanks of 30 m2 each being used; the surface load reaches as high as 4 m3/(m2·h), while the SS concentration in the coal washing wastewater is 26.67 g/L. In Japan, some coal washing plants use two deep-cone thickeners, with the overflow water being further treated in sedimentation tanks; others incorporate inclined plates in the rake thickeners, reducing the SS concentration in the overflow water from 150 g/L to 50 g/L. It can be seen that when the natural sedimentation method is used to treat coal washing wastewater, the surface load is low and the land area required is large; moreover, the concentration of suspended solids in the treated wastewater remains high, failing to meet the concentration standards for wastewater discharge. However, the coagulation sedimentation method and gravity concentration sedimentation method also have certain drawbacks; therefore, selecting an effective sedimentation and concentration technique is also a key issue in properly treating coal washing wastewater and achieving closed-loop wastewater recycling.