In coal chemical production operations, the wastewater generated is complex in composition. If not treated and discharged directly, it can lead to severe consequences, such as severe pollution. Therefore, effective treatment measures must be strengthened. This article takes a coal chemical company as an example, using an activated carbon scheme for pre-treatment of the wastewater produced during production. Additionally, considering the company's specific situation, a more targeted and effective solid waste treatment improvement method is proposed. By reasonably constructing a closed-loop treatment process, the goal of zero wastewater discharge in coal chemical production is achieved for this company.

1. Byproduct separation process

The raw wastewater from coal chemical gasification and washing processes is first directed to the wastewater sump. After natural sedimentation, mechanical impurities and oil are separated. The wastewater is then pumped and pressurized, dividing into two streams which enter the tower separately. Inside the tower, de-acidification is performed, followed by de-ammoniation. For one stream, it undergoes heat exchange through a heat exchanger, implementing a water circulation until the temperature reaches 35°C. The treated water is then reprocessed for de-acidification and de-ammoniation before being returned to the tower for continuous feeding. This process allows for precise and in-depth adjustment and control of the tower's top temperature. For the other stream, it undergoes three heat exchange operations, reaching a temperature of 150°C, which is then used as a hot feed for the stripping tower. The acidic gases, such as H2S and CO2, collected at the tower top are cooled and sent to a separator tank for liquid separation. The resulting gas is directed to the flare, while the separated liquid returns to the phenol water tank. If the gaseous phase leaving the tower has low ammonia and water content, it can be sent directly to the flare or gas cabinet without cooling.

2. Issues Identified

After a period of operation, instability was identified during the process, particularly with the heat exchanger section, which exhibited abnormal scaling conditions. Additionally, the temperature did not reach the originally set target temperature. Furthermore, the specific steam consumption experienced continuous fluctuations, showing a continuous and unstable upward trend. From the perspective of the de-acidification and de-ammoniation tower, the internal scaling was severe, leading to significant blockages in the float valve tower components. This, in turn, impacted the initial water quality treatment, causing research and development delays. This unit operated for less than one month, with a progressively shorter operational cycle. Analysis revealed that the poor quality coal used was the cause. The coal quality led to a sustained increase in ash content in the gas, resulting in a continuous rise in organic suspended solids and dust in the wastewater. During heating, most surfaces of the heat exchange equipment experienced varying degrees of deposition, forming complex scale. As this scale accumulated, it blocked the heat exchangers, rendering them non-flush and severely affecting the normal operation of the unit.

3. Solution

In terms of solutions, some relatively new tower internals can be selected for replacement. For heat exchangers, timely cleaning is necessary. Additionally, detailed identification is required for scaling temperatures, and a thorough assessment of the conditions for scaling occurrence. In actual operation, deep pre-treatment methods can be chosen to forcibly treat the filtration equipment, significantly reducing or lowering inorganic salts in the water. Moreover, effective measures should be taken to minimize the scaling of suspended solids and convert some indirect heating to direct heating.

4. Fundamental Principle Analysis

4.1 Deep Preprocessing Mandatory Filtration Device

Currently, among the commonly used activated carbons, the structural characteristics of activated carbon焦, with its well-developed mesopores, are particularly highlighted by a reduced iodine value and a significantly increased sugar molasses value and methylene blue value. In practical applications, its standout features include the ability to adsorb macromolecules and long-chain organic substances. Due to its inherent resource advantages in this area, it has certain advantages in production efficiency and cost compared to crushed carbon, with pricing only a fraction of that of activated carbon. Consequently, when considering raw material costs, it can achieve a substantial reduction in process operation costs. Activated carbon焦 can continuously adsorb solutes in water until it reaches a corresponding equilibrium state. Analyzing from the perspective of temperature, if it remains constant, the unit weight of activated carbon焦 adsorption, when the adsorption operation is performed and the system is at equilibrium, forms a curve relationship with the solute concentration in water, which is commonly referred to in the industry as the adsorption isotherm. The curve formula is: x/m = kc^1/n, where m represents the weight of added activated carbon焦, x denotes the amount of solute adsorbed by the activated carbon, and k and n are constants obtained from the experiment, while c represents the solute concentration in water.

4.2 Application of Activated Carbon in Water Treatment

For activated carbon, its application in potable water is primarily for odor removal. When reservoirs and lakes remain stagnant for extended periods, they develop odors, while swamp water takes on a muddy taste. Activated carbon effectively eliminates these smells. Currently, powdered activated carbon is most commonly used, which is added to coagulation and sedimentation ponds and then discharged through specific pipelines along with sludge. Activated carbon can remove organic matter and odor-causing substances from water, such as detergents, phenols, benzene, chlorine, and others. Additionally, it exhibits good adsorption capabilities for ions like bismuth, tin, mercury, lead, chromate, cyanide, and antimony. In this process, granular activated carbon is used as a filter medium, and it requires regular and repeated flushing during operation to remove suspended particles and prevent head loss. The spent carbon in the moving bed is discharged through the bottom of the pool, while fresh activated carbon is replenished promptly. After the adsorption capacity of granular activated carbon is exhausted, regeneration is typically achieved through heating, drying the waste carbon, and then subjecting it to a detailed regeneration roasting process at 850°C. For granular activated carbon, the loss is approximately 5-10% each time, and its adsorption capacity gradually decreases. Activated carbon can reduce the amount of suspended solids entering the heat exchanger and also decrease the organic content, thereby playing a pre-treatment protective role, ensuring the continuous proper operation of the wastewater treatment core facilities. Moreover, activated carbon that is converted into solid waste is also a good circulating fluidized bed fuel, which can effectively eliminate environmental pollution.

5. Conclusion

In summary, following the aforementioned modifications, the equipment has operated relatively stably, with minimal initial investment costs. After the plastic molds undergo pretreatment operations, the water quality at the outlet is quite excellent, meeting relevant regulations and requirements. This lays a solid foundation for meeting emission standards. This article, by appropriately improving the pretreatment process of coal chemical wastewater, has significantly enhanced both processing efficiency and quality outcomes, demonstrating good application value.