How to utilize the waste residues from calcium carbide production

For cement, you can get in touch with Xinjiang Tianye

All these materials can be used as raw materials for cement; Tianye produces 120 tons of calcium carbide slag each year, and all of it cannot be utilized

Our company used to produce cement, and it is now used for polyether saponification.

The Hefei Institute and the Xinjiang Institute have carried out designs for calcium carbide slag cement, with practical project examples available

It can also be used for landfilling to build roads: lol

Used to neutralize acidic wastewater; the titanium dioxide industry will be interested in your product

There are usually accompanying projects; nowadays, most of them come with cement production lines

Last edited by That makes sense on 2012-7-11 21:07. Treatment and Comprehensive Utilization Methods for Calcium Carbide Waste. Abstract: This article explains the formation mechanism of calcium carbide waste generated during the production of PVC using the calcium carbide-acetylene method, the composition and properties of dry calcium carbide waste, as well as the methods for treating this waste； It also provides a comprehensive discussion on the advantages and disadvantages of various treatment methods. Calcium carbide waste and its leachate contain toxic and harmful substances; they should be classified as Category II general industrial solid wastes. When stored or disposed of, anti-seepage measures must be taken and the waste must be landfilled. Keywords: calcium carbide slag; treatment; comprehensive utilization; production of propylene oxide; potassium chlorate I. Overview Acetylene (C2H2) is one of the important raw materials in the basic organic synthesis industry. The process of producing acetylene from calcium carbide (CaC2) by adding water (wet method) is simple and well-established; it has been in use industrially for over 60 years, and it still plays a significant role in China today. By adding water to 1 ton of calcium carbide, more than 300 kg of acetylene gas can be produced, along with 10 tons of industrial waste liquid containing about 12% solids, commonly known as calcium carbide slurry. Its disposal has been a headache for the manufacturing plant. The AirReduction Plant in Louisville, Kentucky, United States, has been producing commercial acetylene since 1941, with a production volume of 600,000 pounds of C2H2 per day in 1963. The calcium carbide slurry yard of this plant covers an area of 100 acres, with a pile height of 100 feet. It suddenly collapsed in the winter of 1963, with the calcium carbide slurry burying the PVC production equipment located next to the famous Goodrich plant; this incident became a major news story in the United States that year, resulting in compensation payments of $1 million. Acetylene is the main raw material for producing polyvinyl chloride resin (PVC). Based on production experience, 1.5–1.6 tons of calcium carbide are required to produce 1 ton of PVC products. Meanwhile, 1.2 tons of calcium carbide slag (on a dry basis) are generated per ton of calcium carbide. Assuming that the moisture content of this calcium carbide slag is 90%, approximately 20 tons of calcium carbide slag slurry are produced per ton of PVC manufactured. It can be seen that the amount of calcium carbide slurry produced **exceeds the production volume of PVC. Most PVC manufacturers recycle the supernatant after separating the calcium carbide slurry through gravity sedimentation ； After further dehydration, the calcium carbide slag still has a moisture content of 40% to 50%, giving it a paste-like consistency; it tends to leak and contaminate road surfaces during transportation. Piling it up over time not only occupies large amounts of land but also causes severe erosion of the soil. To resolve the problem fundamentally, it is necessary to achieve technological breakthroughs, seek new treatment processes, make comprehensive use of resources, and turn harm into benefit as well as waste into treasure. II. Mechanism of formation of calcium carbide waste residue In the production of polyvinyl chloride (PVC) using the calcium carbide-acetylene method, calcium carbide (CaC2) reacts with water to produce acetylene and calcium hydroxide. The main chemical reaction is as follows: CaC2 + 2H2O → C2H2 + Ca(OH)2 + 127.3 kJ/mole. While calcium carbide reacts with water, the impurities present in it also participate in the reaction to form calcium hydroxide and other gases: CaO + H2O → Ca(OH)2; CaS + 2H2O → Ca(OH)2 + H2S↑; Ca3N2 + 6H2O → 3Ca(OH)2 + 2NH3↑; Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3↑; Ca2Si + 4H2O → 2Ca(OH)2 + SiH4↑; Ca3As2 + 6H2O → 3Ca(OH)2 + 2AsH3↑. Calcium hydroxide has low solubility in water, so solid Ca(OH)2 particles gradually precipitate out of the solution. The entire system transitions from a true solution to a colloidal solution and then to a coarse dispersion; the microparticles gradually merge, coagulate, and precipitate. During this precipitation process, collisions and compression between particles cause the particles to further aggregate, grow in size, and lose water, resulting in the precipitate becoming increasingly viscous – this substance is commonly known as calcium carbide slurry. In addition, solid impurities in calcium carbide that do not participate in the reaction, such as ferrosilicon and coke, are also mixed in the slurry. The gas generated as a side reaction partially enters the acetylene gas, while partially dissolves in the slurry. Calcium carbide slurry is a gray-brown, turbid liquid. After standing, it is divided into three parts: the clear liquid, the solid sediment layer, and the intermediate colloidal transition layer. The ratio of the three varies reversibly with standing time and environmental conditions. Solid sediment is what we commonly refer to as calcium carbide waste residue. III. Composition and properties of calcium carbide waste residue: After the calcium carbide slurry is fed into a concentration tank for sedimentation, the supernatant is taken for analysis; its composition is shown in Table 1. Table 1: Composition of the carbide slag supernatant – mass concentration, mg/l: PHSS, COD, S2-, PC2H2, Ca(OH)2. On December 14th, the values were 500 for PHSS, 200–400 for COD, 100–400 for S2-, 100–400 for PC2H2, and 0.1–1.0 for Ca(OH)2; meanwhile, the values for 500 were 150–200, and those for 900 were 700–1700. As can be seen from Table 2, the carbide slag slurry wastewater is highly alkaline; it also contains toxic and harmful substances such as sulfides and phosphides, with several parameters exceeding the **emission standards. Among them, after sulfides are discharged into rivers, they react with dissolved oxygen in water in the presence of bacteria to form sulfates: S2- + O2 → SO42-. The content of Ca(OH)2 in calcium carbide waste residue is greater than 95%. Its composition is similar to that of ordinary slaked lime. Sidhehurst Chemical Company believes it fully meets Sidhe’s standards. The composition of the dried calcium carbide waste is detailed in Table 2. Table 2: Composition of Dry Calcium Carbide Waste (% wt, on a dry basis) | Component | Content | Ca(OH)2 | 96.3 | SiO2 | 1.41 | Al2O3 | 1.33 | CaSO4 | 0.34 | C | 0.14 | Fe2O3 | 0.12 | CaS | 0.08 | CNS | –0.01 | CaCO3 | Trace amounts | Cl | Trace amounts | P | Trace amounts. As can be seen from this table, dry calcium carbide waste mainly contains Ca(OH)2, which can be used as a substitute for slaked lime and is widely applied in industries such as construction, chemicals, metallurgy, and agriculture. However, when the moisture content of calcium carbide waste slag exceeds 50%, it takes on a thick paste-like form, making storage and transportation difficult and causing inconvenience to users. Many factories also cause great annoyance by contaminating road surfaces during transportation. Therefore, the key to the comprehensive utilization of calcium carbide waste residue is controlling the moisture content. Calcium carbide waste residues and leachates containing a certain amount of water are also highly alkaline, and they contain toxic and harmful substances such as sulfides and phosphides. According to the \"Criteria for Identifying Hazardous Wastes\" (GB5085—1996), calcium carbide waste belongs to Category II general industrial solid wastes ； If it is discharged directly into seawalls or valleys and piled up in a systematic manner by filling in land or ditches, according to the \"Design Specifications for Chemical Waste Landfills\" HG20504—92, for Class II general industrial solid waste, anti-seepage measures must be taken and the waste must be disposed of through landfilling. IV. Methods for treating calcium carbide waste 4.1 Landfilling and filling in ditches: Some factories located in coastal or mountainous areas have traditionally discharged calcium carbide waste directly into sea dikes or valleys, where it is piled up in an orderly manner with almost no anti-seepage measures taken. This method requires a large amount of space and causes severe pollution. 4.2 Sale after natural settlement: Most factories use the natural settlement method. The calcium carbide slurry is discharged into a sedimentation tank or a low-lying area, where it evaporates naturally until the slurry settles; thereafter, it is excavated manually or using loaders and grabbers for sale. The stacking area also lacks anti-seepage treatment. The treatment effect of the natural settlement method is unstable and is affected by environmental and meteorological conditions. Especially in the south, where rainfall is high and evaporation is low, the sediment during the rainy season contains a high amount of water, typically 50% to 60%, and appears as a thick paste. It is simply impossible to excavate and utilize. 4.3 Using calcium carbide waste residue instead of limestone in cement production: There are many established enterprises in China that utilize calcium carbide waste residue for cement production, such as Jilin Chemical Plant, Tianjin Chemical Plant, Guizhou Organic Chemicals Factory, and Shanxi Chemical Plant. Some of these enterprises built industrial-scale facilities as early as the 1970s, with dedicated cement production lines designed to process calcium carbide waste residue. For example, Jihua Company uses concentration tanks to increase the slurry concentration from 5%–8% to 35%, then feeds it into a feed tank. After separating some of the supernatant, it is mixed with sandstone and clay slurry to form cement raw material, which is subsequently sent to a rotary kiln for cement production. According to investigations, there are currently many production lines in operation across the country. The wet process is generally used for production (with a few facilities employing vertical kilns). Compared to the wet process that typically uses limestone as the main raw material, since calcium carbide slag has a high water content and poor fluidity, its water content is kept at around 56% in order to ensure the fluidity of the slurry when it enters the kiln. The moisture content of the raw material used in wet-process kilns with this type of limestone mixture is 50%–55% higher than that of ordinary limestone mixtures; as a result, the heat consumption is about 20% higher than in conventional wet-process systems. For example, the cement plant of Jihua Company produces 100,000 to 200,000 tons of cement per year, with a clinker firing heat consumption as high as 7,955 kJ/kg.cl. Furthermore, due to the high moisture content of the feed slurry entering the kiln, the preheating section of the kiln has to operate under heavy load; as a result, the temperature of the exhaust gas at the exit of the kiln is 50–60°C lower than that in conventional wet-process production lines. Consequently, the output of this kiln is 20%–25% lower compared to production lines using limestone as the raw material and having similar specifications. Furthermore, the decomposition of Ca(OH)2 produces water vapor, which increases the water vapor content in the exhaust gases at the exit of the kiln. This affects the efficiency and lifespan of electrostatic precipitators. As a result, in areas rich in limestone, cement produced from calcium carbide waste is less popular in the market compared to cement produced from limestone. In accordance with the requirements set out in Document No. 49 issued by the General Office of the State Council in 1999, no new cement enterprises are to be established. A policy of \"total volume control and structural adjustment\" has been implemented within the building materials industry. This indicates that, first, the cement market is already saturated; second, building materials and the chemical industry are distinct sectors. Under structural adjustment, the cement market in the building materials sector does not give way to the cement market in the chemical industry. There is no such industrial policy – instead, companies must compete with those that are more competitive. In an environment where the market is already saturated and competitors are stronger, it is difficult to survive. Due to the complex technology, high energy consumption, large land requirement, substantial investment, market saturation, and weak competitiveness associated with cement production, in a market economy it is the market that determines whether a company can survive or not. Once the market conditions deteriorate, production becomes impossible; therefore, the use of calcium carbide slag in cement production introduces additional constraints that hinder the development of such companies. 4.4 Production of quicklime as a raw material for calcium carbide: As mentioned earlier, the AirReduction Plant in Louisville, Kentucky, United States, realized early on the urgency of dealing with calcium carbide sludge. In 1948, a test unit capable of producing 60 tons of quicklime per day was built. Two 330 t/a quicklime production units were built between 1959 and 1962. It operates safely, with nearly 350 operating days per year. Process for producing lime from calcium carbide: After dehydration, calcium carbide waste with a solid content of 60% is obtained; it is conveyed using a screw conveyor and distributed evenly within the granulator at three-quarters of its length. Granulation takes place to form spheres of various sizes ranging from 5 to 20 mm. These spheres are then dried in a pneumatic dryer at 350°C, and subsequently calcined in a rotary furnace at 900–1000°C. The drying of the material inside the dryer is accomplished using the hot exhaust gas from the rotary furnace. The calcined recycled lime flows into the discharge hopper and is loaded onto vehicles for transport to the calcium carbide plant as raw material for calcium carbide production. The specifications for lime products are as follows: CaO content of not less than 86%, CO2 content of not less than 1.0%, moisture content of 0.5%, and impurities (Fe2O3, H2, SiO3) at not more than 13%. The particle size ranges from 5 to 20 mm. As for raw materials and energy consumption (for producing 1 ton of lime), the requirements are as follows: calcium carbide slag – 1.33 tons, water – 8 m3, electricity – 37 Kwh, steam – 0.16 tons, coal powder – 0.111 tons, nitrogen gas – 3 m3, fuel gas – 388 m3, and fuel oil – 0.001 tons. This technical approach is feasible; it represents the best method for producing lime. The reasons for this are as follows: first, the investment required for producing lime is less than one-tenth of that needed for producing cement; second, lime serves as a raw material in calcium carbide production, so there is no need to find an alternative market, allowing for a closed-loop cycle of calcium carbide waste – lime – calcium carbide – calcium carbide waste; third, by reducing constraints, the PVC production process using calcium carbide can be scaled up further, thereby enhancing competitiveness. At the same time, this method also helps to protect limestone resources. The economic and social benefits associated with producing lime from calcium carbide waste far exceed those of other treatment methods. However, it consumes a lot of energy, and the recycled lime that can be used as raw material for calcium carbide can only account for 20% of the total calcium carbide raw material – too much of it cannot be used, as the recycled lime contains many sulfur and phosphorus impurities, which affect the quality of calcium carbide. 4.5 Production of lightweight bricks: Shandong Cement Products Factory has successfully developed a method for producing lightweight cinder bricks using carbide waste residues, and the quality of these bricks meets the standards of similar products. This brick is made primarily from concentrated waste calcium carbide slag (with 39.6% moisture content), with a small amount of cement added, along with crushed cinder (particle size