In 2016, the Ministry of Ecology and Environment of the People's Republic of China issued the "Technical Policy for Pollution Prevention and Control of Solid Waste Co-processing in Cement Kilns":

　　"This technical policy is formulated to implement laws and regulations such as the 'Environmental Protection Law of the People's Republic of China,' prevent environmental pollution, safeguard ecological security and human health, standardize pollution control and management practices, promote the advancement of technologies and equipment for solid waste co-processing in cement kilns and pollution prevention and control technologies, and facilitate the green, circular, and low-carbon development of the cement industry."

　　What is co-processing of solid waste in cement kilns?

　　Co-processing of waste in cement kilns refers to the process where the cement industry utilizes modern cement production technology to produce clinker while simultaneously introducing solid waste that meets or has been pre-treated to meet the requirements for kiln entry. This process achieves the harmless disposal of solid waste while producing cement clinker.

　　What types of solid waste can be co-processed in cement kilns?

　　Waste that can be processed in cement kilns includes municipal solid waste, various types of sludge (sewage sludge, paper mill sludge, river sludge, wastewater treatment plant sludge), industrial hazardous waste, and various organic wastes (waste tires, waste rubber, waste plastics, waste oil, etc.).

　　What are the uses of solid waste in the cement production process?

　　Depending on their composition and properties, different types of waste have different uses in the cement production process, mainly including the following four aspects:

　　1. Alternative fuel: mainly high-calorific value organic waste;

　　2. Alternative raw materials: mainly low-calorific value inorganic mineral materials that can be used as raw materials for cement production;

　　3. Blending materials: waste with a single component suitable for addition during the cement grinding stage to improve certain properties of cement, adjust the strength grade of cement, increase cement production, and reduce cement production costs;

　　4. Process materials: waste that can be used as industrial materials in certain stages of cement production, such as flame cooling and exhaust gas treatment.

　　What are the effects of sulfur, chlorine, and alkali metals in solid waste on cement production?

　　• Adverse effects on cement calcination:

　　1. After decomposition in the high-temperature environment of the cement kiln, chlorine-containing dust from solid waste can come into contact with the metal structure of the kiln through gaps, damaging the kiln structure;

　　2. Alkali metals, sulfur, and chlorine elements in solid waste circulate and accumulate in the cement kiln, causing scaling in the pre-calcination system at the kiln end due to cooling and melting. In mild cases, this affects the normal production of the kiln and the quality of cement products; in severe cases, it can lead to kiln shutdown. • Impact on Clinker Quality

　　Among the elements in solid waste, sulfur, chlorine, and alkali metals, the ones that primarily harm eco-cement clinker are sulfur and alkali metals. Their harmful effects manifest in influencing clinker quality, mainly in the following aspects:

　　1. Influence of Sulfur on Clinker Strength

　　Sulfur acts in the clinker crystals in the form of SO3. On the one hand, it can lower the appearance temperature and viscosity of the clinker liquid phase, slowing down the rate of crystal nucleation while accelerating crystal growth. On the other hand, excessive SO3 easily reacts with tricalcium aluminate in the clinker to form hydrated calcium sulfoaluminate, which expands in volume, thus increasing the early strength of the cement clinker.

　　2. Influence of Alkali Metals on Clinker Strength

　　Alkali metals act in the clinker crystals in the form of R2O. They affect the strength of the cement clinker by changing the setting time, hydration effect, and rheological properties of the slurry.