A sludge dehydration conditioning method for cement kiln fuel

Abstract

The present invention discloses a sludge dehydration and conditioning method for cement kiln fuel. The method comprises the following steps: a high-valent metal sulfate solution and modified waste concrete are used to jointly condition municipal sludge with a moisture content of 80%. The conditioned sludge is then mechanically dehydrated for 30 minutes by thin-layer filter pressing. The moisture content of the obtained mud cake can be reduced to below 40% after being placed at room temperature for 12 hours, thereby greatly improving the calorific value of the sludge. The calcined sludge can be used as a cement clinker component, thereby effectively realizing the resource utilization of the municipal sludge.

Description

Technical Field

[0001] The present invention relates to a dehydration and conditioning method for cement kiln fuel sludge. The method adopts a high-valent metal sulfate solution and modified waste concrete for chemical conditioning. The combustion calorific value of the sludge as cement kiln fuel is increased by reducing the sludge moisture content. At the same time, the calcined sludge can be used as a cement clinker component. The method belongs to the field of solid waste resource disposal technology. Background Art

[0002] Cement kiln co-processing is a waste disposal method based on the cement industry, achieving harmless disposal of solid waste while simultaneously producing cement clinker. Cement kilns provide an ideal disposal vehicle for municipal sewage sludge. During sewage sludge co-processing in cement kilns, organic matter can be used as biomass fuel, while the main inorganic elements Si, Fe, Al, and Ca are utilized as cement raw materials. Organic nitrogen and ammonia nitrogen act as NOx reducers, reducing denitrification costs. Other heavy metals in the sludge can be solidified in the cement clinker, achieving high-value conversion of the sludge while completely eliminating its environmental risks. However, current cement kiln co-processing technologies for municipal sewage sludge disposal face challenges in effectively utilizing the sludge's resource properties. For example, moisture in the sludge entering the kiln increases heat consumption, impacts system stability, and reduces clinker production. Excessive iron content in the sludge impairs cement sintering and hardening, thus limiting the amount of sludge that can be co-processed.

[0003] High moisture content in municipal sewage sludge not only increases its volume but also negatively impacts subsequent storage, transportation, processing, disposal, and combustion calorific value. Typically, the moisture content of dehydrated municipal sewage sludge is around 80%. Directly feeding it into cement kilns will not fully burn out, failing to effectively replace fuel and even increasing the system's coal consumption. Statistics show that 120 to 150 kg of coal is required for every ton of sludge with an 80% moisture content. Calorific value within the kiln is only balanced when the sludge moisture content is below 50%. Therefore, deep dehydration of sludge with an 80% moisture content not only reduces its volume but also lowers subsequent processing costs. On the other hand, existing sludge conditioning agents primarily aim to reduce sludge volume through dehydration. However, because the ratios of calcium, aluminum, and iron in the sludge do not meet cement kiln calcination requirements, they cannot effectively convert the sludge into cement clinker. Therefore, the key to improving the efficiency of cement kiln co-processing is to find and develop a low-cost, high-efficiency sludge dewatering and conditioning method for using sludge as cement kiln fuel and cement alternative raw material. Summary of the Invention

[0004] The present invention addresses the problems of the prior art and provides a method for dehydrating and conditioning sludge for use as cement kiln fuel. By using an acidic mixture of high-valent metal sulfates and alkaline waste concrete, the method effectively reduces the sludge's moisture content and increases its calorific value. The sludge, calcined in the kiln, can then be used as a component of cement clinker.

[0005] The sludge dehydration and conditioning method for cement kiln fuel of the present invention adopts a high-valent metal sulfate solution and modified waste concrete to jointly condition municipal sludge with a moisture content of 80%, and then mechanically dehydrates the conditioned sludge by thin-layer filter pressing for 30 minutes. The moisture content of the obtained mud cake can be reduced to below 40% after being placed at room temperature for 12 hours.

[0006] The high-valent metal sulfate solution is a mixed solution of ferric sulfate and aluminum sulfate in a certain proportion with 10% sulfuric acid, and the total metal ion concentration in the mixed solution is 1 mol·L -1 The molar ratio of iron to aluminum is 1:0 to 1, further 3 to 5:1. The total amount of iron and aluminum is 0.5 to 3.0% of the weight of the absolute dry sludge.

[0007] The waste concrete is derived from concrete of an initial grade of C40, with the fine powder having a particle size of less than 80 mesh. The waste concrete is modified by heat treating it in a muffle furnace at 600°C for 2 hours. The amount of the waste concrete used is 6-15% of the weight of the absolute dry sludge.

[0008] The initial thickness of the mud cake of the thin layer filter press is less than 5 mm, and the filter press applies pressure by a gradient pressurization method, that is, starting from 2 MPa, increasing by 1 MPa every 2 minutes, and then maintaining constant pressure at 6 MPa.

[0009] Existing sludge-cement kiln co-treatment technologies mainly treat and dispose of wet sludge (80% moisture content) directly into the kiln or into the kiln after pretreatment. Direct sludge entering the kiln brings in a lot of water, which greatly limits the sludge treatment capacity. Pretreatment generally uses mechanical dehydration or thermal drying. At present, the mechanical dehydration agent for municipal sludge with a moisture content of 80% in China usually uses acidic polymeric iron salts. Due to the large amount of agent used, the acidity of the sludge after treatment is high, which not only increases the risk of equipment corrosion, but also the excessive iron content in the sludge causes cement sintering and hardening. Another commonly used treatment agent is CaO, which is not only expensive, but also combines with water to form Ca(OH)2, which decomposes again to produce water after entering the kiln, causing heat loss in the kiln. In addition, high-calcium sludge is prone to compaction, which also affects the sludge treatment capacity. The sludge dewatering and conditioning method adopted in this application utilizes the complexation of binary high-valent metal ions and the ionic action and adsorption effect of the surface of waste concrete, while utilizing the skeleton effect to construct water channels to enhance the sludge compression and dewatering performance. While reducing the sludge moisture content, it also reduces the amount of single metal used, which can effectively increase the calorific value of the sludge and the amount of disposal in the cement kiln. Alkaline waste concrete and acidic high-valent metal sulfate solution are used in combination, and the pH value of the sludge after treatment is close to neutral, avoiding the risk of acid corrosion of equipment. At the same time, waste concrete can effectively supplement iron, aluminum, and silicon elements to the sludge, and cooperate with the high-valent metal sulfate solution to regulate the iron and aluminum content, thereby eliminating the adverse effects on cement sintering and hardening.

[0010] Compared with the prior art, the beneficial effects of the present invention are embodied in:

[0011] 1. The metal salt selected in the present invention is a simple ionic binary high-valent metal sulfate. Compared with polymeric inorganic polymers, it can more fully utilize the complexation between high-valent metal ions and sludge surface functional groups, has better sludge dewatering performance, and is beneficial to sludge disposal.

[0012] 2. An appropriate amount of sulfuric acid can dissolve high-valent metal ions such as iron and aluminum in sludge and waste concrete, thereby acting as a dehydration conditioner to improve the dehydration performance of sludge, reduce the amount of added metal salts, and avoid defects such as sludge compaction, equipment corrosion, cement sintering and hardening caused by excessive metal content.

[0013] 3. The rich pore structure and surface charge of the modified waste concrete are conducive to its interaction with sludge, improving the sludge dewatering performance while neutralizing the pH value of the sludge. Its calcined product can be used as a component of cement clinker. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1Based on Example 1, the effects of different high-valent metal sulfates on the moisture content of sludge after filter pressing were investigated, using the atomic mass of the metal as the calculation standard. It can be seen that increasing the concentration of high-valent metal ions contributes to a decrease in sludge moisture content. Compared with polyferric sulfate (PFS), simple high-valent metal ions are more easily dispersed into the internal structure of the sludge and interact with it, improving sludge dewatering performance.

[0015] Figure 2 Based on Example 1, the effects of different sulfuric acid concentrations on the moisture content of sludge after filter pressing were investigated. It can be seen that the addition of sulfuric acid facilitates the dissolution of high-valent metal ions from sludge and waste concrete, and complexes the sludge, further reducing the sludge moisture content. However, when the sulfuric acid dosage is too high, the sludge viscosity decreases due to sludge cell rupture, the water channeling effect is weakened, and the sludge specific resistance increases, resulting in a decrease in dewatering performance.

[0016] Figure 3 Based on Example 1, the effects of waste concrete and its modification methods on the moisture content of sludge were investigated. It can be seen that waste concrete after different treatments can reduce the moisture content of sludge after filter pressing to varying degrees. The dissolution of metal ions such as iron and aluminum in waste concrete after acid leaching treatment is reduced, and alkali leaching treatment can also easily reduce the dissolution of aluminum. Therefore, waste concrete after acid or alkali treatment has a poorer effect on improving the dewatering performance of sludge than untreated waste concrete. The waste concrete that has been thermally modified at 600°C activates the iron and aluminum metal compounds on its surface, making it easier to undergo dissolution reactions with acid. The dissolved metal ions can undergo complexation with the sludge to further improve the dewatering performance of the sludge.

[0017] Figure 4 Based on Example 1, the thermal weight loss of the sludge after the combined treatment of single polyferric sulfate (PFS) and the conditioning agent of Example 1 was analyzed. It can be seen that although waste concrete was added to the conditioning agent of Example 1, its thermal weight loss rate was only slightly lower than that of PFS, indicating that the use of single PFS caused partial compaction or agglomeration of the sludge during the high-temperature calcination process, resulting in incomplete combustion, and its combustion calorific value was also significantly lower than that of the conditioning agent of Example 1.

[0018] Figure 5 The thermal weight loss of the original sludge with a moisture content of 80% treated in each embodiment can be seen from its weight loss data (41.8%). It can be seen that the high-water content sludge without any reagents is severely compacted or agglomerated during the high-temperature heat treatment due to rapid heating and dehydration, resulting in incomplete combustion of some organic matter and a significant reduction in calorific value. DETAILED DESCRIPTION

[0019] The following describes the implementation of the present invention in detail in combination with some technical solutions.

[0020] The high-valent metal sulfate mixed solutions used in the following examples all refer to mixed solutions of ferric sulfate, aluminum sulfate and 10% sulfuric acid.

[0021] The modified waste concrete in the following examples is waste concrete derived from concrete of initial grade C40 and having a particle size of less than 80 mesh, which is heat-treated at 600° C. for 2 h in a muffle furnace.

[0022] Example 1:

[0023] Weigh 100g of municipal sludge with a moisture content of 80% and add a mixed solution of high-valent metal sulfates, where the molar ratio of iron to aluminum is 5:1, and the total amount of iron and aluminum is 1.5% of the weight of the absolute dry sludge. After stirring evenly, add 6% of the weight of the absolute dry sludge of modified waste concrete. Continue stirring until evenly, then wrap the sludge with filter cloth and fold it into a bottom area of ​​80cm. 2 The square mud cake was placed in a thin-layer filter press for dehydration. When the pressure reached 2 MPa, the timer began, increasing by 1 MPa every 2 minutes until it reached 6 MPa, where it was maintained for 30 minutes. After air-drying at room temperature for 12 hours, the resulting mud cake had a moisture content of 32.4%, and the pH of the sludge filtrate was 6.5.

[0024] Example 2:

[0025] Weigh 75g of municipal sludge with a moisture content of 80% and add a mixed solution of high-valent metal sulfates (i.e., a mixed solution of iron sulfate and aluminum sulfate with 10% sulfuric acid), where the molar ratio of iron to aluminum is 3:1 and the total amount of iron and aluminum is 2.0% of the weight of the absolute dry sludge. After stirring evenly, add 4% of the weight of the absolute dry sludge of modified waste concrete. Continue stirring until evenly, then wrap the sludge with filter cloth and fold it into a bottom area of ​​80cm. 2 The square mud cake was placed in a thin-layer filter press for filtration and dehydration. When the pressure reached 2MPa, the timing began, and the pressure was increased by 1MPa every 2 minutes until it reached 6MPa, and then the pressure was maintained for 30 minutes. The moisture content of the mud cake obtained after being placed at room temperature to dry for 12 hours was 35.1%, and the pH value of the sludge filtrate was 6.1.

[0026] Example 3:

[0027] Weigh 50g of municipal sludge with a moisture content of 80%, add ferric sulfate mixed solution, the amount of iron is 0.5% of the weight of the absolute dry sludge. After stirring evenly, add 15% of the weight of the absolute dry sludge modified waste concrete, continue stirring evenly, wrap the sludge with filter cloth and fold it into a bottom area of ​​80cm 2The square mud cake was placed in a thin-layer filter press for filtration and dehydration. When the pressure reached 2MPa, the timing started and the pressure was increased by 1MPa every 2 minutes until it reached 6MPa and then the pressure was maintained for 30 minutes. The moisture content of the obtained mud cake after being placed at room temperature to dry for 12 hours was 30.8%, and the pH value of the sludge filtrate was 7.2.

[0028] Example 4:

[0029] Weigh 50g of municipal sludge with a moisture content of 80%, add ferric sulfate mixed solution, the amount of iron used is 0.5% of the weight of the absolute dry sludge. After stirring evenly, wrap the sludge with filter cloth and fold it into a bottom area of ​​80cm 2 The square mud cake was placed in a thin-layer filter press for filtration and dehydration. When the pressure reached 2MPa, the timing started and the pressure was increased by 1MPa every 2 minutes until it reached 6MPa and then the pressure was maintained for 30 minutes. The moisture content of the obtained mud cake after being placed at room temperature to dry for 12 hours was 48.3%, and the pH value of the sludge filtrate was 4.6.

[0030] Example 5:

[0031] Weigh 75g of municipal sludge with a moisture content of 80% and add a mixed solution of high-valent metal sulfates (i.e., a mixed solution of iron sulfate and aluminum sulfate with 10% sulfuric acid), where the molar ratio of iron to aluminum is 3:1 and the total amount of iron and aluminum is 2.0% of the weight of the absolute dry sludge. After stirring evenly, add 4% of the weight of the absolute dry sludge of modified waste concrete. Continue stirring until evenly, then wrap the sludge with filter cloth and fold it into a bottom area of ​​80cm. 2 The square mud cake was placed in a thin-layer filter press for filtration and dehydration. When the pressure reached 6MPa, the pressure was maintained for 30 minutes. The moisture content of the mud cake after being placed at room temperature to dry for 12 hours was 43.6%, and the pH value of the sludge filtrate was 6.1.

Claims

1. A method for dehydrating and conditioning sludge for cement kiln fuel, characterized in that: Municipal sludge with a moisture content of ≥80% is conditioned by a combination of high-valent metal sulfate solution and modified waste concrete. The conditioned sludge is then mechanically dehydrated by thin-layer filter press. The resulting mud cake has a moisture content of ≤40% after being stored at room temperature for 12 hours. The high-valent metal sulfate solution is a mixed solution of iron sulfate and aluminum sulfate in a certain proportion with 10% sulfuric acid; the total metal ion concentration in the mixed solution is 1 mol / L, the molar ratio of iron to aluminum is 3-5:1; and the total amount of iron and aluminum is 0.5-3.0% of the weight of the absolute dry sludge; The modified waste concrete is a heat-modified product obtained by heat-treating waste concrete with an initial grade of C40 and a particle size of less than 80 mesh in a muffle furnace at 600° C. for 2 hours; The amount of the modified waste concrete is 6-15% of the weight of the absolute dry sludge.

2. The sludge dewatering and conditioning method according to claim 1, characterized in that: The initial thickness of the mud cake of the thin layer filter press is less than 5 mm, and the filter press applies pressure by a gradient pressurization method, that is, starting from 2 MPa, increasing by 1 MPa every 2 minutes, and then maintaining constant pressure at 6 MPa.

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