Method for producing alkali-activated cement by staged calcination with sodium chloride

Abstract

To provide a method for producing an alkali-activated cement, having excellent characteristics such as a wide selection range of raw materials, low cost, low consumption, and low carbon emissions, and capable of being applied in various fields.SOLUTION: A method for producing an alkali-activated cement by stepwise calcination using sodium chloride includes: a step (1) of mixing sodium chloride, silicate, and carbonate, and finely grinding it to obtain a raw material mixture powder S1; a step (2) of firstly calcining the raw material mixture powder S1 at 850 to 1050°C, then heating it until the temperature reaches 1240°C or more to calcine it again, and after completion of calcination, rapidly cooling it to obtain a clinker S2; and a step (3) of mixing the clinker S2 with one or two selected from sodium hydroxide and potassium hydroxide, and finely grinding it to obtain a cement powder.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention is in the field of cement building materials, and in particular relates to a method for producing alkali-activated cement by step-calcination with sodium chloride. [Background technology]

[0002] Portland cement clinker production has significant problems, including high carbon emissions, high energy and material consumption, severe dust and sulfur oxide and nitrogen oxide gas pollution, and poor corrosion resistance and durability. Alkali-activated cement offers excellent early strength, corrosion resistance, and freeze-thaw resistance. It can utilize large amounts of solid waste residues, making it one of the new low-carbon, energy-saving, and environmentally friendly cements. It has the greatest potential to replace Portland cement. However, conventional alkali-activated cement is a two-component cement that hardens by activating activated amorphous aluminosilicate (calcium) salts with strong alkali. It has the following drawbacks: (1) Kaolin resources are scarce, and other major raw materials, such as fly ash and blast furnace slag, are widely used as Portland cement admixtures, driving up their prices. (2) The high usage of industrial alkali activators (3–14 wt% based on Na2O) increases costs and makes it prone to alkalinity, affecting durability. (3) Because the inherent composition of industrial waste residues varies, it is difficult to stably regulate and standardize the properties and manufacturing process of alkali-activated cement produced from them as the main raw material.

[0003] Reducing or eliminating the use of alkali activators can effectively reduce the cost of alkali-activated cement. Patents CN110371140A and CN110451827A disclose the preparation and use of ambient-cured and steam-cured alkali-activated cements, respectively. These cements are produced by mixing and grinding sodium potassium aluminosilicate and calcium raw materials, calcining them at 1250–1300°C, quenching them to obtain clinker, finely grinding the clinker, and uniformly mixing it with water glass. The compressive strength of the cement paste at 28 days exceeds 80 MPa and 110 MPa, respectively. Although the amount of alkali activator used in these two cements is less than that of conventional two-component alkali-activated cements, the cost of alkali-activated cements remains affected by the high cost of industrial alkalis and potassium sodium aluminum silicate used in clinker calcination. Summary of the Invention

[0004] The present invention provides a method for producing alkali-activated cement by step-by-step calcination using sodium chloride, which reduces the cost of producing alkali-activated cement by calcining clinker using less expensive raw materials and reducing the amount of alkali activator used.

[0005] The object of the present invention is achieved by the following technical solutions. 1. A method for producing alkali-activated cement by staged calcination with sodium chloride, comprising the steps of: Step (1): Sodium chloride, silicate, and carbonate are mixed and finely ground to obtain a raw material mixture powder S1; Step (2): First, the raw material mixture powder S1 is fired at 850 to 1050 ° C, and then heated to a temperature of 1240 ° C or higher and fired again. After firing is completed, it is rapidly cooled to obtain clinker S2; Step (3): Mix the clinker S2 with one or both of sodium hydroxide and potassium hydroxide and finely grind it to obtain cement powder; In step (1), the ignition residue obtained by oxidatively igniting a mixture of silicate and carbonate at (950±25)°C has a mass ratio of SiO2 + Al2O3 + CaO + MgO of 90.0% or more, a mass ratio of SiO2 of 31.0 to 47.0%, a mass ratio of Al2O3 of 12.4 to 18.0%, a mass ratio of CaO of 21.3 to 46.3%, and a mass ratio of MgO of 0 to 16.2%, In step (1), the mass ratio of the amount of sodium chloride added to the ignition residue obtained by oxidatively igniting the mixture of silicate and carbonate at (950±25)°C is 8.0 to 20.0%, In step (3), the mass of sodium hydroxide and / or potassium hydroxide is calculated by (NaOH+0.713KOH) / S2=0.5 to 3.0%.

[0006] Preferably, in step (1), sodium chloride, silicate, and carbonate are mixed and finely ground until they all pass through an 80 μm square aperture sieve.

[0007] Preferably, in step (2), the firing time at 850 to 1050° C. is 1 hour or longer.

[0008] Preferably, in step (2), when firing at 850 to 1050°C, the flow rate is 80 g min -1 m -2 The above steam is introduced.

[0009] Preferably, in step (2), the firing time at a temperature of 1240° C. or higher is 1 hour or longer.

[0010] Preferably, in step (2), the rapid cooling is water quench cooling or blast cooling.

[0011] Preferably, in step (3), sodium hydroxide and / or potassium hydroxide is dissolved in water in advance and mixed with clinker powder obtained by finely grinding the clinker S2 alone when the cement is used.

[0012] Preferably, when carrying out step (2), the exhaust gas from the calcination is collected and introduced into water for dissolution, and the aqueous solution of the exhaust gas is post-treated to produce hydrochloric acid or recover valuable metals. [Effects of the Invention]

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) In the process of producing cement clinker, expensive industrial alkali is not required, and the alkali used for activation is further reduced, minimizing the production cost of alkali-activated cement. (2) The general-purpose cement produced is primarily made from aluminosilicate, which not only significantly reduces carbon emissions compared to silicate cement, but also allows for the consumption of large amounts of aluminosilicate solid waste residues. (3) In the cement manufacturing process, valuable metals in solid waste residue can be simultaneously recovered, improving resource recovery rates and realizing effective utilization of solid waste. (4) By removing heavy metals from solid waste residues through chlorine roasting and then solidifying the remaining heavy metals with cement, a double insurance policy can be achieved: the effective use of solid waste containing heavy metals and the elimination of the risk of heavy metal contamination. (5) HCl gas and hydrochloric acid can be produced using exhaust gases emitted during the cement manufacturing process. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be further described with reference to the following examples, but the scope of protection of the present invention is not limited thereto. Steps such as preparation of raw material mixture powder, calcination of the raw material mixture, preparation and curing of cement, and treatment of the solution of the calcination exhaust gas will be described.

[0015] 1. Preparation of raw material mixture powder The main chemical composition of the present invention is silicates of SiO2 and Al2O3. Silicates include granite, kaolin, quartz sandstone, and lithium tailings. Carbonates include limestone and dolomite. Zinc-lead tailings are mixtures of silicates and carbonates. Here, zinc-lead tailings contain small amounts of lead and zinc sulfides. All examples use aluminum calcium magnesium silicate mixtures obtained by mixing the above-mentioned silicates and carbonates or their mixtures in certain amounts (shown in Table 1). The mixture is taken, crushed, and oxidized and ignited at 950°C to obtain the ignition residue. The amount of the aluminum calcium magnesium silicate mixture is adjusted so that the mass ratio of SiO2 + Al2O3 + CaO + MgO in the ignition residue is 90.0% or more, and the mass ratio of the mixture is adjusted so that the mass ratio of SiO2 is 31.0-47.0%, the mass ratio of Al2O3 is 12.4-18.0%, the mass ratio of CaO is 21.3-46.3%, and the mass ratio of MgO is 0-16.2%. The mass of sodium chloride added is 8.0-20.0% of the ignition residue of the aluminum calcium magnesium silicate mixture, and the sodium chloride and aluminum calcium magnesium silicate mixture are mixed to obtain a raw material mixture. The raw material mixture is finely ground until all of it passes through an 80 μm square-hole sieve to obtain raw material mixture powder S1.

[0016] Table 1. Mixture of silicate and carbonate raw materials in the raw material mixture for each example. Unit: % [Table 1]

[0017] 2. Calcination of raw material mixture The raw material powder mixture S1 is placed in a boat-shaped corundum crucible and then placed in the corundum vacuum tube of a silicon carbide tube-type atmosphere furnace. An acid-resistant quartz flange is attached to the outlet end of the atmosphere furnace. If necessary, an air or steam atmosphere is introduced into the inlet end, and a vent pipe at the outlet end is used for water. The tube furnace is heated to 500°C at a heating rate of 5°C / min and then connected to the atmosphere. The air atmosphere is sufficient to maintain unidirectional gas flow within the tube, and the steam flow rate is 80g·min-1·m-2 or higher. The operating temperature is raised to 850-1050°C, and the temperature is maintained constant for 1.0-4.0 hours. After the temperature is reached, the temperature is increased to 1240°C or higher at a heating rate of 5°C / min, and the temperature is maintained constant for 1-3 hours. After the firing is completed, the introduction of steam is stopped, the quartz flange at the outlet end is quickly removed, the crucible is removed from the tube while still hot, and the clinker S2 in the crucible is cooled by air cooling (air cooling) or water cooling (water cooling). The main chemical composition of the ignition residue of the aluminum calcium magnesium silicate mixture, the amount of sodium chloride added, and the firing parameters for each example are shown in Table 2.

[0018] Table 2. Main chemical composition, sodium chloride content and firing parameters [Table 2] a Unit: g·min -1 m -2

[0019] 3. Cement preparation and curing S2 was ground to a cement-size powder, and NaOH and / or KOH were dissolved in as little water as possible and cooled to room temperature. The mass ratio of NaOH and KOH was determined so that (NaOH + 0.713KOH) / S2 = 0.5-3.0. The alkaline solution and clinker powder were mixed and stirred for 2-5 minutes. Water was added during stirring to reduce the consistency of the slurry so that it could be liquefied during subsequent vibration. The slurry was then transferred to a 40x40x40 cubic steel mold and vibrated to compact it. The mold and the mold were then transferred to a standard cement curing box and cured at 20°C and ≥90% humidity for one day before being demolded to obtain cement paste test blocks. If the strength did not meet the demolding requirements after one day of curing, demolding was delayed and the blocks were subsequently cured in humidity for three, seven, or 28 days. The unconfined compressive strength of the test blocks was measured on the third, seventh, and 28th days, respectively. The cement paste mix proportions and compressive strength are shown in Table 3. As can be seen from Table 3, the strength of the cement increases as the alkali content increases. The maximum compressive strength of the cement paste reached 82.5 MPa after 28 days.

[0020] Table 3. Chlorine content of cement clinker, cement slurry blend ratio and compressive strength [Table 3]

[0021] 4. Treatment of firing exhaust gas solution The chemical composition of cooled clinker S2 was determined, and the residual Cl ion content in the calcination residue ranged from 0.08% to 1.35% (see Table 3 for details). While most of the clinker was close to the standard chloride content (0.10%) for general-purpose silicate cement, it would be desirable to reduce the chloride content to the cement standard by extending the second-stage calcination time (see Example 12). The lead and zinc contents of Example 2 were significantly lower than those of the zinc-lead tailings. The Pb removal rate was 90.5%, and the Zn removal rate was 66.4%. The metallic lead and zinc discharged in this example were dissolved in the water into which the exhaust gas was introduced and could be treated by physical and chemical treatment. The exhaust gas solution of other examples, which does not contain valuable metals or heavy metals, could be distilled to obtain purified HCl gas, which could then be dissolved in water to obtain hydrochloric acid.

Claims

1. 1. A method for producing alkali-activated cement by staged calcination with sodium chloride, comprising the steps of: Step (1): Sodium chloride, silicates of SiO 2 and Al 2 O 3 , and carbonate are mixed and finely ground to obtain a raw material mixture powder S1; Step (2): First, the raw material mixture powder S1 is fired at 850 to 1050°C, and then heated and fired until the temperature reaches 1240°C or higher. After the firing is completed, the raw material mixture powder S1 is rapidly cooled to obtain clinker S2. Step (3): Mix the clinker S2 with one or two of sodium hydroxide and potassium hydroxide, and finely grind it to obtain cement powder; In step (1), the silicate and carbonate mixture is oxidatively ignited at (950±25)°C to obtain an ignition residue, and SiO ２ +Al ２ O ３ +CaO+MgO mass ratio is 90.0% or more, and SiO ２ is 31.0 to 47.0%, and Al ２ O ３ is 12.4 to 18.0%, CaO is 21.3 to 46.3%, MgO is 0 to 16.2%, In step (1), the mass ratio of the amount of sodium chloride added to the ignition residue obtained by oxidatively igniting the mixture of silicate and carbonate at (950±25)°C is 8.0 to 20.0%, In step (3), the mass of sodium hydroxide and / or potassium hydroxide is calculated so that (NaOH + 0.713KOH) / S2 = 0.5 to 3.0%.

2. In step (1), sodium chloride, silicate, and carbonate are mixed and ground to a fine powder that can pass through a 80 μm square-hole sieve.

2. The method of claim 1 .

3. In step (2), the firing time at 850 to 1050°C is 1 hour or more.

2. The method of claim 1 .

4. In step (2), when firing at 850 to 1050°C, the flow rate is 80 g min －１ ・m －２ Introduce the above steam 2. The method of claim 1 .

5. In step (2), the firing time at 1240°C or higher is 1 hour or longer.

2. The method of claim 1 .

6. In step (2), the rapid cooling is water quench cooling or blast cooling.

2. The method of claim 1 .

7. In step (3), sodium hydroxide and / or potassium hydroxide is dissolved in water in advance, and when the cement is used, the solution is mixed with the powder obtained by finely grinding the clinker S2 alone.

2. The method of claim 1 .

8. In step (2), the exhaust gas from the calcination is collected and dissolved in water, and the aqueous solution of the exhaust gas is post-treated to produce hydrochloric acid or recover valuable metals.

5. The method according to claim 1 or claim 4.

Images