Magnesium-calcium microporous aggregate and preparation method thereof

Abstract

The invention discloses a magnesium-calcium microporous aggregate and a preparation method thereof. The method comprises the following steps: calcining magnesite powder in a muffle furnace to obtain light calcined magnesite powder; adding water into the light-burned dolomite powder for digestion; uniformly mixing the digested light-burned dolomite powder, the digested light-burned magnesite powder, the digested basic magnesium sulfate whisker and the digested binder, and aging; performing compression molding on the magnesium-calcium mixture, and putting the molded magnesium-calcium mixture into a drying oven; and sintering the dried magnesium-calcium green body in a box-type resistance furnace, cooling along with the furnace, taking out, crushing and screening to obtain the magnesium-calcium microporous aggregate. The aggregate can meet the requirement of reducing the energy consumption of a cement kiln through the lightweight magnesium-calcium refractory material and the excellent comprehensive performance of the magnesium-calcium refractory material, and has the advantages of simple preparation method, good thermal shock resistance, high compressive strength, high breaking strength and the like.

Description

Technical Field

[0001] This invention relates to the field of materials technology, specifically to a magnesium-calcium microporous aggregate and its preparation method. Background Technology

[0003] Cr in magnesium-chromium refractories 3+ Under high temperature and alkaline conditions, it will oxidize into toxic Cr. 6+ This causes serious environmental pollution. Magnesium-iron-aluminum spinel refractories are widely used in the firing and transition zones of cement rotary kilns in China due to their excellent kiln lining performance and thermal shock resistance. However, pure magnesium-aluminum spinel refractories have poor kiln lining performance, are easily eroded by cement clinker, and have high material costs. Magnesium-iron-aluminum refractories have high thermal conductivity and thermal expansion coefficients, resulting in high energy consumption and high costs during use. The repeated changes in the valence state of iron ions cause volume effects in the refractories, leading to easy damage. Especially when co-processing waste-derived fuels, the frequent fluctuations in the kiln atmosphere make the volume effect more pronounced, severely affecting their performance and service life.

[0004] Magnesia-calcium refractories possess advantages such as high refractoriness, good stability, excellent kiln coating performance, good thermal shock resistance, resistance to cement clinker erosion, and abundant raw materials. However, the susceptibility of magnesia-calcium refractories to hydration limits their widespread application. Therefore, to build a chromium-free high-temperature industry, researching and developing "high-performance / low-cost / long-life" refractories for cement kilns that can replace magnesia-chromium and magnesia-ferroaluminum spinel is of great significance to the efficient development of my country's refractory materials and cement industries. Summary of the Invention

[0005] The purpose of this invention is to provide a magnesium-calcium microporous aggregate and its preparation method, in order to solve the problems of poor kiln coating performance and environmental pollution of existing refractory materials used in cement kilns.

[0006] To achieve the above objectives, embodiments of the present invention provide a magnesium-calcium microporous aggregate, wherein the magnesium-calcium mixture for preparing the magnesium-calcium microporous aggregate comprises:

[0007] Lightly calcined dolomite powder and lightly calcined magnesite powder;

[0008] Basic magnesium sulfate whiskers;

[0009] Polyvinyl alcohol solution.

[0010] Another aspect of the present invention provides a method for preparing magnesium-calcium microporous aggregate, the method comprising:

[0011] a. Calcining magnesite powder yields lightly calcined magnesite powder.

[0012] b. Digest the lightly calcined dolomite with water.

[0013] c. Prepare magnesium-calcium mixture.

[0014] d. Press the magnesium-calcium mixture into shape.

[0015] e. After sintering the magnesium-calcium green blanks, crush and screen them to obtain magnesium-calcium microporous aggregate.

[0016] In one embodiment of the present invention, the lightly calcined magnesite powder in step a is obtained by placing magnesite powder in a muffle furnace, holding it at 900°C for 3 hours, and then cooling it with the furnace.

[0017] In one embodiment of the present invention, the digested light-burned dolomite powder in step b is obtained by digesting light-burned dolomite with 30wt% deionized water for 2 hours.

[0018] In one embodiment of the present invention, the magnesium-calcium mixture in step c is obtained by adding digested light-burned dolomite powder to light-burned magnesite powder (with CaO content controlled at 40 wt%), 4 wt% basic magnesium sulfate whiskers and 5 wt% polyvinyl alcohol solution (3 wt%), stirring thoroughly, and aging for 14 hours.

[0019] In one embodiment of the present invention, the molding pressure in step d is 65 MPa, the holding time is 60 s, and the drying time is 0.5 hours.

[0020] In one embodiment of the present invention, the sintering temperature in step e is 1450-1600°C and the sintering time is 3 hours.

[0021] The embodiments of the present invention have the following advantages:

[0022] This invention utilizes the in-situ decomposition of basic magnesium sulfate whiskers to prepare magnesium-calcium microporous aggregate, completely avoiding the introduction of impurity phases that reduce the sintering performance of the aggregate. At the same time, it improves the hydration resistance of the magnesium-calcium microporous aggregate to a certain extent. This aggregate has advantages such as good thermal shock resistance, good hydration resistance, and good mechanical properties. Attached Figure Description

[0023] Figure 1 SEM images of magnesium-calcium microporous aggregates prepared with different contents of basic magnesium sulfate whiskers as provided in Example 1 of the present invention.

[0024] Figure 2 The XRD energy spectrum of magnesium-calcium microporous aggregates prepared with different contents of basic magnesium sulfate whiskers as provided in Example 1 of the present invention.

[0025] Figure 3The graph shows the bulk density, apparent porosity, water absorption, compressive strength, and thermal shock resistance of magnesium-calcium microporous aggregates prepared with different contents of basic magnesium sulfate whiskers as provided in Example 1 of the present invention.

[0026] Figure 4 SEM images of magnesium-calcium microporous aggregates prepared at different sintering temperatures according to Example 2 of the present invention.

[0027] Figure 5 The XRD spectra of magnesium-calcium microporous aggregates prepared at different sintering temperatures are shown in Example 2 of this invention.

[0028] Figure 6 The graph shows the bulk density, apparent porosity, water absorption, compressive strength, and thermal shock resistance of magnesium-calcium microporous aggregates prepared at different sintering temperatures according to Example 2 of the present invention.

[0029] Figure 7 SEM image of the magnesium-calcium microporous aggregate prepared by the two-step sintering process provided in Embodiment 3 of the present invention.

[0030] Figure 8 The XRD spectrum of the magnesium-calcium microporous aggregate prepared by the two-step sintering process provided in Example 3 of the present invention.

[0031] Figure 9 The graph shows the bulk density, apparent porosity, water absorption, compressive strength, and thermal shock resistance of the magnesium-calcium microporous aggregate prepared by the two-step sintering process provided in Example 3 of this invention.

[0032] Figure 10 Pore ​​size distribution diagrams of magnesium-calcium microporous aggregates prepared at sintering temperatures of 1550℃ and 1600℃ as provided in Example 2 of the present invention.

[0033] Figure 11 This is an analysis diagram of the in-situ decomposition and pore-forming mechanism of basic magnesium sulfate whiskers provided in Example 1 of the present invention. Detailed Implementation

[0034] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the invention.

[0035] Example 1

[0036] A magnesium-calcium microporous aggregate, the preparation method of which is as follows:

[0037] First, magnesite powder was placed in a muffle furnace and held at 900℃ for 3 hours, then cooled to room temperature to obtain light-burned magnesite powder. Then, light-burned dolomite powder was digested with 30wt% water for 2 hours and set aside. The digested light-burned dolomite powder and light-burned magnesite powder were mixed in a ratio (40wt% CaO), and then different mass fractions of basic magnesium sulfate whiskers and 5wt% PVA solution were added and mixed evenly. The mixture was aged for 14 hours to obtain a magnesium-calcium mixture. The magnesium-calcium mixture was pressed into shape under 65MPa for 60s and placed in an oven for 30 minutes. The dried magnesium-calcium green was placed in a box-type resistance furnace and held at 1600℃ for 3 hours, then cooled with the furnace. After removal, it was crushed and sieved to obtain magnesium-calcium microporous aggregate. The performance of the prepared magnesium-calcium microporous aggregate was tested, and its SEM image is shown below. Figure 1 As shown, the phase composition is as follows Figure 2 As shown, the bulk density, apparent porosity, water absorption, compressive strength, and thermal shock resistance are as follows: Figure 3 As shown.

[0038] Example 2

[0039] A magnesium-calcium microporous aggregate, the preparation method of which is as follows:

[0040] First, magnesite powder was placed in a muffle furnace and held at 900℃ for 3 hours, then cooled to room temperature to obtain light-burned magnesite powder. Then, light-burned dolomite powder was digested with 30wt% water for 2 hours and set aside. The digested light-burned dolomite powder and light-burned magnesite powder were mixed in a ratio (40wt% CaO), and then 4wt% basic magnesium sulfate whiskers and 5wt% PVA solution were added and mixed thoroughly. The mixture was aged for 14 hours to obtain a magnesium-calcium mixture. The magnesium-calcium mixture was pressed into shape under 65MPa for 60s and placed in an oven for 30 minutes. The dried magnesium-calcium green was placed in a box-type resistance furnace and held at 1450–1600℃ for 3 hours, then cooled with the furnace. After removal, it was crushed and sieved to obtain magnesium-calcium microporous aggregate. The properties of the prepared magnesium-calcium microporous aggregate were tested, and its SEM image is shown below. Figure 4 As shown, the phase composition is as follows Figure 5 As shown, the bulk density, apparent porosity, water absorption, compressive strength, and thermal shock resistance are as follows: Figure 6 As shown.

[0041] Example 3

[0042] A magnesium-calcium microporous aggregate was prepared using the same method as in Example 2, except that the magnesium-calcium green body was sintered in a two-step process (first held at 1000℃ for 1 hour, then held at 1550℃ or 1600℃ for 1 or 2 hours respectively). The performance of the prepared magnesium-calcium microporous aggregate was tested, and its SEM image is shown below. Figure 7 As shown, the phase composition is as follows Figure 8 As shown, the bulk density, apparent porosity, water absorption, compressive strength, and thermal shock resistance are as follows: Figure 9 As shown.

[0043] Example 4

[0044] The pore size distribution of the magnesium-calcium microporous aggregates prepared at sintering temperatures of 1500℃ and 1600℃ in Example 2 above was tested, and the test results are as follows: Figure 10 As shown. The results showed that the most probable pore size of the samples prepared at both sintering temperatures was approximately 2.80 μm, with most pore sizes concentrated in the range of 1 μm to 10 μm. The in-situ decomposition pore-forming mechanism of basic magnesium sulfate whiskers in Example 1 was analyzed, as follows... Figure 11 As shown.

Claims

1. A magnesium-calcium microporous aggregate, wherein the magnesium-calcium mixture for preparing the magnesium-calcium microporous aggregate comprises: light-burned dolomite powder, light-burned magnesite powder, a pore-forming agent, and a binder; wherein the average particle size of the light-burned dolomite powder is 6.67 mm; the average particle size of the light-burned magnesite powder is 24.09 mm; the pore-forming agent is basic magnesium sulfate whiskers (D < 1.0 μm, L = 10-60 μm, L / D > 30); and the binder is a polyvinyl alcohol solution (type 1788, 3 wt%).

2. A method for preparing magnesium-calcium microporous aggregate, the method comprising: a) Calcining magnesite powder to obtain light-calcined magnesite powder; b) Digesting light-calcined dolomite with water to obtain digested light-calcined dolomite powder; c) Mixing digested light-calcined dolomite powder, light-calcined magnesite powder, pore-forming agent, and binder evenly and aging to obtain a magnesium-calcium mixture; d) Pressing the magnesium-calcium mixture into shape and drying; e) Sintering the magnesium-calcium green blank, cooling it in the furnace, removing it, crushing and screening it to obtain magnesium-calcium microporous aggregate.

3. The preparation method according to claim 2, wherein the lightly calcined magnesite powder in step a is obtained by placing magnesite powder in a muffle furnace, holding it at 900°C for 3 hours, and then cooling it with the furnace.

4. The preparation method according to claim 2, wherein the digested light-burned dolomite powder in step b is obtained by digesting light-burned dolomite with 30wt% deionized water for 2 hours.

5. The preparation method according to claim 2, wherein the magnesium-calcium mixture in step c is obtained by mixing digested light-burned dolomite powder with light-burned magnesite powder (controlling the CaO content to 40wt%), 4wt% basic magnesium sulfate whiskers and 5wt% polyvinyl alcohol solution (3wt%), and aging for 14 hours.

6. The preparation method according to claim 2, wherein the molding pressure in step d is 65 MPa, the holding time is 60 s, and the drying time is 0.5 hours.

7. The preparation method according to claim 2, wherein the sintering temperature in step e is 1450-1600℃ and the sintering time is 3 hours.

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