A method for forming a modified magnesium walnut shell composite board with salt lake brine

Abstract

The present application relates to the technical field of inorganic cementitious composites, and particularly relates to a forming method of a salt lake brine modified magnesium walnut shell composite board, comprising the following steps: step S1, providing the following raw materials: light-burned magnesium oxide, the active magnesium oxide content of which is 60%-65%, and the activity is determined according to the citric acid method stipulated in GB / T 22705-2008; salt lake brine, the Baume degree of which is 32-35 DEG B at 25 DEG C. The present application realizes the synergistic improvement of the strength, anti-returning brine property and volume stability of the board by precisely controlling the molar ratio of active magnesium oxide and magnesium chloride and combining with the filling and curing process optimization. The macroscopic performance comparison and phase analysis are consistent, which shows that under the optimized ratio, each component of the system tends to be completely converted into stable 5•1•8 crystal phase, and the free harmful phase is basically eliminated. The present application systematically solves the problems of traditional chloromagnesite products, such as easy returning brine, easy warping and insufficient strength, and significantly improves the durability and use reliability of the products while maintaining the low cost advantage.

Description

Technical Field

[0001] This invention relates to the field of inorganic cementitious composite materials technology, and in particular to a method for molding magnesium walnut shell composite boards modified by salt lake brine. Background Technology

[0002] Magnesium oxychloride cement is an air-hardening cementing system with magnesium oxide and magnesium chloride solution as the main cementing materials. It has the advantages of early strength, high strength, rapid setting, strong adhesion, low alkalinity, wear resistance and fire resistance, and has been widely used in the field of building materials.

[0003] However, traditional magnesium oxychloride cement production generally relies on industrial magnesium chloride as the chlorine source, resulting in high costs and the large-scale use of industrial salt places a burden on the environment. Meanwhile, Northwest my country is rich in salt lake resources; for example, the Lop Nur Salt Lake has built the world's largest single potassium sulfate production facility. However, the remaining brine after potassium extraction is rich in magnesium chloride and various trace elements, leading to low comprehensive utilization rates. Much of it is discharged or stored, causing resource waste and environmental risks. Furthermore, my country generates a large amount of agricultural and forestry solid waste such as walnut shells annually, which is typically used as low-value fuel or stockpiled, failing to achieve high-value utilization.

[0004] Existing technologies attempt to composite magnesium oxychloride cement with agricultural and forestry waste to prepare boards. For example, patent CN105110758A uses corn stalks and magnesium oxychloride cement to prepare thermal insulation materials, but still uses industrial magnesium chloride and does not involve the resource utilization of salt lake brine; patent CN112250416A uses artificially mixed brine and wood powder to prepare fireproof boards, but does not involve the synergistic modification of walnut shells and real salt lake brine; patent CN101817670A uses industrial magnesium chloride and straw / ash residue, and does not include a heated static pressing molding process. The above technologies generally have the following shortcomings: (1) Relying on industrial magnesium chloride, the high-value utilization of salt lake brine has not been realized; (2) Insufficient density and interfacial bonding of the board limits its mechanical properties; (3) There is a lack of effective solutions to problems such as low interfacial strength and easy moisture absorption and efflorescence in the walnut shell and magnesium oxychloride cement system.

[0005] Therefore, this invention proposes a method for molding magnesium-modified walnut shell composite boards made from salt lake brine. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for molding magnesium-modified walnut shell composite boards made from salt lake brine.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for molding magnesium-modified walnut shell composite boards made from salt lake brine includes the following steps: Step S1: Provide the following raw materials: lightly calcined magnesium oxide, with an active magnesium oxide content of 60%-65%, and the activity is determined according to the citric acid method specified in GB / T22705-2008; salt lake brine, with a Baume degree of 32-35°Bé at 25 ℃, mainly containing Mg. 2+ Cl - SO4 2- B 3+ Li + Walnut shell powder, trisodium phosphate; Step S2: Dry mix the lightly calcined magnesium oxide, walnut shell powder and trisodium phosphate evenly to obtain a dry mixture; Step S3: Add the salt lake brine and optional clean water to the dry mixture, and stir to form a slurry with a fluidity of 140-160 mm. The molar ratio of effective magnesium oxide to magnesium chloride in the brine is controlled to be 6:1. The amount of walnut shell powder is 20%-35% of the mass of light-burned magnesium oxide, and the amount of trisodium phosphate is 1.5%-2.5% of the mass of light-burned magnesium oxide. Step S4: Inject the slurry into the mold and statically press it for 4-6 hours at a temperature of 55-65℃ and a pressure of 2.5-3.5 MPa. After demolding, allow it to cure naturally to obtain the magnesium oxychloride walnut shell board.

[0008] As a preferred technical solution of this application, the brine in step S1 is the old brine after potassium extraction from the Lop Nur salt lake, and its main ion content is: Mg 2+ 92.5 g / L, Cl - 265.8 g / L, SO4 2- 19.2 g / L, B 3+ 1.6 g / L, Li + 0.9 g / L, the remainder being Na + K + and trace amounts of Ca² + .

[0009] As a preferred technical solution of this application, the fineness of the lightly calcined magnesium oxide in step S1 is ≤10% residue on an 180-mesh sieve; the particle size of the walnut shell powder is 0.5-2 mm, the bulk density is 0.45-0.55 g / cm³, and the moisture content is ≤8%.

[0010] As a preferred technical solution of this application, the amount of walnut shell powder in step S3 is 25%-30% of the mass of lightly calcined magnesium oxide.

[0011] As a preferred technical solution of this application, the amount of trisodium phosphate in step S3 is 1.8%-2.2% of the mass of lightly calcined magnesium oxide.

[0012] As a preferred technical solution of this application, the static pressure curing temperature in step S4 is 60°C, the pressure is 3.0 MPa, and the time is 5 hours.

[0013] As a preferred technical solution of this application, the molar ratio of effective magnesium oxide to magnesium chloride in the brine in step S3, which is 6:1, is determined as follows: the stoichiometric ratio of Mg from MgO to Mg from MgCl2 in the main strength phase 5Mg(OH)2·MgCl2·8H2O of magnesium oxychloride cement is 5:1. Combined with the compensation for the utilization rate of lightly calcined magnesium oxide and the zero-tolerance redundancy of free MgCl2, it is adjusted to 6:1 to ensure that MgCl2 is completely fixed.

[0014] As a preferred technical solution of this application, the dry density is 0.9-1.1 g / cm³, the flexural strength is ≥18 MPa, the compressive strength is ≥45 MPa, the water absorption thickness expansion rate is ≤0.8%, the internal bonding force is ≥1.2 MPa, the single-point hanging force is ≥1200 N, and the combustion performance is A1 grade.

[0015] The beneficial effects of this invention are as follows: 1. Under the preferred molar ratio, the flexural strength of the prepared plate is much higher than that of the sample deviating from this ratio, which can meet the requirements of high load-bearing applications. The strength improvement is attributed to the full conversion of the active component in the reaction system into a high-crystallinity strength-contributing phase, resulting in a dense matrix with few defects. 2. The surface of the board with the preferred formula remains dry in a high humidity environment and there is no moisture absorption or deliquescence; while the sample deviating from the preferred ratio shows varying degrees of efflorescence. The reason is that under the preferred ratio, almost all magnesium chloride participates in the reaction, and there is no free moisture-absorbing residual salt in the matrix. 3. The drying shrinkage value of the plates prepared by the optimized scheme is much lower than that of the under- or over-mixed samples, demonstrating good dimensional retention. This is due to the absence of expansive products generated by the hydration of excess magnesium oxide in the system, which reduces the generation of internal microcracks and macroscopic deformation from the source. 4. The product is mainly composed of the 5•1•8 crystalline phase. X-ray diffraction analysis did not detect free magnesium chloride or residual magnesium hydroxide impurities, indicating that the reaction proceeded relatively completely. This phase combination balances chemical stability and physical density, providing microstructural assurance for macroscopic properties. 5. There is a clear performance plateau near the optimal molar ratio, and slight deviations in the batching will not cause a sharp deterioration in performance, resulting in a high tolerance for errors in continuous production; 6. Utilizing salt lake brine to replace industrial magnesium chloride eliminates the significant expenditure on purchasing fine chemical raw materials, resulting in extremely low chlorine source costs. Secondly, using agricultural and forestry waste such as walnut shell powder as filler is not only inexpensive and widely available but also transforms waste into treasure, avoiding the consumption of forest resources by traditional fillers such as sawdust. Overall, the total cost of raw materials for producing the same volume of boards is much lower than that of traditional processes, resulting in significant cost reduction. Furthermore, it helps potassium extraction enterprises in the salt lake area effectively consume old brine, reducing storage and environmental management costs and expanding the utilization of by-products. Moreover, the boards fundamentally solve the long-standing problems of brine return, warping, and corner chipping associated with traditional magnesium oxychloride boards, giving them excellent commercial competitiveness and sustainable development prospects.

[0016] In summary, this invention achieves a synergistic improvement in the strength, resistance to efflorescence, and volume stability of the board by precisely controlling the molar ratio of active magnesium oxide to magnesium chloride and optimizing the filling and curing processes. Macroscopic performance comparison and phase analysis show that under the optimized ratio, all components of the system tend to be completely transformed into a stable 5•1•8 crystalline phase, and the free harmful phase is basically eliminated. From the microscopic to the macroscopic level, this invention systematically solves the prominent problems of traditional magnesium oxychloride products such as easy efflorescence, easy warping, and insufficient strength. While maintaining the advantage of low cost, it significantly improves the durability and reliability of the product. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0018] I. Testing Standards and Equipment Mechanical property testing: Following GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)," specimen dimensions were 40 mm × 40 mm × 160 mm. An ETM series microcomputer-controlled electronic universal testing machine from Shenzhen WanCe Testing Equipment Co., Ltd. was used, with a maximum test force of 300 kN and an accuracy of 0.5 grade. Compressive strength testing was conducted using a broken half-prism on a 40 mm × 40 mm compression surface, with a loading rate of (2.4 ± 0.2) kN / s. The arithmetic mean of six specimens was taken.

[0019] Density test: Referring to the principle of GB / T 208-2014 "Method for Determination of Cement Density", the mass and volume of 3 specimens were measured by the drainage method, and the average apparent density was calculated.

[0020] Evaluation of efflorescence: Place the specimen in a constant temperature and humidity chamber with an ambient humidity of 85%RH and a temperature of 25℃, and observe the surface moisture absorption status after 48 hours. A dry surface with no visible water droplets or deliquescence is considered "no efflorescence"; a damp surface with no flowing water droplets is considered "slight efflorescence"; and obvious water droplets or solution seepage on the surface is considered "severe efflorescence".

[0021] Mixing equipment: JJ-5 cement mortar mixer from Wuxi Jianyi Instrument Machinery Co., Ltd., with low-speed rotation (140±5) r / min and high-speed rotation (285±10) r / min.

[0022] Hot pressing equipment: Qingdao Huabo Machinery Technology Co., Ltd. HBDR-570 laboratory flat vulcanizing machine / hot press, nominal clamping force 500kN, temperature control accuracy ±2 ℃, platen size 350 mm×350 mm.

[0023] Curing equipment: SHBY-40B standard constant temperature and humidity curing chamber from Cangzhou Huayi Test Instrument Co., Ltd., with temperature and humidity control accuracy of ±1 ℃ and ±2% RH respectively.

[0024] X-ray diffraction testing: Powder X-ray diffractometer, GB / T 5618-2014, using Cu Kα radiation (λ=0.15406 nm), tube voltage 40 kV, tube current 40 mA, 2θ scan range 5°-70°, step size 0.02°, scan rate 4° / min. The sample powder was pressed into a pellet for testing after passing through a 200-mesh sieve.

[0025] II. Raw Material Sources and Specifications Lightly calcined magnesium oxide: active magnesium oxide content 60%-65%, activity determined according to the citric acid method specified in GB / T 22705-2008; fineness ≤10% residue on 180 mesh sieve.

[0026] Salt lake brine: A by-product of potassium extraction from Lop Nur Salt Lake in Xinjiang, with a Baume degree of 32-35°Bé at 25℃. The main ion content is: Mg 2+ 92.5 g / L, Cl - 265.8 g / L, SO4 2 19.2 g / L, B 3+ 1.6 g / L, Li + 0.9 g / L, the remainder being Na + K + and trace amounts of Ca 2+ .

[0027] Walnut shells: Produced in Aksu, Xinjiang. After crushing, the shells are sieved to obtain particles with a diameter of 0.5-2 mm and a bulk density of 0.45-0.55 g / cm³. 3 Moisture content ≤8%.

[0028] Trisodium phosphate: Industrial grade, Na3PO4·12H2O, content ≥98%.

[0029] Water: Laboratory deionized water.

[0030] III. Examples and Comparative Examples Example 1 A method for molding magnesium-modified walnut shell composite boards made from salt lake brine includes the following steps: Step S1: Provide the following raw materials: 100 kg of lightly calcined magnesium oxide, with an active magnesium oxide content of 65%, and the activity is determined according to the citric acid method specified in GB / T 22705-2008; 94 kg of salt lake brine, with a Baume degree of 33.5°Bé at 25 ℃, and a MgCl2 content of approximately 360 g / L; 30 kg of walnut shell powder; and 2.0 kg of trisodium phosphate. Step S2: Dry mix the lightly calcined magnesium oxide, walnut shell powder and trisodium phosphate evenly to obtain a dry mixture; Step S3: Add the salt lake brine to the dry mixture and stir to form a slurry with a fluidity of 155 mm; Step S4: Inject the slurry into the mold and statically press it for 5 hours at a temperature of 60°C and a pressure of 3.0 MPa. After demolding, allow it to cure naturally for 28 days to obtain the magnesium oxychloride walnut shell board.

[0031] Example 2 A method for molding magnesium-modified walnut shell composite boards made from salt lake brine includes the following steps: Step S1: Provide the following raw materials: 100 kg of lightly calcined magnesium oxide, with an active magnesium oxide content of 60.8%, and the activity is determined according to the citric acid method specified in GB / T 22705-2008; 88 kg of salt lake brine, with a Baume degree of 33.5°Bé at 25 ℃, and a MgCl2 content of approximately 360 g / L; 20 kg of walnut shell powder; and 1.5 kg of trisodium phosphate. Step S2: Dry mix the lightly calcined magnesium oxide, walnut shell powder and trisodium phosphate evenly to obtain a dry mixture; Step S3: Add the salt lake brine to the dry mixture and stir to form a slurry with a fluidity of 148 mm; Step S4: Inject the slurry into the mold and statically press it for 6 hours at a temperature of 55°C and a pressure of 2.8 MPa. After demolding, allow it to cure naturally for 28 days to obtain the magnesium oxychloride walnut shell board.

[0032] Example 3 A method for molding magnesium-modified walnut shell composite boards made from salt lake brine includes the following steps: Step S1: Provide the following raw materials: 100 kg of lightly calcined magnesium oxide, with an active magnesium oxide content of 63.5%, and the activity is determined according to the citric acid method specified in GB / T 22705-2008; 92 kg of salt lake brine, with a Baume degree of 33.5°Bé at 25 ℃, and a MgCl2 content of approximately 360 g / L; 35 kg of walnut shell powder; and 2.5 kg of trisodium phosphate. Step S2: Dry mix the lightly calcined magnesium oxide, walnut shell powder and trisodium phosphate evenly to obtain a dry mixture; Step S3: Add the salt lake brine to the dry mixture and stir to form a slurry with a fluidity of 152 mm; Step S4: Inject the slurry into the mold and statically press it for 4 hours at a temperature of 65°C and a pressure of 3.2 MPa. After demolding, allow it to cure naturally for 28 days to obtain the magnesium oxychloride walnut shell board.

[0033] Comparative Example 1 A method for molding magnesium-modified walnut shell composite boards made from salt lake brine includes the following steps: Step S1: Provide the following raw materials: 100 kg of lightly calcined magnesium oxide, with an active magnesium oxide content of 65%, and the activity is determined according to the citric acid method specified in GB / T 22705-2008; 56.6 kg of industrial magnesium chloride hexahydrate solution, and add water to make up the total mass of the solution to 94 kg; 30 kg of walnut shell powder; and 2.0 kg of trisodium phosphate. Step S2: Dry mix the lightly calcined magnesium oxide, walnut shell powder and trisodium phosphate evenly to obtain a dry mixture; Step S3: Add the industrial magnesium chloride hexahydrate solution to the dry mixture and stir to form a slurry with a flowability of 155 mm; Step S4: Inject the slurry into the mold and statically press it for 5 hours at a temperature of 60°C and a pressure of 3.0 MPa. After demolding, allow it to cure naturally for 28 days to obtain the magnesium oxychloride walnut shell board.

[0034] Comparative Example 2 A method for molding magnesium-modified walnut shell composite boards made from salt lake brine includes the following steps: Step S1: Provide the following raw materials: 100 kg of lightly calcined magnesium oxide, with an active magnesium oxide content of 65%, and the activity is determined according to the citric acid method specified in GB / T 22705-2008; 94 kg of salt lake brine, with a Baume degree of 33.5°Bé at 25 ℃, and a MgCl2 content of approximately 360 g / L; 30 kg of walnut shell powder; and 2.0 kg of trisodium phosphate. Step S2: Dry mix the lightly calcined magnesium oxide, walnut shell powder and trisodium phosphate evenly to obtain a dry mixture; Step S3: Add the salt lake brine to the dry mixture and stir to form a slurry with a fluidity of 155 mm; Step S4: Inject the slurry into the mold, vibrate it using a room temperature vibration table, and allow it to cure naturally for 28 days after demolding to obtain the magnesium oxychloride walnut shell board.

[0035] IV. Results and Analysis Table 1. Performance comparison of magnesium oxychloride walnut shell boards prepared in the examples and comparative examples.

[0036] Results Analysis: As shown in Table 1, the flexural strength of the boards obtained by the method of this invention in Examples 1-3 is all above 16.8 MPa, with a maximum of 18.2 MPa. This is more than 100% higher than that of Comparative Example 1 using industrial magnesium chloride, and about 170% higher than that of Comparative Example 2 without heating and static pressing. The internal bonding strength is increased by 2-3 times, and the water absorption thickness expansion rate is significantly reduced from 5.2% to below 0.8%. This fully demonstrates that the synergistic whisker regulation by trace components of salt lake brine and trisodium phosphate, as well as the dual effect of heating and static pressing densification, are the key to the performance leap.

[0037] Table 2 Comparison of performance with different molar ratios of effective MgO to MgCl2

[0038] In Table 2, the intensity change is calculated based on the 28-day intensity of Example 1. Results Analysis: Table 2 shows that the molar ratio of active MgO to MgCl2 has a significant impact on the mechanical properties, efflorescence tendency, and volume stability of the board, with a molar ratio of 6:1 exhibiting the best overall performance. Specifically, the 28-day flexural strength of the 6:1 sample reached 18.2 MPa, an increase of 78.4% compared to the 4:1 sample (10.2 MPa) and 45.6% compared to the 7:1 sample (12.5 MPa). Regarding efflorescence, the 6:1 sample showed no efflorescence, while the 4:1 sample experienced severe efflorescence, and the 5:1 sample showed slight efflorescence. In terms of volume stability, the 6:1 sample showed no cracking, while the 7:1 sample exhibited expansion cracking. All three macroscopic performance indicators consistently point to a molar ratio of 6:1 as the optimal ratio.

[0039] To further investigate the phase nature of the performance differences, XRD was used to identify the phase of the above samples (test conditions: Cu Kα radiation, 2θ scan range 5°-70°, step size 0.02°). The analysis results show that the diffraction pattern of the 6:1 molar ratio sample exhibits strong characteristic diffraction peaks of the 5•1•8 phase (5Mg(OH)₂·MgCl₂·8H₂O), with a smooth baseline. No diffraction peaks of free MgCl₂ or Mg(OH)₂ were detected, indicating that the active MgO and MgCl₂ in the brine are almost completely converted into a stable 5•1•8 crystalline phase, resulting in a dense matrix structure. The 4:1 molar ratio sample showed detected diffraction peaks of free MgCl₂, and the intensity of the 5•1•8 phase peaks decreased significantly, indicating that residual MgCl₂ was the direct cause of hygroscopic return. The 7:1 molar ratio sample showed strong Mg(OH)₂ diffraction peaks, while the 5•1•8 phase peaks weakened, indicating that excess MgO hydrated alone to generate expansive Mg(OH)₂, leading to a loose matrix and causing volume expansion and cracking. These phase evolution patterns are completely consistent with the macroscopic performance data, further confirming the rationality of the 6:1 molar ratio as the optimal ratio from a mechanistic perspective.

[0040] V. Economic Analysis To further verify the industrialization value of this invention, using Example 1 and Comparative Example 1 as examples, the raw material cost was calculated based on the production of 1 cubic meter of finished board material. The reference market prices for each raw material are as follows: Lightly calcined magnesium oxide: 600 yuan / ton Industrial magnesium chloride hexahydrate: 800 yuan / ton Salt lake brine: Only transportation and pretreatment costs are included, equivalent to 30 yuan / ton. Walnut shell powder: Processing cost of agricultural and forestry waste, 300 yuan / ton Trisodium phosphate: 3500 yuan / ton Water: Negligible Table 3 Comparison of raw material costs per unit area of ​​sheet metal

[0041] Results Analysis: As shown in the table above, under the premise of achieving the same or even higher mechanical properties and durability, the unit raw material cost of Example 1 of this invention is RMB 268.0, which is about 35% lower than the RMB 412.3 of Comparative Example 1 using industrial magnesium chloride. The cost advantage mainly comes from the substitution of industrial magnesium chloride with salt lake brine, transforming the originally low-value-added salt lake by-product brine into the main chlorine source, significantly reducing the expenditure on purchased chemical raw materials. The utilization of walnut shell powder as agricultural and forestry waste has a cost far lower than that of traditional wood fillers, further enhancing its economic efficiency. Considering the process energy consumption, the solution of this invention still has a significant cost-performance advantage, and is particularly suitable for industrial promotion in areas surrounding salt lake resources, realizing the synergistic production of solid waste resource utilization and low-cost high-performance boards.

[0042] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for molding magnesium-modified walnut shell composite boards made from salt lake brine, characterized in that, Includes the following steps: Step S1: Provide the following raw materials: lightly calcined magnesium oxide, with an active magnesium oxide content of 60%-65%, and the activity is determined according to the citric acid method specified in GB / T22705-2008; salt lake brine, with a Baume degree of 32-35°Bé at 25 ℃, mainly containing Mg. 2+ Cl - SO4 2- B 3+ Li + Walnut shell powder, trisodium phosphate; Step S2: Dry mix the lightly calcined magnesium oxide, walnut shell powder and trisodium phosphate evenly to obtain a dry mixture; Step S3: Add the salt lake brine and optional water to the dry mixture, and stir to form a slurry with a fluidity of 140-160 mm. The molar ratio of effective magnesium oxide to magnesium chloride in the brine is controlled to be 6:

1. The amount of walnut shell powder is 20%-35% of the mass of light-burned magnesium oxide, and the amount of trisodium phosphate is 1.5%-2.5% of the mass of light-burned magnesium oxide. Step S4: Inject the slurry into the mold and statically press it for 4-6 hours at a temperature of 55-65℃ and a pressure of 2.5-3.5 MPa. After demolding, allow it to cure naturally to obtain the magnesium oxychloride walnut shell board.

2. The method for molding a magnesium-modified walnut shell composite board made from salt lake brine according to claim 1, characterized in that, The brine mentioned in step S1 is the old brine after potassium extraction from the Lop Nur salt lake, and its main ion content is: Mg 2+ 92.5 g / L, Cl - 265.8 g / L, SO4 2- 19.2 g / L, B 3+ 1.6 g / L, Li + 0.9 g / L, the remainder being Na + K + and trace amounts of Ca² + .

3. The method for molding a magnesium-modified walnut shell composite board using salt lake brine according to claim 1, characterized in that, The fineness of the lightly calcined magnesium oxide in step S1 is ≤10% residue on an 180-mesh sieve; the particle size of the walnut shell powder is 0.5-2 mm, the bulk density is 0.45-0.55 g / cm³, and the moisture content is ≤8%.

4. The method for molding a magnesium-modified walnut shell composite board made from salt lake brine according to claim 1, characterized in that, The amount of walnut shell powder added in step S3 is 25%-30% of the mass of lightly calcined magnesium oxide.

5. The method for molding a magnesium-modified walnut shell composite board made from salt lake brine according to claim 1, characterized in that, In step S3, the amount of trisodium phosphate added is 1.8%-2.2% of the mass of lightly calcined magnesium oxide.

6. The method for molding a magnesium-modified walnut shell composite board made from salt lake brine according to claim 1, characterized in that, The static pressure curing temperature in step S4 is 60°C, the pressure is 3.0 MPa, and the time is 5 hours.

7. The method for molding a magnesium-modified walnut shell composite board made from salt lake brine according to claim 1, characterized in that, The molar ratio of effective magnesium oxide to magnesium chloride in brine, 6:1, in step S3 is determined as follows: the stoichiometric ratio of Mg from MgO to Mg from MgCl2 in the main strength phase 5Mg(OH)2·MgCl2·8H2O of magnesium oxychloride cement is 5:

1. Combined with the compensation for the utilization rate of lightly calcined magnesium oxide and the zero-tolerance redundancy of free MgCl2, it is adjusted to 6:1 to ensure that MgCl2 is completely fixed.

8. The magnesium oxychloride walnut shell board prepared by the molding method of salt lake brine modified magnesium walnut shell composite board according to claims 1-7, is characterized in that, Dry density is 0.9-1.1 g / cm³, flexural strength ≥18 MPa, compressive strength ≥45 MPa, water absorption thickness expansion rate ≤0.8%, internal bonding strength ≥1.2 MPa, single-point hanging force ≥1200 N, and fire performance is A1 grade.

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