A recyclable low-expansion precoated sand and a method for preparing the same

Abstract

The present application relates to a kind of renewable low expansion coated sand and its preparation method, belong to coated sand technical field.The present application provides a kind of renewable low expansion coated sand, the coated sand includes the following components: the aggregate, sawdust, phenolic resin, first curing agent, second curing agent, the mass percentage of lubricant is 95-99%, 0.07-0.3%, 1.5-3.5%, 0.12-0.6%, 0.005-0.03%, 0.12-0.3%;Wherein, the mass ratio of phenolic resin, first curing agent, second curing agent is (130-200) :(15-30) :1.The present application further provides a kind of preparation method of renewable low expansion coated sand, the preparation method includes: after preheating aggregate is put into mixing equipment, sawdust, phenolic resin, first curing agent, second curing agent, lubricant is added in turn, and after stirring, crushing and screening, coated sand finished product is obtained.

Description

Technical Field

[0001] This invention belongs to the field of coated sand technology, and relates to a renewable low-expansion coated sand and its preparation method. Background Technology

[0002] The coated sand core process has been researched and applied in China since the 1950s. Currently, the low-expansion process of coated sand is mainly solved by artificial sand (beaded sand). Due to its characteristics of rounded particles, angularity coefficient ≤1.10, and high refractory temperature, beaded sand molds have been widely used in engine blocks, cylinder heads, and precision castings. However, beaded sand is produced through high-temperature calcination, electrofusion, and granulation of bauxite, resulting in high energy consumption. Furthermore, a certain quantity of beaded sand needs to be stockpiled for individual regeneration. In addition, because beaded sand particles are denser than natural silica sand, achieving the desired regeneration quality requires increasing the airflow at the bottom of the calcining furnace, making production costs several times higher than ordinary sand. To save costs and increase market share, natural silica sand is now commonly used as raw material. Natural silica sand is mainly composed of SiO2, with mostly elliptical particle shapes and a high expansion coefficient. Its applications typically avoid castings with complex shapes and intricate structures. However, coated sand produced using existing processes from natural silica sand suffers from problems such as a high coefficient of expansion and low precision. Current processes cannot simultaneously achieve the goals of low coefficient of expansion, low cost, and low energy consumption for coated sand. Summary of the Invention

[0003] The purpose of this invention is to address the aforementioned problems in the existing technology by proposing a renewable low-expansion coated sand and its preparation method, which enables low-cost coated sand to achieve a low expansion coefficient and achieves green and environmentally friendly production process.

[0004] The objective of this invention can be achieved through the following technical solutions: A renewable low-expansion coated sand, the coated sand comprising the following components: the aggregate, wood chips, phenolic resin, first curing agent, second curing agent, and lubricant in the following mass percentages: 95-99%, 0.07-0.3%, 1.5-3.5%, 0.12-0.6%, 0.005-0.03%, and 0.12-0.3%, respectively.

[0005] Preferably, the amount of wood chips added is 0.08-0.3% of the weight of the aggregate; the amount of phenolic resin added is 1.7-3.0% of the weight of the aggregate; and the amount of lubricant added is 0.14-0.25% of the weight of the aggregate.

[0006] Further preferably, the mass ratio of the phenolic resin, the first curing agent, and the second curing agent is (130-200):(15-30):1.

[0007] More preferably, the amount of the first curing agent added is 9-17% of the amount of phenolic resin added; the amount of the second curing agent added is 0.5-1% of the amount of phenolic resin added.

[0008] Phenolic resin undergoes expansion upon heating after curing and reverse molding, affecting dimensional accuracy. Therefore, this invention employs a low-expansion, fast-polymerizing phenolic resin. Modified low-molecular-weight polyamide accelerates curing. The abundance of active hydrogen atoms at the para or ortho positions of the phenol nucleus in the resin, or the presence of a suitable amount of moisture, promotes methylene formation, accelerating the cross-linking of methylene groups and increasing the curing speed. Therefore, this invention uses a mixture of low-expansion, fast-polymerizing phenolic resin, modified low-molecular-weight polyamide, and hexamethylenetetramine to ensure the coated sand fully cures during the molding process, preventing uncured reactions in thicker areas of the mold.

[0009] Phenolic resin is the main binder for coated sand; without it, the product will not produce effective performance. The first curing agent is hexamethylenetetramine, which decomposes at temperatures above 100°C, forming dimethylamine and releasing formaldehyde and ammonia, thus undergoing a cross-linking reaction with the linear resin. The amount added varies depending on the type of phenolic resin. For low-expansion coated sand, the optimal range is 9-17% of the total phenolic resin content. Insufficient addition will result in slow curing, affecting efficiency; excessive addition will not affect the curing rate or strength but will instead generate large amounts of ammonia and formaldehyde. The second curing agent promotes the curing process, but excessive addition will not increase the curing rate.

[0010] Preferably, the aggregate is one or more of calcined sand and recycled sand.

[0011] Further preferred, the method for preparing the calcined sand includes: calcining the raw sand at 800-900℃ for 20-30 hours.

[0012] Preferably, the method for preparing the recycled sand includes: collecting waste sand generated during the steel casting process of coated sand, and regenerating the waste sand at high temperature to obtain recycled sand aggregate.

[0013] Further preferably, the recycled sand aggregate is obtained by regenerating foundry waste sand.

[0014] Foundry waste sand, after being recycled, yields recycled sand aggregate. The expansion rate of foundry waste sand used for high-temperature casting does not change significantly. Furthermore, when recycled sand is used as aggregate, since it has already been roasted during the recycling process, it does not need to be roasted again and can be directly used in the coating process.

[0015] Preferably, the recycling process includes placing waste sand in a recycling device and heating it at 650-700°C for 6 hours to obtain recycled sand aggregate.

[0016] Preferably, the wood chips are 70-140 mesh.

[0017] During the sand mold curing process, sawdust is positioned in the gaps between the sand particles. When the sand mold expands due to heat, the shrinkage properties of the sawdust can offset the linear displacement caused by the expansion of the sand particles. Furthermore, during the coating process, while increasing the amount of sawdust slightly increases the gas generation and rising rate, it reduces strength, flowability, and the rate of thermal expansion. When the amount of sawdust added is 0.2-0.3% of the total raw material mass, the rate of thermal expansion changes very little, remaining at a low value. However, excessive sawdust will affect the properties of the coated sand after the resin melts, leading to a decrease in product strength.

[0018] Preferably, the first curing agent is hexamethylenetetramine.

[0019] Further preferably, the first curing agent needs to be mixed evenly with the solvent before being added to the raw material, and the mass ratio of the first curing agent to the solvent is 1:(1.5-3).

[0020] More preferably, the solvent is water.

[0021] Preferably, the second curing agent is a modified low-molecular-weight polyamide.

[0022] Modified low-molecular-weight polyamides can provide sufficient hydroxyl bonds for the reaction of phenolic resins, thereby increasing the rate of polycondensation.

[0023] Preferably, the lubricant is 400-mesh calcium stearate.

[0024] The present invention also provides a method for preparing renewable low-expansion coated sand, the method comprising: feeding preheated aggregate into a mixing equipment, sequentially adding wood chips, phenolic resin, a first curing agent, a second curing agent, and a lubricant, then stirring, crushing and sieving to obtain the finished coated sand product.

[0025] The raw sand (Inner Mongolia scrubbed sand) undergoes a high-temperature roasting process to remove crystal water, organic pollutants, carbonates, oxides, and other contaminants from the sand grains. Compared to natural scrubbed sand, it features more rounded grains, lower gas evolution, lower loss on ignition, and lower mud content. The phase transformation of quartz under high-temperature conditions is an irreversible process. During repeated high-temperature casting, regeneration, and roasting cycles, the linear expansion rate of the sand grains decreases within a certain range, resulting in performance close to that of granulated sand. The thermal expansion rate of conventionally roasted sand is ≤1.3%, and that of high-quality roasted sand is ≤0.95%. The regenerated sand obtained in this invention, after coating, casting, and regeneration processes, exhibits a gradually decreasing thermal expansion rate, reaching a high-quality grade.

[0026] The expansion rate of silica sand changes because its main component is quartz, which has various crystal systems and structures, and its volume expands with temperature changes. At room temperature, silica sand is α-quartz with a trigonal crystal system. When the temperature reaches 870℃, it transforms into β-quartz with a hexagonal crystal system, and its volume expands. This crystal system transformation process is irreversible. After repeated calcination and regeneration, the expansion of the sand grains decreases until all sand grains have undergone the crystal system transformation.

[0027] Preferably, the preheating temperature is 140-160℃.

[0028] Preferably, the current in the mixing equipment reaches its maximum value after the curing reaction.

[0029] Further preferably, the maximum current is 120-140A.

[0030] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses sawdust added during the coating process to counteract the linear displacement caused by the expansion of sand particles and to reduce the thermal expansion rate.

[0031] 2. In this invention, the curing reaction is completed through the combined action of phenolic resin, the first curing agent, and the second curing agent.

[0032] 3. The preparation method of the present invention realizes the recycling of materials, reduces costs, is environmentally friendly and energy-efficient, and is conducive to industrial production.

[0033] 4. The performance data of coated sand prepared by calcined sand and coated sand prepared by recycled sand are similar, which proves that the waste sand collected during the production of calcined coated sand can be reused to produce coated sand after being recycled.

[0034] 5. The finished products obtained by coating recycled sand and calcined sand using the method of this invention have similar fluidity and thermal expansion rate to the finished products coated with pearl sand. Detailed Implementation

[0035] The following are specific embodiments of the present invention, which further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

[0036] Example 1

[0037] After calcining the Inner Mongolian scrubbing sand at 870℃ for 26 hours, collect calcined sand of 50-140 mesh for later use. The performance data of the calcined sand is shown in Table 1. 500 kg of calcined sand preheated to 145℃ is added to a mixing equipment. 0.5 kg of sawdust is added and stirred for 10 seconds. Then, 9 kg of phenolic resin is added and stirred for 35 seconds. Next, 3.4 L of an aqueous solution containing 1.4 kg of hexamethylenetetramine is added and stirred until completely mixed. Then, 60 g of modified low-molecular-weight polyamide is added and stirred until completely cured. At this point, the current of the mixing equipment reaches its maximum value of 130 A. Then, 1 kg of calcium stearate is added and stirred until the lumps are dispersed to form coated particles. After mixing, the particles are crushed and sieved to obtain the finished calcined coated sand. The performance data of the finished coated sand is shown in Table 2.

[0038] Example 2

[0039] The coated sand is used to create casting sand molds for later use in casting. Molten steel is poured into the molds, and after the castings are formed, cooled, cleaned, and polished, the waste sand is collected. The waste sand is then recycled at 680℃ and mechanically ground and sieved to obtain 50-140 mesh recycled sand. The performance data of the recycled sand is shown in Table 1. The recycled sand is used as aggregate. 500 kg of recycled sand preheated to 150℃ is added to a mixing plant. 0.5 kg of sawdust is added and stirred for 12 seconds. Then, 9 kg of phenolic resin is added and stirred for 35 seconds. Next, 4 L of an aqueous solution containing 1.6 kg of hexamethylenetetramine is added and stirred until completely mixed. Then, 58 g of modified low-molecular-weight polyamide is added and stirred until completely solidified. Finally, 0.9 kg of calcium stearate is added and stirred until the lumps are dispersed to form coated particles. After mixing, the particles are crushed and sieved to obtain the recycled coated sand. The performance data of the coated sand is shown in Table 2.

[0040] Example 3

[0041] 500 kg of recycled sand, preheated to 150°C, was added to a mixing equipment. 1 kg of sawdust was added and stirred for 12 seconds. Then, 12 kg of phenolic resin was added and stirred for 35 seconds. Next, 3.5 L of an aqueous solution containing 1.3 kg of hexamethylenetetramine was added and stirred until completely mixed. Then, 65 g of modified low-molecular-weight polyamide was added and stirred until completely cured. Finally, 1 kg of calcium stearate was added and stirred until the lumps were dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished recycled coated sand. Performance data of the finished coated sand are shown in Table 2.

[0042] Example 4

[0043] 500 kg of recycled sand, preheated to 150°C, was added to a mixing equipment. 1.5 kg of sawdust was added and stirred for 13 seconds. Then, 10 kg of phenolic resin was added and stirred for 30 seconds. Next, 3.2 L of an aqueous solution containing 1.1 kg of hexamethylenetetramine was added and stirred until completely mixed. Then, 65 g of modified low-molecular-weight polyamide was added and stirred until completely cured. Finally, 1.1 kg of calcium stearate was added and stirred until the lumps were dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished recycled coated sand. The performance data of the finished coated sand are shown in Table 2.

[0044] Comparative Example 1 500 kg of calcined sand, preheated to 150°C, was added to a mixing apparatus. 11 kg of phenolic resin was added and stirred for 40 seconds. Then, 3.6 L of an aqueous solution containing 1.4 kg of hexamethylenetetramine was added and stirred until completely mixed. Next, 60 g of modified low-molecular-weight polyamide was added and stirred until completely cured. Then, 1 kg of calcium stearate was added and stirred until the lumps were dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished calcined coated sand. Performance data of the finished coated sand are shown in Table 2.

[0045] Comparative Example 2 500 kg of calcined sand, preheated to 155℃, was added to a mixing apparatus. 1.2 kg of sawdust was added and stirred for 10 seconds. Then, 11 kg of phenolic resin was weighed out and stirred for 35 seconds. Next, 3.3 L of an aqueous solution containing 1.2 kg of hexamethylenetetramine was added and stirred until completely mixed. Then, 1.2 kg of calcium stearate was added and stirred until the lumps dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished calcined coated sand. Performance data of the finished coated sand are shown in Table 2.

[0046] Comparative Example 3 500 kg of calcined sand, preheated to 145℃, was added to a mixing apparatus. 10 kg of phenolic resin was added and stirred for 35 seconds. Then, 4 L of an aqueous solution containing 1.3 kg of hexamethylenetetramine was added and stirred until completely mixed. Next, 62 g of modified low-molecular-weight polyamide was added and stirred until completely cured. Finally, 1 kg of calcium stearate was added and stirred until the lumps were dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished calcined coated sand. Performance data of the finished coated sand are shown in Table 2.

[0047] Comparative Example 4 500 kg of recycled sand, preheated to 150°C, was added to a mixing equipment. 2 kg of sawdust was added and stirred for 12 seconds, followed by 10 kg of phenolic resin and stirring for 35 seconds. Then, 3.5 L of an aqueous solution containing 1.17 kg of hexamethylenetetramine was added and stirred until completely mixed. Next, 65 g of modified low-molecular-weight polyamide was added and stirred until completely cured. Finally, 1 kg of calcium stearate was added and stirred until the lumps were dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished recycled coated sand. Performance data of the finished coated sand are shown in Table 2.

[0048] Comparative Example 5 The performance data of the abrasive sand are shown in Table 1. 500 kg of abrasive sand preheated to 150℃ was added to a mixing equipment, along with 10 kg of phenolic resin and stirred for 40 seconds. Then, 2.9 L of an aqueous solution containing 0.97 kg of hexamethylenetetramine was added and stirred until completely mixed. Next, 65 g of modified low-molecular-weight polyamide was added and stirred until completely cured. Then, 1 kg of calcium stearate was added and stirred until the lumps were dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished coated sand. The performance data of the finished coated sand are shown in Table 2.

[0049] Comparative Example 6 500 kg of preheated alumina sand (preheated to 150°C) was added to a mixing equipment. 0.6 kg of sawdust was added and stirred for 12 seconds, followed by 9 kg of phenolic resin and stirring for 40 seconds. Then, 3.2 L of an aqueous solution containing 1.1 kg of hexamethylenetetramine was added and stirred until completely mixed. Next, 65 g of modified low-molecular-weight polyamide was added and stirred until completely cured. Finally, 0.9 kg of calcium stearate was added and stirred until the lumps were dispersed to form coated particles. After mixing, the particles were crushed and sieved to obtain the finished coated alumina sand. Performance data of the finished coated alumina sand are shown in Table 2.

[0050] Table 1. Aggregate Performance Data

[0051] Table 2. Performance data of coated sand (finished product)

[0052] In summary, the preparation method of the present invention enables the low-cost, low-energy-consumption preparation of low-expansion renewable coated sand, and allows for the recycling of waste sand.

[0053] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A renewable, low-expansion coated sand, characterized in that, The raw materials for the coated sand include the following components: aggregate, wood chips, phenolic resin, first curing agent, second curing agent, and lubricant; The amount of wood chips added is 0.08-0.3% of the weight of the aggregate; the amount of phenolic resin added is 1.7-3.0% of the weight of the aggregate; and the amount of lubricant added is 0.14-0.25% of the weight of the aggregate. The mass ratio of the phenolic resin, the first curing agent, and the second curing agent is (130-200):(15-30):1; The phenolic resin is a low-expansion, fast-polymerization type phenolic resin. The aggregate is one or more of calcined sand and recycled sand; The method for preparing the calcined sand includes: calcining the raw sand at 800-900℃ for 20-30 hours; The method for preparing the recycled sand includes: collecting waste sand generated during the steel casting process of coated sand, and regenerating the waste sand at high temperature to obtain recycled sand aggregate; wherein, the regeneration process includes placing the waste sand in a regeneration device and heating it at 650-700℃ for 6 hours to obtain recycled sand aggregate; The first curing agent is hexamethylenetetramine, and the second curing agent is modified low-molecular-weight polyamide; The first curing agent and solvent are mixed evenly before being added to the raw materials. The mass ratio of the first curing agent to the solvent is 1:(1.5-3).

2. A method for preparing renewable low-expansion coated sand as described in claim 1, characterized in that, The preparation method of the coated sand includes: putting preheated aggregate into a mixing equipment, adding sawdust, phenolic resin, first curing agent, second curing agent and lubricant in sequence, stirring evenly, crushing and sieving to obtain the finished coated sand.

3. The preparation method according to claim 2, characterized in that, The preheating temperature is 140-160℃.

4. The preparation method according to claim 2, characterized in that, The first curing agent and solvent are mixed evenly before being added to the raw materials. The mass ratio of the first curing agent to the solvent is 1:(1.5-3).

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