A metallurgical pellet bentonite binder composite and its application

Abstract

This invention provides a metallurgical pellet bentonite binder and reinforcing agent composite and its application. The composite, by weight, comprises the following raw materials: 10-15 parts of ammonium orthophosphate, 15-20 parts of ammonium polyphosphate, 15-20 parts of aluminum polyphosphate, 40-50 parts of aluminum hydroxide, and 10-15 parts of calcined boromagnesite; the general formula of the ammonium orthophosphate is (NH4). n H 3‑n PO4, where n≤3; the general formula of the ammonium polyphosphate salt is H (n‑m)+2 (NH4) m P n O 3n+1 In the formula, 4≤n≤10, m≤n+2; the condensed aluminum phosphate salt is a linear chain-like aluminum phosphate salt with the following general formula: in the formula, 4≤n≤10. The composite is mixed with bentonite pellet powder to obtain a metallurgical pellet bentonite binder. This binder significantly improves the wet-bulb strength, high-temperature strength, and explosion resistance temperature of the pellet bentonite, while greatly reducing the amount of pellet bentonite used and significantly increasing the iron grade of the pellets.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical raw material pretreatment technology, and relates to a metallurgical pellet bentonite binder and reinforcing agent composite and its application. Technical Background

[0002] Oxidized iron ore pellets possess advantages such as uniform particle size, high cold strength, high iron content, and good reducibility, making them a high-quality blast furnace feedstock. To improve pelletizing performance and product quality, pellet production typically involves adding binders. A range of inorganic binders have been used, including clay, diatomaceous earth, lime, cement, water glass, and borates. However, all inorganic binders have certain side effects that degrade the metallurgical properties of the pellets, forcing researchers to develop new binders. Bentonite was ultimately found to be the most suitable pellet binder. Bentonite improves both the drop strength and burst temperature of green pellets. Adding bentonite to pellets significantly improves the technical and economic indicators of pellet plants.

[0003] Bentonite is a clay primarily composed of montmorillonite-like minerals. It possesses properties such as swelling, suspension, dispersibility, and hydrophilicity. Due to the small particle size of montmorillonite, the number of negative charges within its internal unit cells is the same, resulting in like charges repelling each other and exhibiting excellent suspension properties, along with strong hydrophilicity and dispersibility. Furthermore, bentonite is a highly dispersible substance; its addition improves the binding properties of pelletizing materials, reducing the capillary diameter within green pellets and increasing capillary force. On the other hand, after absorbing water, bentonite forms colloidal particles that fill the spaces between green pellet particles, increasing inter-particle adhesion and thus improving green pellet strength. The addition of bentonite to pellet production increases the compressive strength of the pellets, allowing for particle sliding when green pellets are subjected to external impact. Therefore, its effect on improving the drop strength of green pellets is even more significant.

[0004] Increasing the crack resistance temperature of green pellets is another reason for choosing bentonite in the pelleting industry. The drying time of green pellets often accounts for more than a quarter of the entire roasting process. The final quality of the pellets is also affected to some extent by the quality of green pellet drying. Therefore, efforts are always made to increase the drying speed of green pellets without affecting their crack resistance. The crack resistance temperature of green pellets mainly depends on the rate of water evaporation and diffusion. When the diffusion rate is slower than the evaporation rate, the pellets are prone to cracking. Due to the strong affinity between bentonite and water, the moisture inside the green pellets gradually migrates to the surface for evaporation. This results in slower water loss during pellet drying, thus reducing the rate of water evaporation, increasing the crack resistance temperature, and significantly reducing the likelihood of cracking during drying. According to production experience in pelleting, switching from adding hydrated lime to adding bentonite increased the green pellet crack resistance temperature from 450℃ to over 700℃, improved pellet quality, and increased yield by more than 60%.

[0005] Using bentonite as a pellet binder can also have adverse effects. Bentonite's main component is SiO2, with a content exceeding 60%. Excessive addition of bentonite not only slows down the growth rate of green pellets but also reduces the iron grade of the pellets, negatively impacting blast furnace ironmaking. Experience shows that for every 1% increase in the bentonite proportion, the pellet grade decreases by 0.6 percentage points, while the SiO2 content of the finished ore increases significantly. As a harmful component in pelletizing, SiO2 should be minimized when adding it to the furnace, while ensuring a certain pellet strength.

[0006] However, according to decades of domestic production data, foreign pelletizing companies use less than 1% bentonite in their pellet production, while Chinese pelletizing companies use an average of 2.5-3.5%, a significant difference compared to international levels. Therefore, our company conducted a series of experimental studies on the pelletizing of domestically produced iron ore powder. We found that when using bentonite from Wyoming pelletizing plants (USA), the bentonite content is generally around 1%, and all pellet indicators meet the requirements. However, when using purified bentonite from Tianzheng Company and various additives such as CMC, PAA, and HPAM in pelletizing experiments, the bentonite content exceeds 2%. Even when using pure natural sodium-based bentonite from Wyoming and the aforementioned additives, the 1% content level was not reached. Pellets produced in this way have low iron content and high SiO2 content, far below international standards. The main reason for this is the excessively high bentonite content during pelletizing. According to statistics, the current bentonite usage in domestic pellet plants is generally around 2.5-3.5%, significantly lower than the international level (1%). Even in the Jianping Shahai bentonite mine area where our company is located, which is recognized domestically as producing high-quality bentonite for pelletizing and casting, the bentonite usage in iron ore pelletizing only reaches 2%. Moreover, with the increasing scarcity of iron ore resources in recent years and the deterioration of the quality of iron concentrate used for pelletizing, the usage of bentonite is likely to increase. Excessive bentonite usage not only increases the production cost of pellets but, more importantly, reduces the iron grade of the pellet ore. Furthermore, using such pellets in blast furnaces leads to a decrease in pig iron production and an increase in the coke ratio, severely impacting economic efficiency.

[0007] Adding a certain amount of bentonite during the metallurgical pelletizing process can significantly improve the drop strength, compressive strength, bursting temperature, and dry pellet strength of green pellets. However, bentonite contains a large amount of impurities such as SiO2 and Al2O3, which reduce the iron grade of the pellets and affect the quality of the pellets. For many years, researchers have been dedicated to the research and development of organic binders, aiming to reduce the amount of bentonite used, or even eliminate it altogether, and to improve the iron grade of the pellets by utilizing the lower impurity content of organic binders. Research on the use of organic binders in pellet preparation began both domestically and internationally in the 1980s. However, despite decades of research on organic binders, bentonite remains the dominant binder in pellet production. Organic binders have not yet achieved widespread application due to various reasons, including issues with addition and mixing uniformity, pelletizing kinetics, green pellet thermal properties, pellet roasting, and cost.

[0008] The bentonite addition rate in my country's pellet production is generally high, primarily due to the iron ore powder raw material. Domestic iron content is typically around 62-63%, 2-3 percentage points lower than international advanced levels. Furthermore, compared to foreign iron ore powder, which has a finer particle size (generally over 85% of particles smaller than 0.074mm), domestic iron ore powder has a coarser particle size (generally below 75% of particles smaller than 0.074mm), a significant difference. Using this type of iron concentrate for pelletizing requires a higher proportion of bentonite (2.5-3.5% or even higher) to meet the requirements for pellet strength and explosion resistance temperature. Another, and more important, reason for the generally high bentonite addition rate in my country's pellet production is the significant gap in bentonite quality used in pelletizing. Specifically, the additives used in pelletizing bentonite differ considerably from those used abroad. This necessitates a high proportion of bentonite to ensure the required pellet strength and anti-burst temperature, resulting in increased impurities such as SiO2 and Al2O3 in the pellets and a substantial reduction in the grade of the finished pellets. Therefore, given that the current iron ore powder raw material content, grade, and particle size cannot be changed, the only way to improve pellet quality and reduce the bentonite content is to develop bentonite additives for pelletizing. Consequently, domestic pellet and bentonite pellet manufacturers are actively taking various measures to reduce the bentonite ratio while meeting the needs of pellet production. Among these measures, researching and developing pelletizing bentonite reinforcing agents to improve the binding properties of bentonite and replace conventional bentonite to reduce the bentonite content is of great practical significance. Summary of the Invention

[0009] Accordingly, the applicant invented a metallurgical pellet bentonite binder and used it to conduct research and production verification on the partial replacement of bentonite.

[0010] This invention aims to provide a bentonite binder for metallurgical pellets, partially replacing bentonite in pellet production to compensate for the deficiencies in properties such as bonding strength of the bentonite itself. This achieves the desired pellet strength and bursting temperature using a lower bentonite ratio. The improvement in metallurgical pellet strength involves both wet-state bonding strength and high-temperature strength of iron ore pellets. The use of this metallurgical bentonite binder significantly reduces the proportion of bentonite in the pellets, avoiding impurities introduced by excessive bentonite use, thereby improving the iron grade of the pellets and effectively saving on iron ore pellet production costs.

[0011] Studies have found that combinations of inorganic compounds with specific elemental compositions and molecular structures, such as ammonium orthophosphates, ammonium polyphosphates, aluminum condensate polyphosphates, aluminum hydroxide, and calcined borosilicate, when incorporated into bentonite pellets, significantly improve the wet-bulb strength, dry-bulb strength, and bursting temperature of metallurgical pellets. The metallurgical pellet bentonite binder composite of this invention comprises a composition of these inorganic compounds.

[0012] The technical solution of the present invention is as follows:

[0013] A metallurgical pellet bentonite binder and reinforcing agent composite, comprising the following raw materials in parts by weight:

[0014] 10-15 parts of ammonium orthophosphate, 15-20 parts of ammonium polyphosphate, 15-20 parts of aluminum polyphosphate, 40-50 parts of aluminum hydroxide, and 10-15 parts of calcined boromagnesia.

[0015] The general formula for the ammonium orthophosphate salts is (NH4). n H 3-n PO4, where n≤3; the general formula of the ammonium polyphosphate salt is H (n-m)+2 (NH4) m P n O 3n+1 In the formula, 4≤n≤10, m≤n+2;

[0016] The condensed aluminum phosphate salts are linear chain-like aluminum phosphate salts having the following general formula:

[0017] In the formula, 4 ≤ n ≤ 10.

[0018] Ammonium orthophosphate salts are one or more of NH4H2PO4, (NH4)2HPO4, and (NH4)3PO4. Ammonium orthophosphate compounds are unstable compounds. They undergo hydrolysis when exposed to water and decomposition, dehydration, and condensation when heated. At room temperature, they generate H3PO4. For example, NH4H2PO4 will undergo the corresponding chemical reaction: NH4H2PO4 → NH3 + H3PO4. The newly generated H3PO4 reacts with another component of this invention, Al(OH)3, to generate Al(H2PO4)3. The generated Al(H2PO4)3 slowly condenses at room temperature to gradually generate the adhesive Al2(H2P2O7)3. The generated Al(H2PO4)3 and Al2(H2P2O7)3 can also react with the curing agent calcined borosilicate to generate Mg(H2PO4)2, MgH2P2O7, and MgB(H2PO4)5, etc. The substances produced by the reaction have good binding capacity, providing iron powder with binding force other than bentonite, and increasing the low-temperature strength, i.e. wet-bulb strength, of the iron ore pellets.

[0019] As described above, the main factor considered in selecting ammonium orthophosphate salts in this invention is that they should be able to provide as much nascent H3PO4 as possible for the neutralization reaction with Al(OH)3, while also minimizing the release of ammonia. Therefore, this invention preferentially selects NH4H2PO4.

[0020] The ammonium polyphosphates involved in this invention are long-chain or branched compounds with molecular structures composed of -POP- chains, and their molecular weights can be very high, reaching over 100,000. These ammonium polyphosphates are also thermally unstable compounds, especially at high temperatures, where they can undergo chemical reactions such as decomposition, dehydration, deamination, and condensation to generate various larger molecular weight ammonium polyphosphates. As the temperature continues to rise, the larger molecular weight ammonium polyphosphates generated will decompose to form polyphosphoric acid and lose ammonia and water. This process of deamination and water removal lowers the internal temperature of the pellets, allowing ammonia and water to escape through the existing pores of the pellets. At this point, the pellets already have a certain strength, and the small amount of ammonia and water leaving the pellets will not cause damage. Simultaneously, the generated high molecular weight polyphosphoric acid reacts with Al(OH)3 to form aluminum polyphosphate with good binding properties, further increasing the compressive strength of the pellets. The unreacted high molecular weight ammonium polyphosphates decompose completely at approximately 750°C, leaving almost no residue within the pellets.

[0021] Because high-polymerization-degree ammonium polyphosphates have better thermal stability than low-polymerization-degree ammonium polyphosphates—for example, polyphosphates with n≥20 only decompose significantly above 350℃, meaning they only provide binding strength to pellets above 350℃—this invention preferentially selects low-polymerization-degree ammonium polyphosphates with n=4-10 and incomplete replacement of hydrogen atoms in the chain with ammonium, especially those with hydrogen atoms at the end groups. These are more prone to a series of reactions such as deammoniation, dehydration, and condensation, producing binding substances, under metallurgical pelletizing and drying sintering conditions. These low-molecular-weight ammonium polyphosphates with hydrogen atoms at the end groups are:

[0022] (NH4)4H2P4O 13 (NH4)3H3P4O 13 (NH4)2H4P4O 13 NH4H5P4O 13 (NH4)5H2P5O 16 (NH4)4H3P5O 16 (NH4)3H4P5O 16 (NH4)2H5P5O 16 NH4H6P5O 16 (NH4)6H2P6O 19 (NH4)5H3P6O 19 (NH4)4H4P6O 19 (NH4)3H5P6O 19 (NH4)2H6P6O 19 (NH4)H7P6O 19 (NH4)7H2P7O 22 (NH4)6H3P7O 22 (NH4)5H4P7O 22 (NH4)4H5P7O 22 (NH4)3H6P7O 22 (NH4)2H7P7O 22 (NH4)H8P7O 22 (NH4)8H2P8O 25 (NH4)7H3P8O 25 (NH4)6H4P8O 25 (NH4)5H5P8O 25 (NH4)4H6P8O 25 (NH4)3H7P8O 25 (NH4)2H8P8O 25 (NH4)H9P8O 25 (NH4)9H2P9O 28 (NH4)8H3P9O28 (NH4)7H4P9O 28 (NH4)6H5P9O 28 (NH4)5H6P9O 28 (NH4)4H7P9O 28 (NH4)3H8P9O 28 (NH4)2H9P9O 28 (NH4)H 10 P9O 28 (NH4) 10 H2P 10 O 31 (NH4)9H3P 10 O 31 (NH4)8H4P 10 O 31 (NH4)7H5P 10 O 31 (NH4)6H6P 10 O 31 (NH4)5H7P 10 O 31 (NH4)4H8P 10 O 31 (NH4)3H9P 10 O 31 (NH4)2H 10 P 10 O 31 and (NH4)H 11 P 10 O 31 One or more of them.

[0023] The low molecular weight polyphosphate ammonium salt with hydrogen atoms at the end group involved in this invention is a white crystal or amorphous fine powder with low solubility in water and suitable thermal stability. It begins to slowly decompose into polyphosphate at about 150°C, and at the same time, adhesive aluminum polyphosphate will gradually be produced.

[0024] The aluminum polyphosphate salt involved in this invention is a balanced mixture of aluminum pyrophosphate and higher-chain aluminum polyphosphate salts, and may contain trace amounts of aluminum orthophosphate as impurities. The aluminum polyphosphate consists of two or more anions. A mixture of relatively complex polymers formed by the condensation of tetrahedral groups connected at their apexes, i.e., these mixtures all have a recurring tetrahedral structure. and They are connected by a POP bond through a shared oxygen atom at the apex.

[0025] The phosphorus atoms in the aluminum polyphosphate described in this invention are in a fully oxidized state, so its chemical properties are relatively stable. However, as is well known, aluminum polyphosphate is not very stable to hydrolysis. In the presence of water and other suitable conditions, all POP bonds may be broken and react with themselves or with other substances to form a substance with binding properties.

[0026] The hydrolysis pathway and rate of aluminum polyphosphate vary depending on the type of aluminum polyphosphate anion, and those containing more linked ions are more prone to hydrolysis. The tetrahedral condensed aluminum phosphate structure has relatively few interconnections. Tetrahedral condensed aluminum phosphate hydrolyzes much faster, especially hyperaluminum phosphate salts. Tetrahedral aluminum phosphates are numerous and always exhibit branched structures, making them the most unstable type of condensed aluminum phosphate. A key factor considered in selecting condensed aluminum phosphate salts for metallurgical pelletizing in this invention is the undesirability of excessively rapid reactions. Therefore, this invention preferentially selects linear condensed aluminum phosphate salts with a degree of polymerization n = 4-10, where the hydrogen atoms on the chain are not completely substituted, and the terminal groups are low molecular weight condensed aluminum phosphate salts with two hydrogen atoms. The specific low molecular weight condensed aluminum phosphate salts preferentially selected in this invention are as follows:

[0027] Al4(H2P4O 13 3. AlH3P4O 13 Al2(H4P4O) 13 3. Al(H5P4O) 13 3. Al5(H2P5O) 16 3. Al4(H3P5O) 16 3. AlH4P5O 16 Al2(H5P5O) 16 3. Al(H6P5O) 16 3. Al2H2P6O 19 Al5(H3P6O) 19 3. Al4(H4P6O) 19 3. AlH5P6O 19 Al2(H6P6O) 19 3. Al(H7P6O) 19 3. Al7(H2P7O) 22 3. Al2H3P7O 22 Al5(H4P7O) 22 3. Al4(H5P7O) 22 3. AlH6P7O 22 Al2(H7P7O) 22 3. Al(H8P7O) 22 3. Al8(H2P8O) 253. Al7(H3P8O) 25 3. Al2H4P8O 25 Al5(H5P8O) 25 3. Al4(H6P8O) 25 3. AlH7P8O 25 Al2(H8P8O) 25 3. Al(H9P8O) 25 3. Al3H2P9O 28 Al8(H3P9O) 28 3. Al7(H4P9O) 28 3. Al2H5P9O 28 Al5(H6P9O) 28 3. Al4(H7P9O) 28 3. AlH8P9O 28 Al2(H9P9O) 28 3. Al(H) 10 P9O 28 3. Al 10 (H2P 10 O 31 3. Al3H3P 10 O 31 Al2H4P 10 O 31 Al7(H5P) 10 O 31 3. Al2H6P 10 O 31 Al5(H7P) 10 O 31 3. Al4(H8P) 10 O 31 3. AlH9P 10 O 31 Al2(H) 10 P 10 O 31 )3 and Al(H 11 P 10 O 31 One or more of 3.

[0028] Compared to orthophosphates, aluminum polyphosphate, in addition to having the acidic -POO group, also contains other beneficial compounds. -In addition, there is the main chain of polyphosphoric acid -PO-O-PO-, which gives it its own unique characteristics. In chemical reactions, aluminum condensate salts have strong dehydration condensation properties, gradually polymerizing from smaller molecules into aluminum condensate with larger molecular weights. As mentioned above, the condensation properties of aluminum condensate with high P2O5 content are stronger than those with low P2O5 content. Heating can further accelerate the reaction rate of aluminum condensate salt hydrolysis.

[0029] The aluminum hydroxide involved in the metallurgical pellet bentonite binder of this invention is aluminum hydroxide trihydrate Al(OH)3, which appears as a white powder and can be either crystalline or amorphous. The crystal structure of aluminum hydroxide trihydrate consists of closely packed hydroxyl ions arranged in an AB double layer, with aluminum ions situated within these hydroxyl ions. Two-thirds of the octahedral voids are occupied by aluminum ions, leaving the remaining voids empty. Each aluminum ion coordinates to six -OH ions, and this close-packed hydroxyl ion structure forms a layered structure, with adjacent layers linked by hydrogen bonds formed by hydroxyl ions. This layered structure is readily wetted and penetrated by phosphoric acid and condensed phosphoric acid produced from the decomposition of ammonium orthophosphate and ammonium polyphosphate, reacting to generate the corresponding binding material for aluminum.

[0030] The calcined boromagnesia stone involved in the metallurgical pellet bentonite binder of this invention is produced by calcining and pulverizing natural boromagnesia stone ore at 700-850℃. It exhibits an alkaline reaction upon contact with water. Its composition and structure after calcination are complex, mainly containing MgO, B2O3, and a boron-magnesium complex, with the complex likely being the main component. Recent research has found that calcined boromagnesia stone, when used as a curing agent in aluminum phosphate binders, exhibits higher activity than calcined magnesium oxide, and can be used for both room temperature and high-temperature curing. Considering that metallurgical pellets involve both wet-bulb strength during the pelletizing stage and high-temperature strength during the drying and sintering stage, this invention employs calcined boromagnesia stone as both a room-temperature and high-temperature curing agent for the metallurgical pellet bentonite binder.

[0031] As can be seen from the roles of each component in the metallurgical pelletizing bentonite binder of this invention, the ammonium orthophosphate component mainly provides wet-bulb strength to the pellets. The ammonium polyphosphate and aluminum condensate polyphosphate components primarily provide high-temperature strength and anti-crash temperature to the pellets. The aluminum hydroxide and calcined borosilicate components can react with the above three components at both room temperature and high temperature to form binder substances, which can be said to provide wet-bulb strength and high-temperature strength to the pellets.

[0032] The metallurgical pellet bentonite binder and reinforcing agent composite of the present invention is composed of ammonium orthophosphate salts, ammonium polyphosphate salts, aluminum polyphosphate salts, calcined borosilicate, and aluminum hydroxide. From the mechanism of action of each component of the metallurgical pellet bentonite binder and reinforcing agent composite of the present invention, it can be seen that some components can undergo decomposition and condensation reactions to provide a binding effect, some components react with each other to provide a binding effect, and some components must react with calcined borosilicate or aluminum hydroxide to provide a binding effect; some reactions occur at room temperature and produce a binding effect, while others occur at high temperatures and produce a high-temperature binding effect. Therefore, the reinforcing system composed of all the above five types of components has a variety of substances, with multiple components capable of simultaneously providing both low-temperature and high-temperature strength. The components in the composition have synergistic and complementary effects, strong anti-interference ability, and can still exert a reinforcing effect even when pelletizing conditions fluctuate to a certain extent.

[0033] The preferred embodiment of this invention is a thickening system composed of ammonium orthophosphate, ammonium polyphosphate, aluminum condensate, calcined boromagnesia, and aluminum hydroxide, with the following mass ratio between the components:

[0034] Aluminum hydroxide: Calcined boromagnesia: Ammonium orthophosphate: Ammonium polyphosphate: Aluminum condensate = 40-50: 10-15: 10-15: 15-20: 15-20

[0035] The preferred degree of polymerization for ammonium polyphosphate salts is 4-10; the preferred degree of polymerization for aluminum polyphosphate salts is 4-10.

[0036] The five components are mixed evenly and sealed in a package to obtain the metallurgical pellet bentonite binder and reinforcing agent composite of the present invention.

[0037] The metallurgical pellet bentonite binder of this invention provides additional (auxiliary) bonding effect for iron ore powder bound to pellet bentonite, which is achieved through the following chemical processes.

[0038] (1) The mechanism by which the bentonite binder-reinforcing compound for metallurgical pellets provides wet-bulb strength to the pellets is roughly as follows:

[0039] The iron ore pellet binder composite, composed of ammonium polyphosphates, especially ammonium orthophosphates, dispersed in bentonite pellets, undergoes hydrolysis upon contact with water sprayed during pelleting to generate phosphoric acid. This phosphoric acid then reacts with aluminum hydroxide to form a series of binding substances that provide strength to the wet pellets. For example, the hydrolysis of NH4H2PO4 to H3PO4:

[0040] NH4H2PO4→H3PO4+NH3↑

[0041] The newly formed H3PO4 reacts with another component in the binder, Al(OH)3, in a neutralization reaction to form Al(H2PO4)3.

[0042] Al(OH)3+3H3PO4→Al(H2PO4)3+3H2O

[0043] The newly formed Al(H2PO4)3 has adhesive properties, and it can also slowly condense to gradually form an adhesive Al2H2P6O. 19 wait.

[0044] The generated Al(H2PO4)3 and Al2H2P6O 19 It can also react with MgO in calcined boromagnesite to produce Mg(H2PO4)2 and MgAlHP6O. 19 And MgB(H2PO4)5, these substances produced by the reactions can all provide the pellets with room temperature strength.

[0045] Some components of the aluminum polyphosphate in the reinforcing agent complex can dissociate into chelating groups upon contact with water, which then complex with the iron element in the iron powder, giving the pellets room-temperature strength. For example, component AlH6P7O... 22 Upon contact with the water injected during pelleting, it dissociates to release AlH4P7O, which has a strong complexing ability. 22 2- ion:

[0046] AlH6P7O 22 →AlH4P7O 22 2- ++2H +

[0047] AlH4P7O 22 2- Ions penetrate the surface of iron oxide powder and combine with iron to form FeAlH4P7O 22 These FeAlH4P7O 22 A bonding effect gradually forms between the iron powder particles:

[0048] AlH4P7O 22 2- +Fe 2+ →FeAlH4P7O 22

[0049] The reaction products are Al(H2PO4)3 and FeAlH4P7O. 22 These components all act as binders here, providing a binding effect for iron powder pellets in addition to the binding effect of bentonite, thereby increasing the wet pellet strength of iron ore pellets.

[0050] (2) The mechanism by which bentonite binder, a metallurgical pelletizing agent, provides high-temperature strength to the pellets is as follows:

[0051] The aluminum condensate salt in the metallurgical pellet bentonite binder composite of this invention has a unique structure, namely, it simultaneously contains acidic groups -POO. - The relatively long chain backbone -PO-O-PO- exhibits strong dehydration condensation properties during pellet drying and sintering, gradually polymerizing from smaller molecules into larger molecular weight condensed aluminum phosphate with excellent bonding properties, thereby improving the high-temperature strength and bursting temperature of the pellets. In practice, during pellet drying and sintering, as the temperature increases, each condensed aluminum phosphate molecule in the metallurgical pellet bentonite binder composite of this invention may undergo intermolecular dehydration polymerization under high temperature. For example, each AlH4P5O... 16 Both the "O" and "OH" radicals on the molecule can each fuse with another adjacent AlH4P5O group. 16 The "OH" and "O" groups on the molecules of aluminum phosphate, or other different types of condensed aluminum phosphate molecules, combine to form a water molecule, undergoing a condensation polymerization reaction. These AlH4P5O... 16 They undergo a shrinkage polymerization reaction in both planar and spatial dimensions to form long-chain or three-dimensional network macromolecules, which generate adhesive forces between iron oxide powder particles, increasing the high-temperature strength and crack resistance of iron ore pellets.

[0052] The aluminum polyphosphate in the bentonite binder and reinforcing agent composite for metallurgical pellets of this invention can also dissociate into complexing groups when heated in the presence of water, and then complex with the iron element on the surface of iron powder to strengthen the pellets. For example, AlH4P5O is one component of the bentonite binder and reinforcing agent for metallurgical pellets of this invention. 16 When heated, it can dissociate to release AlH2P5O, which has complexing ability. 2- 16 ion:

[0053] AlH4P5O 16 →AlH2P5O 2- 16 ++2H +

[0054] AlH2P5O 2- 16 Ions penetrate the surface of iron oxide powder and combine with iron to form FeAlH2P5O 16 These FeAlH2P5O 16 A bonding effect gradually forms between the iron powder particles:

[0055] AlP3O 10 2- +Fe 2+ →FeAlH2P5O 16

[0056] The reaction product is FeAlH2P5O16 These components all act as binders here, providing the iron powder with binding force beyond that of bentonite, which can contribute to the high-temperature strength of the pellets.

[0057] The ammonium polyphosphate salt in the bentonite binder composite for metallurgical pellets of this invention undergoes dehydration, deammoniation, and condensation reactions during high-temperature heating in pellet drying and sintering, generating polyphosphoric acid. This polyphosphoric acid reacts with Al(OH)3 to form aluminum polyphosphate, which exhibits good binding properties, increasing the high-temperature strength of the pellets and raising their bursting temperature. For example:

[0058] (NH4)H7P6O 19 →NH3+H8P6O 19

[0059] H8P6O 19 +Al(OH)3→AlH5P6O 19 +3H2O

[0060] The Al(H2PO4)3 and Al2H2P6O generated above 19 It can also react with MgO in calcined boromagnesite to produce Mg(H2PO4)2 and MgAlHP6O. 19 And MgB(H2PO4)5, these substances generated by the reactions can all provide the pellets with room temperature strength. The mechanism by which many phosphates and polyphosphates in this invention can act as binders and reinforcing agents for bentonite pellets also includes the fact that the phosphate ions contained in the polyphosphate components of the reinforcing agent complex can undergo a condensation reaction under certain temperature conditions to generate macromolecules. In the initial stage, the PO4 present in the phosphate binder... 3- When boromagnesia stone is calcined with a curing agent, it gradually undergoes condensation polymerization to generate PO4. 2- Thus, it becomes the "endpoint" of chain polymerization; as the reaction progresses, PO4... 2- After the ionic reaction, it becomes PO4. - Many PO4s - The ends combine in pairs to form a series of linear polyphosphates. Subsequently, the linear polyphosphates gradually branch out and evolve in a three-dimensional direction, thus forming a three-dimensional network structure, and finally becoming a polymer with a certain bonding strength, binding the iron oxide powder together.

[0061] It should be noted that, since the aluminum polyphosphate salts and ammonium polyphosphate salts involved in this invention are complex compounds, they may undergo various other chemical reactions under pelletizing conditions. Overall, these reaction products are beneficial to the strength of the pellets.

[0062] One of the significant features of this invention is that the wet-bulb strength of the pellets formed using the bentonite binder-reinforcing compound of this invention is mainly provided by bentonite, while the reinforcing compound of this invention also plays a certain role in wet-bulb strength, thereby improving the wet-bulb strength of the pellets. Because the montmorillonite particles in the pellets have very small sizes and negative charges within their internal unit cells, the repulsion between like charges between particles gives them good dispersibility. Adding bentonite improves the bonding properties of the pelletizing material, reducing the capillary diameter within the green pellets and increasing capillary force. On the other hand, after absorbing water, bentonite forms colloidal particles that fill the spaces between the green pellet particles, improving the connection between material particles and acting as a transfer agent between particle molecules, increasing the bonding force between particles. The greater the bonding force, the greater the mechanical strength of the green pellets. The addition of bentonite to pellet production increases the compressive strength of the pellet ore. When green pellets are subjected to external impact, sliding can occur between the particles, thus significantly improving the drop strength of the green pellets. The metallurgical pellet bentonite binder composite of the present invention, such as the H3PO4 generated by the room temperature hydrolysis of NH4H2PO4 and the AlH2PO4 and its condensate generated by the reaction of Al(OH)3, and the products generated by the room temperature hydrolysis of aluminum condensate salt and the reaction with iron powder, all have good binding properties at room temperature, which further increases the wet ball strength of the pellets.

[0063] Another significant feature of this invention is that the high-temperature strength and burst resistance of the pellets come from two parts. One part comes from the strength generated by bentonite, but this is insufficient; using bentonite alone requires adding 2%-2.5% or even higher to meet the requirements. The other part comes from the metallurgical pellet bentonite binder and reinforcing agent composite of this invention. Bentonite is a substance with a large specific surface area and high dispersion. Its addition changes the surface properties of the mixture, causing a high-temperature sintered glassy phase to exist between the iron powders in the pellets, thereby improving the high-temperature strength of the pellets. The use of bentonite as a binder can greatly increase the burst resistance because bentonite has a special affinity for water, allowing water inside the pellets to slowly migrate to the surface of the green pellets for evaporation, rather than causing the water inside the pellets to escape too quickly, thus preventing the pellets from bursting due to excessive internal vapor pressure. As mentioned above, the contribution of the metallurgical pellet bentonite binder composite to the high-temperature strength of the pellets is that certain components in the metallurgical pellet bentonite binder composite react before and during the pellet drying and sintering process to generate substances with high-temperature bonding strength, thereby significantly improving the high-temperature strength and bursting temperature of the pellets.

[0064] The pellets made using the bentonite binder and reinforcing agent composite of this invention continuously generate binder substances other than bentonite from the beginning of pelletizing, which is another significant feature of this invention. These binder substances continuously contribute to the strength of the pellets. As the drying and sintering temperature of the pellets increases, new binder substances appear, contributing to the high-temperature strength of the pellets. This reinforcing effect of the reinforcing agent composite of this invention is another auxiliary effect on the binding strength of the pellets based on bentonite. Moreover, even after the temperature rises to 600-700°C and bentonite loses its structural water and no longer has a binding effect, the high-temperature binding effect of this invention continues to play a role. Therefore, these pellets can withstand greater blast stress than pellets bound by bentonite alone. The performance of the bentonite binder and reinforcing agent composite of this invention in continuously increasing the thermal strength of iron ore pellets during heating is beneficial. It enables the dry heat strength of the pellets to increase rapidly during heating, allowing them to withstand greater blast stress. In particular, the performance of maintaining or increasing thermal strength after the loss of the binding effect of bentonite at 600-900°C is even more valuable.

[0065] This invention also provides a specific preparation method for the metallurgical pellet bentonite binder and reinforcing agent of this invention. Another technical problem it aims to solve is to provide a method for preparing the metallurgical pellet bentonite binder and reinforcing agent of this invention. Since the amount of the metallurgical pellet bentonite binder and reinforcing agent composite is relatively small, and the pellet bentonite grinding process is a continuous operation, a relatively large amount of carrier is needed to ensure the uniform distribution of the metallurgical pellet bentonite binder and reinforcing agent composite in the bentonite. The specific preparation method of the metallurgical pellet bentonite binder and reinforcing agent of this invention is as follows: aluminum hydroxide trihydrate, calcined boromagnesia, ammonium orthophosphate salts, ammonium polyphosphate, and aluminum condensate polyphosphate salts are added in proportion and mixed evenly to obtain the metallurgical pellet bentonite binder and reinforcing agent composite of this invention; then, this composite is mixed evenly with pellet bentonite powder as a carrier at a mass ratio of 1:1 to 5 to obtain the metallurgical pellet bentonite binder and reinforcing agent of this invention.

[0066] The pelletized bentonite binder of this invention is a yellowish-brown powder. Industrial experiments have proven that this product has excellent binding and reinforcing effects on both low- and high-blue-absorption bentonite. In particular, it enables the use of widely available low-blue-absorption bentonite in the production and processing of pelletized bentonite, which is significant for the full utilization of bentonite resources and reducing the production cost of pelletized bentonite. The pelletized bentonite carrier in this invention is not an inert carrier; its main function is as a carrier for the pelletized bentonite binder. They are mixed evenly to facilitate the uniform addition and dispersion of the metallurgical pelletized bentonite binder in the pelletized bentonite. Furthermore, the carrier itself is pelletized bentonite, and therefore still has a binding effect on iron powder. In other words, the non-inert carrier in this invention will not affect the quality of the foundry bentonite and will not produce adverse consequences.

[0067] The most significant beneficial effect of this invention is:

[0068] (1) Significantly improves the wet-bulb strength of bentonite pellets

[0069] The reaction products of bentonite binder in metallurgical pellets possess binding properties and effectively improve the compressive strength of green pellets. With a constant total bentonite content of 1.3%, as the binder content increases from 0.1% to 0.3%, the drop strength and compressive strength of magnetite green pellets increase from 2.4 drops / pelle to 6.0 drops / pelle, and the compressive strength increases from 10.30 N / pelle to 14.20 N / pelle. This is because the binder components generated by the binder reaction are initially viscous, forming viscous bridges between iron powder particles. With increasing proportions of bentonite binder in metallurgical pellets, the viscosity of the bridging fluid increases, thereby enhancing the interparticle adhesion energy. Without breaking the bridging bonds, this significantly increases the dislocation amplitude of the particles, increasing the interparticle resistance to plastic deformation, resulting in a substantial increase in the drop strength and compressive strength of the green pellets.

[0070] (2) The high-temperature strength and anti-burst temperature of the pellets are significantly improved.

[0071] The use of bentonite binder in iron ore pellets also significantly improves the high-temperature strength and anti-blast temperature of the pellets. Using bentonite alone requires a 2.0% admixture to achieve a blast temperature of 600℃, while adding 0.1%, 0.2%, and 0.3% binder to a bentonite base of 1.3% results in blast temperatures exceeding 650℃, 700℃, and 850℃, respectively. The main reason for the good high-temperature strength and anti-blast properties of iron ore pellets using metallurgical bentonite binder is that during the drying and sintering process, as the temperature rises, certain components of the metallurgical bentonite binder react to form compounds such as Al(H2PO4)3 and FeAl(H6P)3. 10 O 31 Under high temperature, these molecules undergo dehydration polymerization, forming long-chain or three-dimensional network macromolecular chains or networks, eventually forming a solid gel. This gel generates adhesive force between iron oxide powder particles, increasing the high-temperature strength and crack resistance of iron ore pellets.

[0072] (3) The amount of bentonite used in pellets was significantly reduced and the iron grade of pellets was significantly increased.

[0073] Without binder, the iron content of the pellets is 64.36%, while with the addition of 2.0% bentonite, the iron content is only 62.26%, a decrease of 2.10%. When 0.10% reinforcing agent and 1.30% bentonite are added for partial substitution, the iron content of the pellets is 63.74%, an increase of 1.48% compared to bentonite pellets. This is because the metallurgical pelletizing bentonite binder of this invention can provide additional low-temperature and high-temperature strength to the pellets, and the amount of bentonite added to the pellets is significantly reduced from 2.0% to 1.3%, which improves the physicochemical properties and pelletizing performance of the raw materials, thereby reducing the introduction of harmful impurities such as silica and significantly improving the quality of the finished pellets. Detailed Implementation

[0074] The following examples further illustrate the features of the present invention. These examples are for illustrative purposes only and do not limit the scope of protection of the present invention.

[0075] This invention (NH4)3H7P8O 25 Polymeric ammonium phosphate with Al8(H2P8O) as the main component and Al8(H2P8O) as the main component 25 Aluminum polyphosphate, mainly composed of 3, is obtained through commercial channels.

[0076] Example 1: Preparation of Bentonite Binder Reinforcing Agent for Metallurgical Pelletizing

[0077] Industrial aluminum hydroxide, calcined boromagnesia, industrial ammonium dihydrogen phosphate, and (NH4)3H7P8O 25 Polymeric ammonium phosphate, mainly composed of Al8(H2P8O) 25 Five components, mainly aluminum polyphosphate (APP) and phosphate (A3), are mixed evenly in a mass ratio of 45:12.5:12:17.5:17.5 to obtain a pellet bentonite binder and reinforcing agent composite. This composite is then mixed evenly with pellet bentonite in a mass ratio of 1:5 to obtain the metallurgical pellet bentonite binder and reinforcing agent A of this invention.

[0078] Example 2: Preparation of Bentonite Binder Reinforcing Agent for Metallurgical Pelletizing

[0079] Industrial aluminum hydroxide, calcined boromagnesia, industrial ammonium dihydrogen phosphate, and (NH4)3H7P8O 25 Polymeric ammonium phosphate, mainly composed of Al8(H2P8O) 25 The five components of aluminum polyphosphate, mainly 3, are mixed evenly in a mass ratio of 40:10:10:15:15 to form a pellet bentonite binder and reinforcing agent mixture. This mixture is then mixed evenly with pellet bentonite in a mass ratio of 1:5 to obtain the metallurgical pellet bentonite binder and reinforcing agent B of this invention.

[0080] Example 3: Preparation of Bentonite Binder Reinforcing Agent for Metallurgical Pelletizing

[0081] Industrial aluminum hydroxide, calcined boromagnesia, industrial ammonium dihydrogen phosphate, and (NH4)3H7P8O 25 Polymeric ammonium phosphate, mainly composed of Al8(H2P8O) 25 The five components of aluminum polyphosphate, mainly 3, are mixed evenly in a mass ratio of 50:15:15:20:20 to form a pellet bentonite binder and reinforcing agent mixture. This mixture is then mixed evenly with pellet bentonite in a mass ratio of 1:5 to obtain the metallurgical pellet bentonite binder and reinforcing agent C of this invention.

[0082] Example 4: Metallurgical pellet bentonite binder replacing high blue absorption bentonite

[0083] Magnetite powder was dried to control its moisture content to 8.4-8.6%. 5 kg of magnetite powder was weighed and mixed with a certain proportion of the metallurgical pelletizing bentonite binder A prepared in Example 1 and pelletizing bentonite H with a blue absorption capacity of 32 as pelletizing binders. Pelletizing was performed on a disc pelletizer with a diameter of 800 mm, a rotation speed of 25 r / min, and an inclination angle of 49°. After pelletizing, 500 g of green pellets was weighed to determine their moisture content, and green pellets of 10-15 mm were taken for testing their bursting temperature, drop strength, and compressive strength. The green pellets were dried at 120°C in a 50 mm diameter iron-chromium-aluminum wire resistance furnace and then calcined in a silicon carbide tube resistance furnace. The compressive strength of the pellets was measured using an intelligent pelletizing press. The pelletizing experimental results are shown in Table 1.

[0084] Table 1. Experimental results of reinforcing agent A combined with high-blue-absorption bentonite.

[0085] Enhancer A / % Bentonite H / % Drop intensity / (times / 0.5m) Compressive strength (N / piece) Bursting temperature / °C 0 2.0 3.3 9.10 ＞600 0 1.3 2.4 5.6 542 0.1 1.3 4.4 10.30 ＞650 0.2 1.3 5.2 12.01 ＞700 0.3 1.3 6.0 14.20 ＞850

[0086] The green pellets produced by reinforcing agent A all exhibited excellent thermal properties, with bursting temperatures exceeding 650℃. Adding 0.10% reinforcing agent A reduced the bentonite dosage to 1.3%, still meeting the green pellet strength requirements of 4.4 pellets / (0.5m). From the pelletizing effect, using reinforcing agent A, for the tested iron concentrate, a dosage of 0.10% reduced the bentonite dosage by 0.7%. This indicates that reinforcing agent A has a high substitution efficiency.

[0087] Example 5: Metallurgical pellet bentonite binder replacing low-blue-absorption bentonite

[0088] All operations were the same as in Example 4, except that bentonite M with a blue absorption capacity of 24 was used instead of bentonite H with a blue absorption capacity of 32 in the pelletizing experiment. The results of the pelletizing experiment are shown in Table 2.

[0089] Table 2. Experimental results of reinforcing agent A combined with low-absorption bentonite pellets.

[0090] Enhancer A / % Bentonite M / % Drop intensity / (times / 0.5m) Compressive strength (N / piece) Bursting temperature / °C 0 3.5 3.0 8.42 ＞600 0 1.7 1.6 4.4 530 0.1 1.7 4.2 8.78 ＞600 0.2 1.7 4.7 10.32 ＞650 0.3 1.7 5.3 12.74 ＞750

[0091] When using low-blue-absorption (24) bentonite alone for pelletizing, the bentonite content needs to reach 3.5% for the bursting temperature to exceed 600℃. However, when reinforcing agent A is added at 0.10%, the amount of low-blue-absorption bentonite is reduced to 1.7%, which can meet the green pellet strength requirements, reaching 4.2 times / (0.5m), with bursting temperatures all above 600℃. From the perspective of pelletizing effect, when using reinforcing agent A, inferior bentonite with low blue absorption can be used as a pellet binder.

[0092] Example 6: Iron grade of pellets when reinforcing agent B replaces bentonite

[0093] High-grade furnace feed is the fundamental guarantee for achieving high output and low consumption in blast furnaces, and it is also a long-term goal pursued by metallurgists. For many years, the pelletizing industry has been committed to the research and development of various binders, aiming to reduce or even eliminate bentonite usage and improve the iron grade of pellets. Therefore, the iron grade of pellets is the main basis for verifying the quality of bentonite additives used in pelletizing.

[0094] Table 3 shows the iron grade analysis results of pellets with different reinforcing agents B. Without binder, the iron grade of the pellets was 64.36%, while with the addition of 2.0% high-blue-absorption bentonite H(32), the iron grade was only 62.26%, a decrease of 2.10%. When 0.30% of reinforcing agent B completely replaced the bentonite, the iron grade of the pellets was 63.57%, only 0.79% lower than the unbindered pellets, but 1.31% higher than the bentonite pellets. When 0.10% of reinforcing agent B and 1.30% bentonite were added for partial replacement, the iron grade of the pellets was 63.74%, 1.48% higher than the bentonite pellets. Therefore, using reinforcing agent B to replace bentonite has a significant effect on improving the iron grade of the pellets.

[0095] Table 3 Iron grade of pellets with different proportions of reinforcing agent B

[0096] Bentonite dosage / % Enhancer B W(TFe)% 0 0 64.36 2.0 0 62.26 0 0.3 63.57 1.3 0.1 63.74

[0097] Example 7: Effect of Bentonite Binder Reinforcing Agent C on Pelletizing Properties of Iron Concentrate

[0098] Without the addition of metallurgical pelletizing bentonite binder, using only 2.0% high-blue-absorption bentonite H results in a green pellet growth rate of 1.46 mm / min. However, when 1.3% bentonite is combined with 0.2% metallurgical pelletizing bentonite binder C, the green pellet growth rate decreases to 1.52 mm / min. This is because the addition of metallurgical pelletizing bentonite binder C to the pellet raw material reduces the bentonite content, leading to a slight increase in the green pellet growth rate.

Claims

1. A metallurgical pellet bentonite binder and reinforcing agent composite, characterized in that: By weight, it includes the following raw materials: 10-15 parts of ammonium orthophosphate, 15-20 parts of ammonium polyphosphate, 15-20 parts of aluminum polyphosphate, 40-50 parts of aluminum hydroxide, and 10-15 parts of calcined boromagnesia. The general formula for the ammonium orthophosphate salts is (NH4). n H 3-n PO4, where n≤3; the general formula of the ammonium polyphosphate salt is H (n-m)+2 (NH4) m P n O 3n+1 In the formula, 4≤n≤10, m≤n+2; The condensed aluminum phosphate salts are linear chain-like aluminum phosphate salts having the following general formula: In the formula, 4≤n≤10; The calcined boromagnesia stone is made by calcining and pulverizing natural boromagnesia stone ore at 700-850℃. It reacts with ammonium polyphosphate salts and aluminum polyphosphate salts under normal and high temperature conditions to produce a bonding effect and solidify. Therefore, calcined boromagnesia stone is used as a room temperature curing agent and a high temperature curing agent for the bonding and reinforcing agent of metallurgical pellet bentonite.

2. The metallurgical pellet bentonite binder composite as described in claim 1, characterized in that: Ammonium orthophosphate salts are one or more of NH4H2PO4, (NH4)2HPO4 and (NH4)3PO4.

3. A method for preparing a metallurgical pellet bentonite binder, characterized in that: The metallurgical pellet bentonite binder and reinforcing agent composite of claim 1 is mixed with pellet bentonite powder as a carrier at a mass ratio of 1:1 to 5 to obtain the metallurgical pellet bentonite binder and reinforcing agent.

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