1 . Introduction:
The preparation of metals and their composite powders has developed numerous methods, and several classifications for these methods have been established.
Depending on the state of the raw material, the methods can be divided into solid, liquid and gaseous methods; Based on the state of the reagents, they can be categorized as wet and dry methods; and according to the production principle, they can be divided into physicochemical and mechanical methods.
Generally, in physicochemical methods, the most important are reduction, combination reduction and electrolysis, while in mechanical methods, atomization and mechanical grinding are more prominent.
The choice of metal powder production method depends on the raw material, powder type, powder material performance requirements and powder production efficiency.
As the application of powder metallurgy products becomes more and more widespread, the requirements for the size, shape and performance of powder particles are becoming increasingly higher.
Therefore, powder preparation technology is continually evolving and innovating to meet particle size and performance requirements.
2. Metal powder preparation methods:
2.1 Physicochemical Methods
2.1.1 Reduction Method
The reduction of metal oxides and salts is a widely used method for preparing powders. Solid carbon can be used to reduce iron and tungsten powder, while hydrogen or decomposed ammonia is used to produce tungsten, molybdenum, iron, copper, cobalt and nickel powders.
Iron powder can also be produced using converted natural gas and coal gas. Sodium, calcium, magnesium and other metals can act as reducing agents to produce tantalum, niobium, titanium, zirconium, thorium, uranium and other rare metal powders.
The basic principle of this reduction method is that the affinity of the reducing agent used for oxygen is greater than that of the metal in the oxide or salt, thus allowing the reduction of the metal by capturing the oxygen in the metal oxide or salt.
Because different metallic elements react differently with oxygen, the stability of the resulting oxides also varies. The degree of stability of the oxide can be characterized by the size of ΔG during the oxidation process. The lower the ΔG value during the reaction, the greater the stability of the oxide, indicating greater affinity for oxygen.
The advantages of this method include its simplicity, easy control of process parameters, high production efficiency and low cost, making it suitable for industrial production.
However, it is only applicable to metallic materials that readily react with hydrogen and become brittle and prone to fracture after absorbing hydrogen.
2.1.2 Thermal Metal Reduction and Reduction Combination Method
The thermal reduction of metal involves the reduction of raw materials that can be solid, gaseous or even molten salts, the latter two having the characteristics of reducing the gas phase and precipitation of the liquid phase.
Common industrial applications of the metal thermal reduction method include using calcium to reduce TiO2, ThO2, UO2 and others; magnesium to reduce TiCl4, ZrCl4, TaCl5 and others; sodium to reduce TiCl4, ZrCl4, K2ZrF6, K2TaF7 and others; and calcium hydride (CaH2) for co-reduction of chromium oxide and nickel oxide for the production of nickel-chromium stainless steel powder.
The reduction combination method refers to the process of obtaining carbides and borides through the reaction of carbon, boron carbide, silicon, nitrogen and refractory metal oxides.
2.1.3 Electrolysis Method
The electrolysis method involves the deposition of metal powder on the cathode through the electrolysis of molten salts or their aqueous solutions. Almost all metal powders can be produced by electrolysis, with copper, silver and tin powders being particularly suitable.
Electrolysis can be divided into aqueous solution electrolysis, organic electrolyte electrolysis, molten salt electrolysis and liquid metal cathode electrolysis.
The advantage of this method is that it produces metal powder with high purity, generally with a purity of 99.7% or more for single element powders. Additionally, electrolysis can precisely control particle size, enabling the production of ultrafine powders.
However, the electrolysis method consumes a large amount of electricity, resulting in higher powder production costs. Aqueous electrolysis can produce Cu, Ni, Fe, Ag, Sn, Fe-Ni and other metal powders (alloy), while molten salt electrolysis can produce Zr, Ta, Ti, Nb and other metal powders.
2.1.4 Hydroxyl Method
Certain metals (such as iron, nickel, etc.) are synthesized with carbon monoxide to form metal carbonyl compounds, which are then thermally decomposed into metal powder and carbon monoxide.
The resulting powder is extremely fine and pure, but the process is expensive. Industrially, it is mainly used to produce fine and ultrafine nickel and iron powders, as well as Fe-Ni, Fe-Co, Ni-Co and other alloy powders.
2.1.5 Chemical Shift Method
The chemical shift method is based on the reactivity of metals, where a more reactive metal displaces a less active metal from its salt solution, producing a metal (metal powder particles) that is subsequently refined by other methods.
This method is mainly used in the preparation of powders from less active metals such as Cu, Ag, Au.
2.2 Mechanical Method
2.2.1 Atomization Method
The atomization method is a mechanical method of powder production, involving the direct spraying of liquid metal or alloy to produce powder. It is widely applied and second only to the downscaling method.
Also known as the pulverizing method, it can be used to produce powders of metals such as lead, tin, aluminum, copper, nickel and iron. It can also be used in the production of alloy powders such as bronze, brass, carbon steel and alloy steel.
Atomization generally involves using high-pressure gas, high-pressure liquid, or high-speed rotating blades to break high-temperature, high-pressure molten metal or alloy into small droplets. These droplets then condense inside a collector to form ultrafine metallic powder, a process that involves no chemical changes.
Atomization is one of the main methods of producing metal powders and alloys. There are many atomization methods, such as dual-flow atomization, centrifugal atomization, multi-stage atomization, ultrasonic atomization technology, tightly coupled atomization technology, high-pressure gas atomization, laminar flow atomization, tightly coupled ultrasonic atomization and gas atomization. hot gas.
Atomized powder has advantages such as high degree of sphericity, controllable powder granularity, low oxygen content, low production cost and adaptability to the production of various metal powders.
It has become the main development direction for special and high-performance alloy powder preparation technology. However, atomization has disadvantages such as low production efficiency, low ultrafine powder yield rate, and relatively high energy consumption.
2.2.2 Mechanical Spraying Method
Mechanical pulverizing of solid metals is a distinct method of powder production, closely associated with the final state of solid deformation and the formation and extension of cracks during pulverizing.
Furthermore, it serves as an indispensable complementary process for some powder production methods, such as grinding electrolytically produced brittle cathode precipitates or grinding sponge-like pieces of metal produced by reduction. Therefore, the mechanical spraying method occupies a significant position in powder production.
The spraying method varies according to the nature of the materials and the degree of spraying required.
Depending on the mode of application of external force, material pulverization generally occurs through compression, impact, crushing and focused splitting. The operating principles of various spray equipment are based on these methods.
Among them, ball mill mainly involves rolling ball and vibrating ball grinding methods. This method uses the mechanism where metal particles decompose into finer substances due to deformation at different strain rates.
Its advantages include low material selectivity, continuous operation, high production efficiency and it is suitable for dry and wet grinding, facilitating the preparation of various metal and alloy powders. The disadvantage is that classification is relatively difficult during the powder preparation process.
2.2.3 Grinding Method
The grinding method involves directing compressed gas through a specialized nozzle into the grinding area, causing materials within this zone to collide and turn into powder.
The expanded airflow rises with the materials to the classification zone, where a turbine classifier separates the materials that have reached the desired granularity.
The remaining coarse powder returns to the grinding area for further grinding until it reaches the necessary granularity for separation. The grinding method, as it is a dry process, eliminates the need for dehydration and drying operations of the material.
The resulting product is of high purity, excellent activity and good dispersibility, with fine granularity and a narrow distribution range. The particles have smooth surfaces and are widely applied in industries such as non-metallic, chemical raw materials, pigments, abrasives, healthcare pharmaceuticals and others for ultra-fine crushing.
However, the grinding method has some disadvantages, such as high equipment manufacturing costs, and in the metal powder production process, a continuous supply of inert gas or nitrogen as a source of compressed gas is required, which leads to consumption substantial amount of gas.
Therefore, it is only suitable for crushing and pulverizing processes of brittle metals and alloys.
3. Summary
With the advancement of technology, metallic powders have been developed and applied in areas such as metallurgy, chemical engineering, electronics, magnetic materials, fine ceramics and sensors, presenting promising application prospects.
Metallic powders tend to have greater purity and superfine (nano) development. Although there are several methods for preparing ultrafine metal powders, each method has its limitations and there are many problems that need to be solved and improved.
Currently, the most used methods for the production of metal powders are reduction, electrolysis and atomization.
Furthermore, improvements in traditional production processes have led to many new production techniques and methods, such as ultrasonic atomization, rotating disc atomization, double and triple roller atomization, multi-stage atomization, plasma rotary electrode process and arc method. electric. .
Among the production methods of metal powders, although many have been put into practical use, there are still two main problems: the scale is small and the production cost is high.
To promote the development and application of metal powder materials, it is necessary to make comprehensive use of different methods, to take advantage of their strengths and compensate for their weaknesses, and to develop processes that produce higher production volumes and lower costs.
