Fine Chemical Formulation Research and Product Preparation Technology (Volume 2)

Author: Huang Yuyuan et al.
Publisher:
Publish Date: 2003-06-27
Features:
Section 2: Mixing refers to the operation of combining materials with different properties or forms through mixing equipment to achieve a random distribution state. Random distribution is a fundamental concept in probability theory and mathematical statistics. In simple terms, if we divide the mixing space into several regions to a certain extent and take samples from any region for measurement, and if the results show that the proportion (by mass or volume) of each component within the region is the same as the feed ratio of the materials into the mixer (or within the allowable error range of the product specifications), then the random distribution state is achieved, and the product can be considered uniformly mixed on a macroscopic level. Mixing operations can be classified based on the form of the materials being mixed into solid-solid, liquid-liquid, solid-liquid, gas-liquid mixing, etc. In fine chemical formulation products, solid-solid, liquid-liquid, and solid-liquid mixing products are most common. Solid-solid mixing typically refers to the mixing of powders and granules. Involving the mixing of solid polymer materials such as plastics and rubber with solid additives, the principles and equipment differ significantly from the mixing of powders and granules. The mixing of rubber, plastics, and additives often requires special equipment called mixing and plasticizing machines, where the polymer materials are heated and melted, and then mixed within the molten region. This operation is often referred to as mixing and plasticizing. This chapter discusses solid-solid mixing, while mixing and plasticizing will be introduced in Chapter 10.
Liquid-liquid mixing can be divided into single-phase mixing of mutually soluble liquid materials and multi-phase mixing of immiscible liquids. The former, such as glycerol and water or alcohol and water, can easily achieve uniform single-phase mixtures through stirring and is commonly completed using stirrer mixers, so it is not discussed in this section. When immiscible liquids are mixed, it is not easy to disperse one liquid into droplet form and uniformly distribute it into another immiscible liquid to achieve a stable dispersion. The mixing of immiscible liquids often requires the addition of surfactants, and sometimes specific emulsification and dispersion equipment must be used to control certain conditions and apply specific experience to achieve the desired result. Emulsification technology will be introduced in Chapter 11.
Solid-liquid mixing can be categorized into different types based on the final product form:
(1) When the mixing involves adding a small amount of liquid to solid mixing components, and the product remains a powdery material, a liquid nozzle can often be added to solid-solid mixing equipment to spray the liquid component into the solid powder mixture during the mixing process. When the mixing involves adding a small amount of liquid to powders to prepare uniform plastic materials or pasty materials, or adding a small amount of powder or liquid additives to high-viscosity materials to form a uniform mixture, a machine called a kneader must be used. The mixing process is also referred to as kneading.
(2) When the mixture is a paste or slurry, wet grinding equipment is typically used, where dispersion and mixing are completed simultaneously during the grinding process. This was introduced in the previous section.
(3) When the mixing involves adding a small amount of solid to liquid materials, if the material is soluble in the liquid, stirring can produce a uniform solution. When the material is dispersed as colloidal particles (particle size 1–100 nm) in the liquid, a colloidal solution is formed. Proteins, animal glue, and soaps, for example, disperse in water to form colloidal solutions when they come into contact with water. Through artificial methods, such as using a colloid mill, substances can be ground to colloidal particle size to obtain a colloidal solution. If the solid particles are not soluble in the liquid and the particle diameter is greater than 100 nm (0.1 μm), the mixture is an unstable suspension. Suspensions can also be prepared using wet grinding equipment, but due to their instability, they are not suitable as commercial products. When necessary, a usage note must be added, such as "shake well before use."
For formulated fine chemical products, uniform mixing of components is a basic requirement. Therefore, the mixing operation is of great significance in the production of formulated products. This section focuses on solid-solid mixing and also covers some aspects of solid-liquid mixing and kneading.
I. Mixing Principles
Solid-solid mixing refers to the mixing of solid powders and granules. The mixing operation involves using mixing equipment with different structures to stir, flip, and push the materials, causing the positions of the components in the material to change and overcoming the tendency of the particles to stratify due to their different properties. This transforms the initial partitioned distribution state of the materials when they are first added to the mixer into a random distribution state where the materials are mixed. In contrast, kneading mixing, which produces dough-like materials, primarily relies on the shearing force generated by the moving parts of the equipment to compress, stretch, tear, and fold the material while promoting relative motion. This process is repeated multiple times to achieve uniform mixing. The mixing principles of the two are different.
II. Factors Affecting Material Mixing
The ease of achieving uniform mixing of solid powders depends greatly on the physical properties of the particles. Uniform-sized particles with similar densities are easier to mix uniformly. Uniform-sized particles with different densities tend to settle at the bottom of the container, showing a stratification tendency. Particles with similar densities but different shapes, such as small, smooth, spherical particles, also tend to settle at the bottom of the container and show a stratification tendency. Particles that are prone to sticking together are also difficult to mix uniformly due to the high resistance to relative motion between particles. To mix particles with different properties uniformly, various types of mixing equipment have been developed for different material properties. The right equipment often yields satisfactory mixing results, so choosing the appropriate mixing equipment is often the key to obtaining a uniform product.
III. Selection of Powder and Granule Mixers
The selection of a mixer considers factors such as the properties of the material, the quality requirements of the mixture, and the requirements of the mixing process.
(1) Different materials require different mixing equipment. Tumbler mixers use the rotation of the mixing chamber to achieve mixing and are primarily suitable for materials with minor property differences and good fluidity. They can also be used for adding a small amount of liquid to solid materials. Rotating mixers use the rotation of components (rotors) within the mixing chamber to achieve mixing and are suitable for materials with certain differences in properties and particle size. Some models of these mixers, such as double helical conical mixers and SCH screw belt conical mixers, can also achieve good mixing results for materials with significant density differences and varying particle sizes. It is clear that different mixers have different applicability to material properties, so the correctness of the selection directly affects the success of mixing. Therefore, it is the first factor to consider when selecting.
(2) The quality of the mixture includes uniformity, cleanliness, and particle size distribution. The uniformity of the mixture primarily depends on the suitability of the selected equipment for the material. A suitable equipment can overcome the stratification tendency of the material, thus achieving a uniform mixture. The cleanliness of the mixture requires that the material not be contaminated during the mixing process. This means that the material of the mixing equipment should not react with the components of the material. The inner wall or rotating parts of the mixing equipment should not contaminate the material due to wear during mixing. For mixers with rotating parts, the lubricating oil used for the rotating shaft should not contaminate the mixture. The mixer should also be easy to clean to prevent residual material from contaminating subsequent products. In fact, to meet cleanliness requirements, for products requiring high cleanliness, it is often better to choose mixers without rotating parts, with well-polished and wear-resistant inner walls, and simple structures. The particle size distribution of the product is often one of the important quality indicators. Different mixers have different effects on the particle size of the material during the mixing process. Even for the same type of mixer, the shape of the mixing chamber or the shape and speed of the agitator can affect the particle size. For products with strict requirements on particle size, it is necessary to choose equipment that does not cause particle crushing or wear during mixing.
(3) The requirements of the material mixing process refer to whether heating or cooling is needed during the mixing process, whether batch addition of components is required, or whether there are specifications on the amount of addition per unit time. In such cases, it is necessary to choose equipment that meets the process requirements. For example, to achieve heating and cooling, a jacketed equipment should be selected. To control the addition of materials, it is preferable to use fixed-container equipment. If a rotating-container equipment is selected, special positioning and braking devices must be installed.