Research on Fine Chemical Formulations and Product Preparation Technology. Volume 2

Author: Huang Yuyuan
Publisher:
Publish Date: 2003-06-01
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 space of a mixer 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 categorized based on the form of the materials being mixed, such as solid-solid mixing, liquid-liquid mixing, solid-liquid mixing, 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 additives with plastics, rubber, and other polymer solid materials, the principles and equipment for this type of mixing 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 commonly referred to as mixing and plasticizing. This chapter discusses solid-solid mixing, while plasticizing and mixing 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 common stirrer mixers can be used for this purpose, 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 techniques will be introduced in Chapter 11.
Solid-liquid mixing can be categorized into various 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 powder or granular material, a liquid can be added to the 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 or paste 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 material, wet grinding equipment is typically used, where dispersion and mixing are completed simultaneously during the grinding process. This was discussed in the previous section.
(3) When the mixing involves adding a small amount of solid material to a liquid material, if the material is soluble in the liquid, stirring alone can achieve a uniform mixture 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. 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 insoluble 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 formulating 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 primarily 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 or pull 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 fed into the mixer into a random distribution state where the materials are uniformly 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 movement. This process is repeated multiple times to achieve uniform mixing. The mixing principles of the two are different.
II. Factors Affecting Material Mixing
Whether solid powders can be mixed uniformly depends greatly on their physical properties. Particles of uniform size and similar density are easier to mix uniformly. Particles of uniform size but different densities tend to settle at the bottom of the container during mixing, exhibiting a stratification tendency. Particles of similar density but different shapes, such as small, smooth, spherical particles, also tend to settle at the bottom of the container and exhibit a stratification tendency. Particles that are prone to sticking together are also difficult to mix uniformly due to the high resistance to relative movement between particles. To mix particles with different properties uniformly, various types of mixing equipment have been developed for different material properties. Appropriate equipment can often achieve satisfactory mixing results, making the selection of the mixing equipment a key factor in obtaining uniform products.
III. Selection of Powder Mixing Machines
The selection of a mixing machine primarily 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. Drum-type mixers use the rotation of the mixing chamber to achieve mixing and are primarily suitable for materials with minor property differences and good flowability. They can also be used for adding a small amount of liquid to solid materials. Rotating-type 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-screw 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 tendency of the material to stratify, thus achieving a uniform mixture. The cleanliness of the mixture requires that the material not be contaminated during the mixing process. This requires the material of the mixing equipment to be chemically inert to 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, high-purity products often benefit from using 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 mixing equipment has different effects on the particle size of the material during mixing. Even for the same type of mixer, the shape of the mixing chamber or the stirrer, as well as the speed, can affect the particle size differently. For products with strict requirements on particle size, it is necessary to select equipment that does not cause particle crushing or wear during the mixing process.
(3) The requirements of the material mixing process refer to whether heating or cooling is needed during mixing, whether batch addition of components is required, or whether there are specifications on the amount of material added per unit time. In such cases, it is necessary to select equipment that meets these 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.