Glass and Crystal Manufacturing in Iran (بلورسازی در ایران)
Solid mineral materials composed of molecules, atoms, or ions that are arranged regularly in space or possess a crystalline structure are called crystals (بلور). Crystallization (تبلور) is a process through which crystalline materials are purified and obtained in a desired size. During crystallization, solid particles are formed from a homogeneous phase. This process may occur from a gas phase, as in the formation of snow, or from a liquid phase, through freezing or precipitation.
In general, crystallization is a solid–liquid separation technique. It is used to separate a dissolved solid completely from its solvent. Crystallization has wide applications in obtaining pure chemical substances under different conditions. One of the reasons for its widespread use is that crystals formed from a solution may contain impurities.
Crystal (بلور)
A crystal is the most highly organized form of inorganic matter. In fact, a crystal is a solid in which atoms, ions, or molecules are arranged in a regular pattern. Within a crystal, the constituent molecules, ions, or atoms are arranged in an orderly manner, and consequently the geometric form of a crystal is independent of its size.
As a crystal grows, each of its surfaces develops according to the same orderly pattern. If, under ideal conditions, a growing crystal maintains its geometric similarity, it is described as a geometrically invariant crystal (بلور تغییرناپذیر).
Crystal Production (تولید بلور)
Crystalline bodies are produced through a thermal process. During this process, changes in concentration and temperature are accompanied by changes in heat. Crystallization may take place either from a solution or from a molten substance.
During dissolution, the number of particles decreases or remains constant. In contrast, during crystallization, the number of particles that precipitate increases.
Stages of Crystal Production (مراحل تولید بلور)
In crystallization vessels, one of the most important factors in the crystallization process is the particle-size distribution (توزیع اندازه ذرات). Through interactions among particles, the rates of nucleation (هستهزایی) and crystal growth (رشد بلور) are determined, independently of the material and energy balances.
Nucleation and crystal growth do not normally occur in an unsaturated or saturated solution. Very small crystals may, however, be produced through abrasion and may subsequently grow when the solution becomes supersaturated.
When the crystal growth rate is low, as may occur in a highly viscous solution, considerable time is required for the solid and liquid phases to reach equilibrium.
Crystal formation takes place in two stages:
Formation of a new particle (ایجاد ذره جدید)
Growth of the particle until it reaches macroscopic size (رشد ذره تا رسیدن به اندازه ماکروسکوپی)
Origin of Crystals (منشاء بلورها)
The formation of crystal nuclei can be divided into three categories. Spontaneous nucleation (هستهزایی خودبهخودی) is a process that occurs with difficulty. Nevertheless, the formation of nuclei depends on random variations in concentration or temperature at the molecular scale.
If cooling occurs slowly, the nucleation rate is low, and the precipitated material is deposited on a small number of nuclei, resulting in relatively large crystals. If cooling is rapid, nucleation increases and a larger number of nuclei are formed, resulting in smaller crystals.
1. Pseudonucleation (هستهزایی کاذب)
Pseudonucleation occurs under conditions of high supersaturation or poor circulation of the magma (ماگما؛ سوسپانسیون بلوری). It is characterized by abnormal needle-like or spine-like projections resembling imperfect crystals at the ends of crystals.
2. Primary Nucleation (هستهزایی اولیه)
Very small crystals formed on the surfaces of crystal particles produced during other crystallization processes disappear rapidly and subsequently grow in a supersaturated solution. This condition is referred to as primary generation (نسل اولیه).
Primary nucleation is divided into t
wo types:
a. Homogeneous Nucleation (هستهزایی همگن)
In this type of nucleation, crystals form within a single phase in the absence of impurities. Nucleation occurs as a result of supersaturation. Except in certain precipitation reactions, this type of nucleation rarely occurs.
b. Heterogeneous Nucleation (هستهزایی ناهمگن)
This type of nucleation occurs in the presence of impurities or a catalyst (کاتالیست) in the solution.
3. Secondary Nucleation (هستهزایی ثانویه)
The formation of nuclei when crystals are already present in the magma is known as secondary nucleation (هستهزایی ثانویه). This type of nucleation is also divided into two categories:
a. Fluid-Shear Nucleation (هستهزایی ناشی از برش سیال)
This type of nucleation is caused by stresses resulting from fluid shear.
b. Contact Nucleation (هستهزایی تماسی)
This type of nucleation occurs when a crystal comes into contact with another object, such as the wall of a crystallizer (کریستالایزر), an agitator, or another crystal. It is the most common type of nucleation in industrial crystallization vessels.
Because contact nucleation occurs at relatively low supersaturation, it is proportional to the first power of supersaturation rather than the twentieth power or higher, as occurs in certain forms of primary nucleation. Consequently, it is easier to control during operation.
Under suitable conditions, nuclei may form simultaneously; in most cases, however, additional small crystalline particles are produced. Even small quantities of impurities may also act as nuclei.
