**1. Introduction**

Crystallization is the process where the molecules or tiny ice crystal are attached to each other to produce bigger formation of molecule or ice layer. Large cluster is stable compared to the smaller cluster as most of the molecules are far from the surface of the solution. It starts to form at the center of the cluster and slowly spread to the outward part of the cluster [1].

Besides, crystallization happens when there is phase change from the liquid to solid or crystalline form. This process happens when concentration of the solute already exceeds the equilibrium concentration or known as the supersaturated solution. The solution provides the driving force for the process and develops the growth of the particle but it not always develops the crystallization process because the supersaturated solution might be in metastable state. The process will happen when the limit of metastable has reached the limit and it depends on the kinetics system [2].

Nucleation is the process that happens in the crystallization process and it can be divided into few categories [2]. Ice nuclei appear at the solid surface when the fusion of heat is transferred by the conduction toward the cooled surface. By this time, the thin film starts to grow thicker until the ice crystal layer is formed. Most of the nuclei are developed in the high supersaturation and it attaches to form the ice crystal [3].

Activated process is known as nucleation because there is a barrier that the growing nucleus needs to be defeated [4]. The nucleation phenomenon is widely known in most aspects of science. Besides, it normally occurs at the beginning of the form of the ice crystal, snow, volcanic and the rainfall [5]. The nucleation process can be divided into a few parts as a different process has different conditions.

Other than that, it is important to understand more detail about the formation of ice for the nucleation and the supercooling condition in a science and technology field [6].

and solubility curve. The kinetics of primary nucleation of NaNO3 for fractional crystallization process of high saline wastewater had been studied by Bian et al. MSZW of NaNO3 was analyzed based on different stirring rate, present of seed

The development of nuclei which refer to the influential of microscopic crystals in the magma is the phenomenon of secondary nucleation [15]. This nucleation happens when the crystal growth is introduced with a contact of other existing crystal or "seeds" [16]. For example, under a certain condition fluid shear forces are sufficient to produce secondary nuclei from an existing crystal surface [17]. Fluid shear nucleation happens when the fluid passes through the crystal with higher speed, sweep away the nuclei that would otherwise be incorporated into a crystal. The scenario led to the formation of a new crystal by the "swept away" nuclei. This contact nucleation

In addition, this secondary nucleation depends on supersaturation. As stated Strickland-Constable that arises since the starting size distribution of potential secondary nuclei is depend on supersaturation. However, the particle produces at critical size ranged. This secondary nucleation has been performed in the agitated system that many been found in industrial crystallizers [18]. Sometimes, for needle breeding appear at high supersaturation where the dendrites may be expanding, or the needle is growing from existing crystal. As a result, when the crystal breaks the new centers for crystal growth are formed and this also refers to collision nucleation where arises from contact between two growing crystals or between a crystal with

The second nucleation gives many advantages includes lower kinetic order and rate-proportional to supersaturation which allows easy control without unstable operation, the nucleation occurs at low supersaturation which the growth rate is optimal for good quality, lower energy needed where the crystal strike avoids the breaking existing crystal to form new crystal, and the quantitative fundamental have already been isolated and are being incorporated into practical [20]. The following model, although somewhat simplified, is often used to model secondary nucleation.

crystals and cooling rate by using ultrasonic velocity sensor [14].

proves to be the most effective and common method in nucleation.

*<sup>B</sup>* <sup>¼</sup> *dN*

limited and chemical reaction controlled [21].

**4. Homogeneous nucleation**

**51**

*dt* <sup>¼</sup> *<sup>k</sup>*1*M<sup>j</sup>*

where k1 is a rate constant, MT is the suspension density, j is an empirical exponent that can range up to 1.5, and b is an empirical exponent that can range up to 5. Xue et al. stated that secondary nucleation is one of the main steps in the crystallization industrial. Most of the researchers are focused on the mechanism and the kinetics of the secondary nucleation of the aluminum hydroxide crystallization. Besides, it is related to the growth of the crystal process through the surface nucleation mechanism. Other than that, secondary nucleation is known as the removal-

Homogeneous nucleation is known when the nucleation is occurring without any favorable nucleation sites. Normally, the nucleation process happens randomly

*<sup>T</sup> <sup>c</sup>* � *<sup>c</sup>* <sup>∗</sup> ð Þ*<sup>b</sup>* (2)

**3. Secondary nucleation**

*DOI: http://dx.doi.org/10.5772/intechopen.90164*

*Nucleation*

another solid surface [19].

#### **2. Primary nucleation**

The supersaturated state achieved is the first step considered in the crystallization process. For supersaturation condition itself are not sufficient for a system to crystallize. The crystal needs to grow where new crystallization centers must exist in the solution. This formation of one phase in another, under conditions where a free energy barrier exists, is known as nucleation [1].

Nucleation is a random process which in two same situations or system nucleation will happen at a different time [4, 7]. Generally, the theory introduces in describing this phenomenon include its nature and behavior of the new thermodynamic phase is called classical nucleation theory (CNT). On the other hand, the experimental result of vapor to liquid nucleation cannot be described using CNT include the Argon by a few orders of magnitude for model substances [8].

For the formation of ice in the water below 0°C, if the system not changing with time and the nucleation occurs in one step. Thus, the probability of nucleation happens is denied through exponential decay as seen in radioactive decay. This scenario can be seen in the nucleation of ice in supercooled [9]. Another example is the self-organization process that forms like amyloid assembly associated with Alzheimer's diseases which the self-assemble system by the energy consumed such as microtubules in cells are referred to the nucleation and growth [10].

There are two types of nucleation namely the homogeneous or spontaneous nucleation and heterogeneous nucleation. This phenomenon happens when nuclei are formed perfectly in a clean solution where there are no any foreign particles. In any practical situation the presence of foreign particles includes impurity molecules, dust particles or ions refer to the heterogeneous nucleation. Both the nucleation process classified as primary nucleation [11].

Besides, primary nucleation refers to the early formation of a crystal in absence of any other crystal or it will not affect or influence the process if there any presence of crystal in a system that will happen in two conditions as stated above. In homogenous nucleation, the nucleation is not affected by solids and this include walls of crystallizer vessel and particles of any substance [12]. While the second one is the heterogeneous nucleation caused by the increasing rate of nucleation by the presence of other substance of solid particles would otherwise not be seen without the existence of these foreign particles [13].

For homogeneous nucleation, it is rarely happening because higher energy is needed to start the nucleation with the absence of solid particles. Primary nucleation has been modeled with the following power-law expression.

$$B = \frac{dN}{dt} = k\_n (c - c^\*)^n \tag{1}$$

Eq. (1) shows that the nucleation requires the rate constant, the instantaneous solute concentration, the concentration at the saturation phase and the empirical exponent which is generally ranges between 3 and 4.

Basically, primary nucleation theories can be used to study the metastable zone width (MSZW) data and kinetic of nucleation for the crystallization process. MSZW is explained as the gap of supersaturation between the supersolubility curve and solubility curve. The kinetics of primary nucleation of NaNO3 for fractional crystallization process of high saline wastewater had been studied by Bian et al. MSZW of NaNO3 was analyzed based on different stirring rate, present of seed crystals and cooling rate by using ultrasonic velocity sensor [14].
