(1) Wetting phenomenon
Wetting is a common phenomenon in nature. For example, dropping a drop of water onto a clean glass will quickly spread out along the surface of the glass, becoming the shape of a planar convex mirror. However, if a drop of water is dropped onto the surface of the paraffin, the water tries to maintain a spherical shape, but due to the influence of gravity, the water droplet forms an elliptical sphere on the paraffin without unfolding. These two different phenomena indicate that the glass can be wetted by water and is a hydrophilic substance; the paraffin cannot be wetted by water and is a hydrophobic substance.
Similarly, dropping a drop of water onto the surface of a dry mineral or introducing a bubble onto the surface of the mineral immersed in water (as shown in Figure 4-6-14) reveals that the surface of the different minerals is wetted by water. On some minerals (such as quartz , feldspar , calcite, etc.) the water droplets are easily spread out, or the bubbles are more difficult to spread on their surface; on the other minerals (such as graphite , molybdenum , etc.) the surface is reversed. The hydrophilicity of these mineral surfaces shown in Figures 4-6-14 gradually increases from right to left, while the hydrophobicity gradually increases from left to right.
From this, it can be seen that in order to occupy a solid surface, there is a competition between the gas phase and the liquid phase. However, the condition that the surface liquid phase of the mineral is replaced by another phase (gas phase or oil phase) is very important. Dupre first applied thermodynamics to study, and Osterhof et al. proposed three basics. Wetting forms, ie adhesion wetting, spreading wetting and immersion wetting, are shown in Figure 4-6-15. The liquid phase in the figure is water and the other phase is air. If oil is used instead of air and water is replaced by other liquids, the relationship remains the same.
Many scholars use the wetting process to explain the principle of flotation. It is believed that: 1 Surface flotation basically depends on whether the air on the mineral surface can be replaced by water. If water cannot replace the air on the mineral surface, the mineral will float on the water surface. 2 Full oil flotation is caused by the lipophilicity and hydrophobicity of the surface of the floating mineral; 3 Foam flotation is caused by the floating mineral being treated by the flotation agent, causing the surface to be hydrophobic and attached to the bubble floating.
The attachment, unfolding or immersion (in a broad sense) that occurs when any two fluids come into contact with a group can be referred to as the wetting process. The result is that one fluid is partially displaced or replaced by another fluid from the solid surface, which is a physical process and is reversible. For example, the flotation process is the regulation of the regularity of a process in which a fluid (such as water) is replaced by another fluid (such as air or oil) on the surface of the mineral (ie, the wetting process).
(2) Contact angle
The so-called "hydrophilic" and "hydrophobic" points on the mineral surface are relative. In order to determine the wettability of the mineral surface, the contact angle θ is commonly used, as shown in Figures 4-6-14 and 4-6-15. A bubble is attached to the surface of a mineral immersed in water. When the balance is reached, the bubble forms a certain contact periphery on the surface of the mineral, which is called a three-phase wetting periphery. Interface free energy exists at any two-phase interface, It represents the interface free energy at the interface of solid water, water, gas and solid gas. The angle enclosed by the two free energy interfaces (including the water phase) of solid water and water-gas is called the contact angle and is expressed by θ. It can be seen that the contact angles on different mineral surfaces are different, and the contact angle can mark the wettability of the mineral surface: if the θ angle formed by the mineral surface is small, it is called a hydrophilic surface; otherwise, when the θ angle is larger , then its hydrophobic surface. The clear boundaries between hydrophilicity and hydrophobicity are non-existent and only relative. The larger the angle θ, the stronger the hydrophobicity of the mineral surface; the smaller the angle θ, the stronger the hydrophilicity of the mineral surface.
The contact angle of the mineral surface is a combined effect of the three-phase interface properties. As shown in Fig. 4 - 6 - 16, when the equilibrium is reached (the wetted periphery is not moving), the three surface tensions acting on the wetted periphery must be zero in the horizontal direction. Then the equilibrium state (Young's Young) equation for:
The above equation shows the relationship between the equilibrium contact angle and the surface tension between the three phase interfaces. The equilibrium contact angle is a function of the interfacial tension of the three phases. The size of the contact angle is not only related to the surface properties of the mineral, but also to the interfacial properties of the liquid phase and the gas phase. Any factor that causes any change in the interfacial tension between the two phases can affect the wettability of the mineral surface. However, the above formula can only be used when the system reaches equilibrium.
It can be seen from the formula [4-6 - 1] that the contact angle 0 value is larger. The smaller the value of 80, the smaller the wettability of the mineral, and the better the floatability. Since the sink value is between -1 and 1, the wettability and floatability of minerals are defined as:
Therefore, the wettability and floatability of the mineral can be roughly evaluated by measuring the mineral contact angle.
(3) Effect of wetting block on flotation
When the droplet moves, the movement of the wetted periphery is hindered, and the equilibrium contact angle at the beginning is changed, and the phenomenon that the movement of the wetted periphery is hindered is called wetting retardation. When a droplet has contacted the mineral and has formed an equilibrium contact angle, such as tilting the solid by an angle α, if α is small, then the wet perimeter does not move, but the contact angle changes, as shown in Figure 4-6- 17 is shown. The contact angle formed by the direction in which the droplets advance is expressed by θ1, which is called the front contact angle and the retardation front angle; the contact angle after the droplet advances is represented by θ2, which is called the contact back angle and the retardation angle. The rake angle is usually greater than the equilibrium contact angle, and the relief angle is usually less than the equilibrium contact angle, often taking half of the sum of the rake angle and the relief angle as the equilibrium contact angle of the mineral. The greater the contact angle of the mineral, the more obvious the difference between the rake angle and the back angle. The size of the wetting block is affected by many factors such as wetting sequence, mineral surface composition, chemical composition, inhomogeneity, roughness and mineral surface wettability. The rake angle and the back angle in the wetting process represent two retarding effects, respectively. The rake angle θ1 represents the retarding effect of water exhaust during wetting; the back angle θ2 represents the retarding effect of gas drainage during wetting. . During the flotation process, when the ore particles adhere to the bubbles, they belong to the gas drainage, which is equivalent to the relief angle in the blocking effect. The back angle is smaller than the equilibrium contact angle, that is, in the case where the mineral itself is floatable, the contact angle becomes smaller, and the adhesion of the ore particles to the bubble is difficult to perform, so the wetting retardation is disadvantageous for the flotation process. of. When the ore particles and bubbles have adhered, the ore particles may fall off the bubbles due to other factors. The process of detaching the ore particles from the bubbles belongs to water exhaust. The retarding effect at this time is the rake angle, and the rake angle is greater than the equilibrium contact angle. In the case where the mineral floatability is constant, it is difficult for the water to discharge the bubbles from the mineral surface. Opening, that is, preventing the ore particles from falling off the bubbles, therefore, the retardation phenomenon is advantageous for the flotation process. For gangue minerals that should not float, the effect of wetting block is just the opposite of the above analysis.
(4) Determination of contact angle
There are many methods for measuring the contact angle, such as observation measurement, slanting plate method, light reflection method, length measurement method, and soaking measurement method, etc., and can refer to surface chemistry information. However, due to the uneven surface and pollution of the mineral surface, it is difficult to accurately measure the contact angle. Coupled with the influence of the wetting block, it is difficult to reach the equilibrium contact angle. Generally, the contact front and back angles are measured, and then the average value is taken. The method serves as a mineral contact angle.
It is worth noting that Young's law has its specificity. Strictly speaking, the law is only suitable for the contact angle of solid surface droplets, and not for the bubble contact angle of solid surfaces in liquid. From the analysis of the force of the two contact angles on the periphery of the three phases, the bubble contact angle has an additional buoyancy of the bubble, which cannot be calculated by the Young's equation, and thus the contact angle and the contact angle of the droplet are different. The scope of its application is also different. Neglecting this in the measurement and application can cause large deviations.
Blowing agents, be defined as compounds which are thermally unstable and decompose to yield gas at the desired polymer processing temperature, are widely used to expand rubber, plastics to create foam. Most of the chemical blowing agents are organic chemicals, while inorganic chemicals are few.
Ac Blowing Agent is an azodicarbonamide blowing agent for rubber and plastics. It has a relatively high decomposition temperature which can be effectively reduced by the addition of small amounts of activators.
Azodicarbonamide, ADCA, ADA, or azo(bis)formamide, is a chemical compound with the molecular formula C2H4O2N4.[1] It is a yellow to orange-red, odorless, crystalline powder. It is sometimes called a 'yoga mat' chemical because of its widespread use in foamed plastics
It is a foaming agent commonly used in industry, which can be used in the production of yoga mats, rubber soles, etc. to increase the elasticity of products, and also can be used in the food industry to increase the strength and flexibility of dough.
Ac Blowing Agent,Adc Blowing Agent,Adc Blowing Agent Tds
HENAN JINHE INDUSTRY CO.,LTD , https://www.hnchromiumoxidegreen.com