Drying the final product (concentrate) one or beneficiation plant process products dehydrated, its role is to further reduce the moisture of the product, or to meet the requirements of the user of the production process.
At present, the requirements of the concentrator sinotrans antifreeze concentrate, and packaging moisture content increasingly stringent, such as tungsten concentrate is 0.05% to 0.8%, less than 4% of molybdenum concentrate, manganese ore 8-9% less than the titanium concentrate 1%, less than 0.5% fluorite ore, copper ore, lead zinc concentrate is less than 8%. Thus, for such products using only dehydration concentration, filtration operation is difficult to meet the requirements, yet to be dehydrated and dried. At the same time, the concentrating plants located in the northeast, northwest and north China regions of China, due to the low temperature in winter, generally below -14 °C, in the case of anti-freezing measures for the storage and transportation of concentrates in the concentrator, should also be considered as needed Set up the drying operation.
Drying is an operation that uses thermal energy to remove some of the moisture (moisture) in the material. The commonly used method is indirect or direct heat transfer drying method, that is, using hot gas as a medium to transfer heat directly or directly to the wet material, so that the moisture on the surface of the wet material vaporizes and diffuses to the airflow body through the gas film at the surface. At the same time, due to the moisture separation on the surface of the material, a moisture difference is generated between the inside and the surface of the material, and the moisture inside the material diffuses to the surface in a gaseous or liquid form, thereby drying the material.
The necessary condition for the drying of the material is that the pressure of the water vapor on the surface of the material (ie, the vaporization strength) must be greater than the partial pressure of the water vapor in the drying medium. The greater the pressure difference between the two, the faster the drying speed, and the drying medium should take the vaporized water vapor away in time. If the pressure difference is zero, the drying operation will stop. It can be seen that drying is a combination of heat transfer and vaporization strength. Figure 1 shows the heat transfer and vaporization strength between hot gas and wet material.
There are two ways to dry the material with hot gas. One is direct drying, that is, the hot gas is directly in contact with the wet material to dry the material; the other is indirect drying, that is, the hot gas is not in direct contact with the wet material, but passes through the wall. Heat is transferred to the dried material to dry it. The direct drying process is a process of heat transfer and taking away moisture simultaneously, that is, using a flowing high-temperature gas as a heat carrier to directly contact the material, transferring heat to the wet material, and taking away the water vaporized from the material. . The indirect drying process is a process in which a vessel is heated by a hot gas to vaporize the moisture of the material in the vessel.
When the material is directly dried with high temperature gas (air or flue gas), the process is more complicated, but the flow of water inside the material and the external conditions (temperature, humidity and flow rate of the gas, contact with the material and gas, The shape and size of the material, etc., are studied in two ways.
The moisture in the loose material consists of unbound moisture or combined moisture. Unbound moisture refers to a type of moisture that exists in the material in a free state, that is, the surface of the material wets moisture and pore water (capillary water), which is easily removed. The combined moisture is the absorbed water in the material, that is, the crystal water, which is difficult to remove. [next]
The various moisture present in the material can be illustrated in Figure 2. Any material can only be dried until it is equilibrated. Balanced water is the moisture content of the material when the vapor pressure on the surface of the material is equal to the partial pressure of steam in the surrounding air.
The drying process is divided into four stages according to the change of drying speed: preheating stage, constant speed stage, deceleration stage and the water content of the material has reached the equilibrium stage under certain conditions, that is, the moisture content in the material is balanced with the humidity of the air. stage.
There are two ways to express the drying speed:
One is expressed in terms of evaporation per unit area of ​​evaporation per unit time, ie:
R=W/(A · t) (1)
Where R———drying speed, kg/(m 2 · h);
w———the amount of water evaporated, kg;
A———unit evaporation area, m 2 ;
t———Unit time, h.
The other is expressed by the reduced moisture content per unit time, ie:
R W =W/(M d · t) (2)
Where R W ———drying speed, kg/(kg · h);
W———the amount of water evaporated, kg;
M d ———material weight, kg;
t———Unit time, h.
The relationship between R and R w is expressed by the following formula:
R=R w (M d /A) (3)
Due to the drying mechanism and the complexity of the drying process, the drying speed is usually determined experimentally. In order to simplify the influencing factors, the drying test should be carried out under constant drying conditions. The experiment is intermittent operation, using a large amount of air to dry a small amount of material. Therefore, the state of the air as it enters and exits the dryer such as temperature, humidity and gas velocity, air The flow pattern with the material is constant.
During the experiment, the change of the weight of the material was measured periodically, that is, the weight change ΔW' of the material in each time interval Δt and the surface temperature of the material were recorded. The experiment was carried out until the weight of the material was constant. At this point, the material and the air have reached a dynamic balance state, and the moisture content in the material is the equilibrium moisture under the condition. Then, when the material is placed in an electric oven and dried to constant weight, the weight of the dry material can be measured. According to the experimental data, the curve is drawn, and the curve drawn according to the relationship between the average water content of the material and the drying time is called a drying curve. The curve drawn according to the drying speed obtained from the change in water content with time is referred to as the drying speed curve. As shown in Figure 3.
Figure 3 shows a typical drying curve and drying speed curve. The relationship between temperature and time of the material is also plotted. In the drying curve (the relationship between material moisture and time), the AB line represents the preheating stage, the BC line is a straight line, indicating that the water content drops uniformly, representing the constant speed stage, and the CD line illustrates the situation of the speed reduction stage. [next]
Figure 4 shows the drying speed curve of the moisture content of the material in relation to the drying speed. From this curve, the characteristics of the four stages in the drying process can be clearly seen.
(1) Preheating stage: the temperature of the material to be dried is very low. When the material is in contact with the drying medium (hot air), the heat of the hot air first raises the temperature of the material and moisture to reach the temperature at which the water vaporizes. The one-stage time is called the warm-up phase. This phase is characterized by a rapid increase in drying speed from zero to a fixed value for a short period of time.
(2) Constant speed stage: As the heat continues to pass, the surface temperature of moisture and material is higher, but the surface moisture is due to the steam pressure (the difference between the vapor pressure at the surface temperature of the material and the vapor pressure in the hot air). The vaporization of the relationship to the air will cause the surface temperature to decrease. However, if the heat transferred to the surface of the material by the hot air is exactly equal to the heat required by the surface water vehicle, the temperature of the surface of the material will remain unchanged. Since the surface temperature of the material is constant, if there is no change in the external drying conditions, when the surface of the material has sufficient moisture (all surfaces have moisture), the vaporization rate must be constant. After the surface water evaporates, if the internal moisture can flow to the surface at a sufficient speed and the amount of vaporized water is replenished in time, the surface can remain wet, so the drying speed does not change. The characteristic of this stage is that the moisture is vaporized by the surface of the material, and the moisture of the inner layer of the material diffuses rapidly to the surface thereof, the surface of the material is always kept moist, the drying speed is constant and has a maximum value.
(3) Deceleration phase: Since the velocity of the internal moisture outflow of the material is lower than the velocity of the surface vaporization, a part of the surface of the material will be in a dry state (especially a portion protruding from the surface), so that the amount of vaporized water is reduced, that is to say: The drying speed will gradually decrease. At this time, it is called the slowdown phase of drying. The characteristics of this stage are the same as those of the second stage. The moisture is vaporized by the surface of the material, while the wet surface is gradually reduced, and the drying speed is uniformly decreased.
(4) The material reaches the equilibrium water content or the critical water content stage: as the drying speed decreases, the temperature at the surface of the material surface will continuously rise, and the heat will be transferred to the inside, so that the evaporation surface will move to the inside, and the water will become steam. Then spread to the surface. In this way, the water content in the material is less and less, and the resistance encountered by the water flow is getting larger and larger, and the drying speed is decreased rapidly. Finally, the moisture content in the material is balanced with the humidity of the air, that is, the water content of the material has reached The equilibrium water content under certain conditions, when the drying speed is equal to zero, the drying process is completely finished. The wet surface is reduced due to the formation of the dry skin of the material, initially limited to the convex portion and continues to increase, gradually covering the surface of the material. When the surface begins to form the outer skin, the water content of the material is referred to as the first intermediate water content, as shown by point C in FIG. When the outer skin is completely formed, the water content of the material is referred to as the second critical moisture content, as shown by point D in FIG. The so-called critical water content refers to the first critical water content. This stage is characterized by evaporation of moisture from the inner layer of the material.
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