RU2560733C1 - Drying method of moulded air brick - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: during the drying operation, to one of the end faces of the arbitrary chosen brick of a batch of bricks arranged in a drier there supplied is a pulse ultrasound source, and to the opposite end face of the same brick there connected is an ultrasound receiver; ultrasonic pulse oscillations are excited in the ultrasound source and time intervals τ of passage by each ultrasound pulse of the distance from one end face of the above brick to the other end face of the same brick are constantly determined. Rate of change of the above intervals $$\frac{d\tau }{dt}$$

is determined during the drying time by differentiation of duration of τ intervals as to time t. Before the first minimum of change rate of intervals as to time $$\frac{d\tau }{dt}=\min 1$$

is achieved, temperature of the heat carrier is linearly increased at the rate of 30-35 deg/h. After the first minimum of change rate of intervals as to time $$\frac{d\tau }{dt}=\min 1$$

is achieved before maximum value of interval τ=max is achieved, heat carrier temperature rise rate is changed again and set within the range of 5-6 deg/h. After the second minimum of change rate of intervals as to time $$\frac{d\tau }{dt}=\min 2$$

is achieved, heat carrier temperature is linearly changed again and set within the range of 8-10 deg/h. After expiry of 2.5-3 h of temperature rise, heat carrier temperature is stabilised at the achieved level; with that, during the brick drying process, continuous measurement of moisture content in a brick is made and the above stabilised temperature value is maintained till the specified final moisture content is achieved; after that, drying is stopped.

EFFECT: reduction of drying time; absence of cracks and increase of average density and compressive strength of brick that passed drying as per the proposed method.

4 dwg, 1 tbl

Description

The invention relates to the production technology of building materials and can be used in the manufacture of products from coarse building ceramics (brick, drainage pipes, etc.).

A known method of drying building products, including the operation of evacuation, necessary to intensify the process [1].

The disadvantages of this method are the complexity of the device for its implementation and increased energy intensity.

A known method of drying building ceramics by convection heating products with a coolant and water vapor removal [2].

However, the drying mode by zones is set without taking into account structural changes that occur in ceramic products during their drying, which leads to a decrease in the drying speed and does not completely eliminate drying cracks on raw brick made by plastic molding, since significant moisture conditions occur in convective drying methods gradients in thickness and volume of the product, especially in its surface zone. The low quality of ceramic products subjected to drying by the analogous method is due to the fact that with convective drying methods, due to the evaporation of moisture from the surface layers of the product without taking into account the structural changes occurring in the raw brick, it is impossible to avoid high humidity and temperature gradients, which lead to cracking.

The closest known method is the drying of building ceramics by convection heating of products with a coolant and the removal of water vapor [3].

The prototype method consists in the fact that during the drying process, the speed of ultrasound passing through the control product is continuously measured and its acceleration is determined by differentiating this value over time, and until the first maximum of ultrasound acceleration occurs, the products are heated at a speed of 0.6-2.0 deg / min , then, until the minimum speed of ultrasound passage, the products are heated at a speed of 0.1-0.5 deg / min, after which the second maximum acceleration of ultrasound of the product is heated at a speed of 0.5-1.5 deg / min and hered at the achieved temperature to achieve the desired final moisture content.

The disadvantage of the prototype method is that in the prototype the parameters by which the drying modes of the product are adjusted are the speed of ultrasound and its acceleration.

The choice of this parameter as a control complicates the control by the fact that to measure the speed of ultrasound, it is necessary to measure with sufficiently high accuracy two parameters: the distance between the source and receiver of ultrasound, and the time it takes for the ultrasound to travel the specified distance.

In addition, in the prototype method, the change in the rate of heating of the coolant at each stage of drying is not optimal, which does not allow sufficiently intensify the drying process and improve the quality of products.

The technical problem within the framework of the present invention is to simplify the method and to intensify the drying process and improve the quality of products.

в процессе времени сушки путем дифференцирования длительности интервалов τ по времени t, при этом до наступления первого минимума скорости изменения интервалов по времени

температуру теплоносителя линейно повышают со скоростью, лежащей в диапазоне 30-35 град/ч, далее после наступления первого минимума скорости изменения интервалов по времени

до наступления максимального значения интервала τ=max вновь изменяют скорость изменения температуры теплоносителя и устанавливают ее в диапазоне 5-6 град/ч, затем после наступления второго минимума скорости изменения интервалов по времени

вновь линейно изменяют температуру теплоносителя и устанавливают ее в диапазоне 8-10 град/ч, затем по истечении 2,5-3 ч подъема температуры, стабилизируют температуру теплоносителя на достигнутом уровне, при этом в процессе сушки кирпичей производят непрерывное измерение влагосодержания в кирпиче и выдерживают упомянутое стабилизированное значение температуры до достижения заданного конечного влагосодержания, после чего сушку прекращают.The problem is solved in that in a method for drying molded raw bricks, including placing raw bricks in a dryer, supplying and selecting a heat carrier, removing water vapor while continuously transmitting ultrasound and monitoring its parameters when passing through a controlled product and adjusting the heating medium heating rate at each drying stage according to the measured ultrasound parameters, and as a controlled parameter, the ultrasound transit time from one end of an arbitrary selected from the batch of ki pulses to the other end of the indicated brick, while a pulsed ultrasound source is supplied to one of the ends of a randomly selected brick from a batch of bricks placed in the dryer, and an ultrasound receiver is connected to the opposite end of the same brick, pulsed ultrasonic vibrations are excited in the ultrasound source and continuously determined the intervals τ of each ultrasound pulse passing the distance from one end of the said brick to the other end of the same brick determine the rate of change of the above ervalov

during the drying time by differentiating the duration of the intervals τ with respect to time t, while before the first minimum of the rate of change of the intervals with time

the coolant temperature is linearly increased at a speed lying in the range of 30-35 deg / h, then after the first minimum of the rate of change of time intervals

before the maximum value of the interval τ = max, the rate of change of the temperature of the coolant is changed again and set in the range of 5-6 deg / h, then after the second minimum of the rate of change of time intervals

again linearly change the temperature of the coolant and set it in the range of 8-10 degrees / h, then after 2.5-3 hours of raising the temperature, stabilize the temperature of the coolant at the achieved level, while in the process of drying the bricks, the moisture content in the brick is continuously measured and maintained said stabilized temperature until a predetermined final moisture content is reached, after which drying is stopped.

In FIG. Figure 1 shows a graph of the change in the time interval τ by the passage of an ultrasound pulse from one end of a brick to another on the drying time t.

скорости изменения интервалов по времени прохождением импульса ультразвука от одного торца кирпича до другого от времени сушки t.In FIG. 2 shows a graph of dependence

the rate of change of time intervals by the passage of an ultrasound pulse from one end of the brick to another from the drying time t.

In FIG. 3 is a graph of drying temperature versus time.

In FIG. 4 is a graph of the moisture content of a brick versus drying time.

The graphs shown in FIG. 1-4, serve to explain the essence of the invention.

The essence of the proposed method is as follows. Ceramic masses used for molding bricks contain one or another amount of moisture. The moisture in the masses is composed of moisture contained in the raw materials (natural humidity) and water introduced into the masses artificially and necessary to give them the ability to form.

Regardless of the origin of the moisture content of the ceramic mass (natural or artificial), this moisture is briefly referred to as ground water. By drying, they also understand the removal of ground water from products that have taken their final shape.

In the claimed method, the drying process of ceramic raw brick can be divided into three stages: the movement of moisture inside the brick to the evaporation surface (internal diffusion); evaporation and absorption of water vapor by ambient air or gases (external diffusion); the removal by gases of water vapor outside the drying space.

An imbalance of internal and external diffusion causes the appearance of a moisture content drop in the drying brick, which, on the one hand, intensifies the internal diffusion, and on the other hand, leads to a drop in shrinkage deformations, which cause stresses and cracks in the drying brick. The optimal drying mode will be one that provides a minimum moisture drop across the thickness of the material. This mode is obtained by increasing the speed of movement of moisture in the raw material or by preventing too sharp drying of the surface layers. The speed of movement of moisture in raw can be increased by increasing its temperature. This can be done by heating the raw material during air drying at elevated temperatures.

However, the data obtained by studying the process of evaporation of moisture from raw materials indicate that these measures do not reach the goal if the temperature of raw materials in the air is increased at a sufficiently high rate. As experiments show, an elevated drying temperature at low air humidity can not only accelerate drying by increasing the speed of water movement, but also slow it down, since an over-dried crust and a sharp temperature drop are formed, inhibiting the movement of moisture to the surface. Therefore, at different stages of drying, it is important to choose the right temperature regimes of drying.

In the process of drying the raw material, structural changes occur in it. These changes can be monitored using ultrasound. For this purpose, a pulsed ultrasound source is supplied to one of the ends of an arbitrarily selected brick from a batch of bricks placed in the dryer, and an ultrasound detector is connected to the opposite end of the same brick. Piezoelectric elements can be used as an ultrasonic sensor and an ultrasound receiver. Piezoelectric emitters are used to generate ultrasounds with frequencies up to 50 MHz. The phenomenon of the inverse piezoelectric effect is the mechanical deformation of some materials (quartz and tourmaline crystals, Rochelle salt, ammonium phosphate, ceramic material based on barium titanate, etc.) under the action of an alternating electric field. If an alternating electric field is brought to certain planes of the crystal, then the crystal contracts or stretches depending on the polarity of the electric field. The main part of such a radiator is a plate or rod of piezoelectric material. Electrodes are applied to the surface of the plate in the form of conductive layers. Under the action of an alternating, in particular pulsed, electric field, the plate vibrates, emitting a mechanical wave of the corresponding frequency.

Piezoelectric ultrasound transducers are used to record and analyze ultrasounds. The piezoelectric sensor uses a direct piezoelectric effect. The direct piezoelectric effect is that during the mechanical deformation of the above crystals in certain directions, electric charges of opposite signs appear on their boundaries, which leads to the generation of an electric field. This phenomenon is due to the deformation of elementary crystalline cells and the shift of the sublattices relative to each other under mechanical action on the crystal. In piezosensors under the influence of registered ultrasonic waves, forced mechanical vibrations (variable deformation) arise in the plate, which lead to the generation of an alternating electric field, the corresponding electric voltage of which can be measured.

The speed of ultrasonic transmission through any substance depends on the structure and physico-chemical properties of the substance. If ultrasound is passed through a body of a fixed length and the structure or physico-chemical parameters of the specified substance are continuously changed, then the time taken by the ultrasound to travel through the specified fixed distance will change synchronously with the change in the properties of the substance. This graphically demonstrates the dependence of the time interval τ passing by the ultrasound pulse from one end of the brick to the other on the drying time t shown in FIG. one.

With a linear change in temperature at the first stage of drying, water vapor is formed inside the raw brick in its pores and capillaries, the pressure of which increases with increasing temperature. This leads to an increase in the speed of ultrasound inside the raw material and, consequently, to a reduction in the time τ of the passage of the ultrasound pulse from one end of the brick to the other. With the wrong mode, changes in the temperature of the coolant temperature already at this first stage of drying can cause microcracks in the raw brick, which lead to marriage in the finished product. The experiments showed that the optimal mode at the first stage of drying is one in which the temperature of the coolant is linearly changed at a speed lying in the range of 30-35 deg / h. When the rate of change of the temperature of the coolant is less than 30 deg / h, the duration of the process increases, and the productivity of drying bricks decreases sharply. In addition, when the rate of change of the temperature of the coolant is less than 30 deg / h, moisture condensation occurs on the surface of the products, which leads to the appearance of small cracks on the surface of the brick.

приведенного на фиг. 2 графика, непрерывно изменяется и по истечении 2 часов достигает своего минимума. С этого момент времени начинается более интенсивное выделение паров воды с поверхности кирпича (см. фиг. 3). Чтобы процесс испарения влаги с поверхности кирпича не приводил к трещинообразованию, необходимо в этот момент времени снизить скорость изменения температуры теплоносителя. Эксперименты показали, что оптимальная скорость изменения температуры теплоносителя после достижения первого минимума величины

(на фиг. 2, при t=2 ч) до наступления максимального значения интервала τ=max (см. фиг. 1, t=10 час) должна лежать в диапазоне 5-6 град/ч. Изменение скорости нарастания температуры теплоносителя в период сушки медленнее чем 5 град/ч, снижает интенсивность сушки и повышает ее продолжительность. Увеличение скорости нарастания температуры теплоносителя за предел 6 град/ч, не позволяет обеспечить плавного удаления влаги с поверхности изделий, что приводит к образованию на упомянутой поверхности микротрещин. В указанном интервале времени сушки, лежащем в диапазоне от 2 до 10 ч, происходит более медленное нарастание температуры сушки, чем это было на предыдущем временном интервале сушки (см. фиг. 3).When the rate of change of the coolant temperature is greater than 35 deg / h, too many water vapor forms in the pores of the raw brick, the pressure of which can be greater than the ultimate strength of the raw material, and cracks may appear in it. A linear change in the temperature of the coolant temperature in the range of 30-35 deg / h is optimal, since the temperature and moisture gradient are relatively small during the residence of the raw material in the first drying zone, which allows the water vapor to gradually diffuse through the capillaries of the raw brick from the inside to the brick surface. In FIG. 1, this first drying step ends after 2 hours of drying, and is indicated in FIG. 1 - 4 dashed line. During this period of time, noticeable moisture separation from the brick surface to the drying chamber still does not occur (see Fig. 3), the speed of ultrasound is slowly but constantly increasing, and its transit time along the length of the brick is also slowly but constantly decreasing (see Fig. 1 ) During this period of time, the slope of the graph (Fig. 1), characterized by the derivative

shown in FIG. 2 graphics, continuously changing and after 2 hours reaches its minimum. From this moment in time, a more intense release of water vapor from the surface of the brick begins (see Fig. 3). So that the process of evaporation of moisture from the brick surface does not lead to cracking, it is necessary at this point in time to reduce the rate of change of the temperature of the coolant. The experiments showed that the optimal rate of change of the temperature of the coolant after reaching the first minimum value

(in Fig. 2, at t = 2 hours), before the maximum interval, τ = max (see Fig. 1, t = 10 hours), should be in the range of 5-6 deg / h. The change in the rate of increase of the temperature of the coolant during the drying period is slower than 5 deg / h, reduces the drying intensity and increases its duration. An increase in the rate of increase in the temperature of the coolant beyond 6 deg / h does not allow for the smooth removal of moisture from the surface of the products, which leads to the formation of microcracks on the mentioned surface. In the indicated drying time interval, lying in the range from 2 to 10 hours, a slower increase in the drying temperature occurs than in the previous drying time interval (see Fig. 3).

The rate of internal diffusion of moisture vapor depends on the viscosity of the water, and its evaporation from the surface of products (external diffusion) depends on the strength of the surface tension of water. An increase in temperature inside a raw brick has a much stronger effect on the viscosity of water than on the strength of its surface tension. With a change in the water temperature in the indicated drying period (from 2 to 10 hours) to 100 ° C, the viscosity of the water decreases by almost 85%, and the surface tension forces - only by 19%. Therefore, the heating of the raw brick material facilitates the movement of water through the capillaries inside the brick, but to a lesser extent contributes to its removal from the surface of the product.

(см. фиг. 2, t=l5 ч) должна лежать в диапазоне 8-9 град/ч. Изменение скорости нарастания температуры теплоносителя в этот период сушки медленнее чем 8 град/ч, снижает интенсивность сушки и повышает ее продолжительность. Увеличение скорости нарастания температуры теплоносителя за предел 9 град/час, не позволяет обеспечить плавного удаления влаги с поверхности изделий, что приводит к образованию, на упомянутой поверхности, микротрещин. На этом этапе сушки наряду с интенсивным удалением влаги из кирпича-сырца, происходит его постоянная усадка, в результате которой его плотность непрерывно возрастает, что приводит к непрерывному возрастанию скорости ультразвука и, следовательно, к снижению времени Т прохождением импульсом ультразвука от одного торца кирпича до другого. По истечении 5-6 ч сушки кирпича в указанном режиме увеличение скорости подъема температуры теплоносителя прекращают, и при достигнутой температуре (на фиг. 3, Т=160°C) осуществляют окончательную сушку изделия до тех пор, пока влагосодержание в сырце, которое непрерывно контролируется влагомером, не достигнет заданного значения.Due to this process, the temperature and pressure gradient inside the brick begins to increase sharply, which leads to a sharp increase in the speed of ultrasound and, consequently, to a sharp decrease in the time it takes to travel along the length of the brick (see Fig. 1). In addition, when moisture is removed from the raw, its volume is reduced. This is due to the fact that as the moisture evaporates in the clay capillaries, increasing surface tension forces arise, creating a uniformly distributed load, tending to bring together the clay clay particles, which leads to the beginning of shrinkage of the product. Shrinkage leads to an increase in the density of the raw material and contributes to a sharp increase in the speed of ultrasound and, consequently, leads to an equally sharp decrease in the time it takes an ultrasound pulse from one end of the brick to the other. When the first maximum of the speed of sound is reached, and therefore the first minimum of the time it takes for an ultrasound pulse from one end of the brick to the other (in Fig. 1 at time t = 5 hours), two competing processes begin to appear in the raw: on the one hand, the increasing temperature raw leads to volumetric thermal expansion of raw, which leads to a decrease in its density. In addition, the removal of moisture from the pores and capillaries of the raw material leads to the formation of voids inside these pores and capillaries, which should also lead to a decrease in the density of the raw material. These two processes lead to a decrease in the speed of ultrasound and, consequently, to an increase in the time it takes for an ultrasound pulse to pass from one end of the brick to the other. On the other hand, as noted above, with an increase in the temperature of the product, partial shrinkage of its material and an increase in the internal pressure gradient of water vapor in the capillaries occur. These two processes lead to an increase in the speed of ultrasound and, consequently, to a decrease in the time it takes for an ultrasound pulse to pass from one end of the brick to the other. At the drying time t = 5 h, these two processes are aligned, and then the first process begins to prevail over the second process, which leads to a decrease in the ultrasound speed and, consequently, to an increase in the time it takes for the ultrasound to travel from one end of the brick to the other. As the temperature rises, both processes continue to compete with each other. At time t = 10 h, both processes balance each other again. The speed of ultrasound in this case reaches its first minimum, and the transit time of an ultrasound pulse from one end of the brick to the other reaches its maximum τ = max (see Fig. 1). At this point in time, a high pressure gradient was created inside the pores and capillaries, and a significant part of the internal moisture diffused to the surface of the brick. In order to prevent pressure gradients arising inside the brick from warping and cracking the raw material, it is necessary at the time point τ = max to increase the rate of moisture removal from the surface of the products in comparison with the previous value. The experiments showed that the optimal rate of change of temperature of the coolant after reaching τ = max (see Fig. 1, at t = 10 h) before the second minimum value of speed

(see Fig. 2, t = l5 h) should be in the range of 8-9 deg / h. The change in the rate of increase of the temperature of the coolant during this drying period is slower than 8 deg / h, reduces the drying intensity and increases its duration. An increase in the rate of increase in the temperature of the coolant beyond 9 deg / h does not allow for the smooth removal of moisture from the surface of the products, which leads to the formation of microcracks on the mentioned surface. At this stage of drying, along with the intensive removal of moisture from the raw brick, it constantly shrinks, as a result of which its density continuously increases, which leads to a continuous increase in the speed of ultrasound and, consequently, to a decrease in the time T passing by the ultrasound pulse from one end of the brick to of another. After 5-6 hours of drying the brick in the specified mode, the increase in the rate of rise of the temperature of the coolant is stopped, and at the temperature reached (in Fig. 3, T = 160 ° C), the product is finally dried until the moisture content in the raw material, which is continuously monitored moisture meter, does not reach the set value.

The speed of ultrasound during the final drying process continuously increases, and the time T by the passage of an ultrasound pulse from one end of the brick to the other decreases with increasing vapor pressure of the water inside the pores and capillaries, which is associated with the shrinkage process that continues during drying. After drying (in Figs. 1-4, t = 20 h), the ultrasound speed reaches its maximum value, and the time τ - the ultrasound pulse passes from one end of the brick to the other end of its minimum value, and stabilizes.

An example of a specific implementation. According to the claimed method and the prototype method, a batch of molded raw bricks was dried. Graphs of changes in temperature conditions and the characteristics of the raw brick dried by the present method are shown in FIG. 1 - 4. The drying process in both cases was carried out until the moisture content of U in the bricks reached a predetermined value of 115%. The difference between the drying methods was that according to the claimed method, the change in temperature conditions was carried out taking into account the structural changes that occur in the products as they are dried. In the first stage of drying, which was carried out for 2 hours, the rate of change of the temperature of the coolant was 30 deg / h, in the second stage of drying it was 5 deg / h, and in the third - 8 deg / h.

The selected temperature conditions led to the uniform removal of moisture from the products during the entire period of their drying (see Fig. 4), which prevented the cracking process. Drying by the prototype method was carried out without taking into account the structural changes that occur in the products during their drying, according to the technological regulations prescribed in traditional convection drying units. In both cases, the brick, dried up to the set condition, was annealed. Annealing of the dried brick by the claimed method and the prototype method was carried out under the same conditions, regulated by standard technological instructions. The test results are shown in table 1.

As follows from table 1, the average density and compressive strength of the brick, dried by the present method, is higher than these characteristics of the brick, dried by the prototype method, while cracking in the brick made by the claimed method is absent, whereas on the brick, dried by the prototype method, they are present. In addition, the drying time and water absorption of a brick prepared by the present method is lower than the water absorption of a brick dried by the prototype method.

Literature

1. USSR copyright certificate No. 316675, cl. C04B 41/30, 1970.

2. Lykov A.V. Theory of drying. - M .: Energy, 1968, p. 136-147.

3. A.S. No. 571682, class F26B3 / 00. Publ. 10/28/1977. - The prototype.

Claims (1)

The method of drying a molded raw brick, including placing it in a dryer, supplying and selecting a heat carrier and water vapor removal while continuously passing ultrasound and monitoring its parameters when passing through a controlled product and adjusting the heating medium heating rate at each drying stage according to the measured ultrasound parameters, characterized in that the ultrasound transit time from one end of an arbitrarily selected brick from a batch of bricks to another end is selected as a controlled parameter a brick, while a pulsed ultrasound source is connected to one of the ends of an arbitrarily selected brick from a batch of bricks placed in the dryer, and an ultrasound receiver is connected to the opposite end of the same brick, pulsed ultrasonic vibrations are excited in the ultrasound source and continuously determine the intervals τ of passage of each the ultrasound pulse of the distance from one end of the said brick to the other end of the same brick, determine the rate of change of the said intervals

during the drying time by differentiating the duration of the intervals τ with respect to time t, while before the first minimum of the rate of change of the intervals with time

the coolant temperature is linearly increased at a speed lying in the range of 30-35 deg / h, then after the first minimum of the rate of change of time intervals

before the maximum value of the interval τ = max, the rate of change of the temperature of the coolant is changed again and set in the range of 5-6 deg / h, then after the second minimum of the rate of change of time intervals

they again linearly change the temperature of the coolant and set it in the range of 8-10 degrees / h, then after 2.5-3 hours of temperature rise, the temperature of the coolant is stabilized at the achieved level, while in the process of drying the bricks, the moisture content in the brick is continuously measured and maintained a stabilized temperature value until a predetermined final moisture content is reached, after which drying is stopped.

Images