RS49726B - PYMETALLURGICAL REACTOR COOLING ELEMENT AND PROCEDURE FOR ITS PRODUCTION - Google Patents

Abstract

A process for the production of a pyrometallurgical reactor cooling plate, wherein the plate is produced by sliding casting of highly thermally conductive copper and has at least one flow channel for cooling water, basically of round or oval cross-section, produced in the cooling plate during casting. In order to improve the heat transfer capacity of the cooling plate, the wall surface of at least one flow channel inside the cooling plate is increased by making one or several grooves inside the flow channel surface by means of a grooved spindle during casting or by machining threads or grooves as in a rifle barrel. The application contains 1 more independent and 2 dependent patent claims.

Description

This invention relates to the process for the production of a cooling element with flow channels, for pyrometallurgical reactors, where this element has at least one flow channel and where the production of the element is achieved by continuous Friction, i.e. slip casting. To improve the element's ability to transfer heat, the outer surface area of the cooling channel is increased given its round or oval cross-sectional shape, without increasing the diameter or length of the flow channel. This invention also relates to an element produced in this way.

Reactor refractories in pyrometallurgical processes are protected by water-cooled cooling elements so that, as a result of cooling, the heat reaching the surface of the refractory material through the water cooling element significantly reduces wall wear compared to an uncooled reactor. The reduced wear is caused by the effect of cooling, which leads to the formation of the so-called autogenous wall, which is attached to the surface of the heat-resistant wall and is formed from slag and other substances deposited in the melting phase.

Commonly, cooling elements are manufactured in two ways: primarily, the elements can be produced by sand casting, where the cooling tubes are made of a highly thermally conductive material, such as copper, placed in a mold formed in sand and cooled by air or water during casting around the tube. The element that is cast around the tube is also made of a highly thermally permeable material. It is recommended that it be copper. This production procedure is described in Great Britain patent no. 1386645. One problem with this process is the uneven attachment of the tubing, which acts as a cooling channel, to the casting material surrounding it because some of the tubing may be completely detached from the cast element around them and part of the tubing may melt completely and thus be damaged. Unless a metallic bond is formed between the cooling tube and the rest of the casting around it, heat transfer will not be effective. Again, if the piping completely melts, it will prevent the flow of cooling water. The casting characteristics of this casting material can be improved, for example by mixing phosphorus with copper, to improve the metallic bond formed between the pipeline and the casting material, but in this case the heat transfer characteristics (thermal conductivity) of copper are significantly weakened even by a small addition. One of the advantages of this procedure that is worth mentioning is its comparatively low production cost and its independence from dimensions.

Another manufacturing process used is to insert a channel-shaped glass tubing into the mold of the cooling element which breaks after casting to form a channel within the element.

United States Patent 4,382,585 describes another widely used method of manufacturing cooling elements, whereby the element is manufactured, for example, from rolled or forged copper plate by machining the necessary channels in it. The advantage of an element made in this way is its dense, strong structure and good heat transfer from the element to a cooling medium such as water. Its disadvantages are dimensional limitations (size) and high cost.

A well-known procedure in the state of the art is the production of a cooling element for a pyrometallurgical reactor by casting a continuous hollow profile such as, for example. slip casting through the spindle. The element is manufactured from a highly thermally conductive metal such as copper. The advantage of this procedure is the dense structure of the casting, good surface quality, and that the cast cooling channel provides good heat transfer from the element to the cooling medium so that no interference with the heat transfer occurs, but the heat coming from the reactor to the cooling element is transferred without any excess heat transfer resistance directly to the surface of the channel and further to the cooling water. The cross-section of the cooling channel is usually oval, and the spindle has a smooth surface. This type of cooling channel is mentioned in United States Patent No. 5,772,955.

However, to improve the heat transfer capability of the cooling element it is recommended to increase the heat transfer surface of the element. As shown in the following explanation, according to the present invention, this occurs by increasing the outer surface area of the flow channel wall without increasing the diameter or adding length. The surface area of the cooling element flow channel wall is increased by forming grooves in the channel wall during casting or by machine adding grooves or threads to the channel after casting so that the cross section of the channel remains necessarily round or oval. As a result of this, with the same amount of heat, a smaller temperature difference between the water and the wall of the flow channel is required and an even smaller temperature of the cooling element. The present invention also relates to cooling elements produced by this process. The important features of this invention will be clearly seen in the appended patent claim.

The ability of the cooling element to receive heat can be represented by the following formula:

O = a x A x AT, where

Q = amount of heat transferred [W]

a = coefficient of heat transfer between the wall of the flow channel and the water [W/Km<2>]

A = heat transfer area [ni<2>]

AT = temperature difference between the wall of the flow channel and the water [K]

The heat transfer coefficient a can be theoretically determined using the formula

Nu= a D/ X

X= thermal conductivity of water [W/mK]

D = hydraulic diameter [m]

OrNu = 0.023 x ReA0. 8Pl* 0. 4,

whereby they are

Re =vvDp/77

w = speed [m/s]

D = hydraulic diameter [m]

p= density of water [kg/m<3>]

77= dynamic viscosity

Pr = Prandtl number []

Therefore, according to the above, it is possible to influence the amount of heat transferred to the cooling element by influencing the temperature difference, the heat transfer coefficient or the heat transfer surface.

The temperature difference between the wall and the pipe is limited by the fact that water boils at 100°C, when the heat transfer components at normal pressure become significantly worse due to boiling. In practice, it is better to work at the lowest possible temperature in the walls of the flow channels.

The heat transfer coefficient can be influenced to a large extent by changing the flow rate, i.e. by affecting the Reynolds number. This is, however, limited by the increased pressure loss in the pipeline when increasing the flow rate, which increases the cost of pumping the cooling water, and the investment costs in the pump also increase significantly after crossing a certain limit.

According to the conventional solution, the heat transfer surface can be influenced either by increasing the diameter of the cooling channel and/or its length. The diameter of the cooling channel cannot be increased indefinitely in such a way as to be justified in terms of economy, because the increase in the diameter of the channel increases the amount of water required to achieve a certain flow rate and, moreover, the energy required for pumping. On the other hand, the channel diameter is limited by the physical size of the cooling element, which, for reasons of reducing investment costs, is recommended to be made as small and light as possible. Another limitation on length is the physical size of the cooling element itself, i.e. the amount of cooling duct that will fit in a given zone.

If one wants to increase the heat transfer area of the cooling element shown here, it is done by changing the shape of the flow channel wall by sliding the cast cooling element to achieve a larger heat transfer area, calculated per unit length of the flow channel, with the same flow cross-section (the same velocity is achieved with the same amount of water). This increase in surface area is achieved, for example, by the following means: At least one flow channel, necessarily round in cross-section, is formed in the slip-cast cooling element during casting, and the threads are machined into the flow channel after casting.

At least one flow channel, necessarily flat in cross-section,

is formed in the cast cooling element during slip casting and the tube grooves are machined into the flow channel after casting. The grooves can be made in a better way by using a so-called expanding spindle that is passed through the flow channel. Grooves are e.g. they can be made in the form of a hole that is closed at one end, in which case the spindle is pulled out. A hole made in the channel, which is open at both ends, is made by either pushing or pulling a tool designed for that purpose through the channel.

The most suitable increase in the surface layer is achieved by forming, during casting, one or more grooved, preferably straight grooved, flow channels in the cooling element, using a specially designed grooved casting spindle. Despite making grooves, the shape of the flow channel is still necessarily round or oval in cross-section. The use of this procedure avoids the stages of mechanical machining after casting.

In all the previously described procedures, it is obvious that, if there are parts of the channel in the flow channel transverse to the casting direction, those parts are machined, for example by drilling, and the openings that do not belong to the channel are plugged.

The advantage of this method of increasing the heat transfer surface, which is described in the present invention, is compared with the prior art method with the help of the example given here. Some diagrams are provided by way of example to illustrate the invention, in which

Figure 1 shows the main drawing of the cooling element used in the tests,

Figure 2 shows the cross-section of the profile of the tested cooling element,

Figures 3a - 3d indicate the temperature inside the element at different measurement points as a function of the melting temperature,

Figure 4 shows the heat transfer coefficient calculated from measurements taken as a function of melting, i

Figure 5 presents the differences in the temperature of the cooling water and the channel wall at different cooling levels for the normalized cooling elements.

Example

Cooling elements related to this invention were tested in practical tests, where the bottoms of these elements A, B, C and D were immersed in molten lead to a depth of about 1 cm. Cooling element A had a conventional flow channel with a smooth surface and this element was used for comparative measurements. The amount of cooling water and the temperature were carefully measured in the tests, both before the introduction of water into the cooling element and afterwards. The temperature of the molten lead and the temperatures inside the cooling element itself were also carefully measured at seven different measurement points.

Figure 1 shows the cooling element 1 used in the tests and the flow channel 2 inside it. The dimensions of the cooling element were as follows: height 300mm, width 400mm and thickness 75mm. The cooling pipe or flow channel was located inside the element as in Figure 1, so that the center of the horizontal part of the pipe in the picture was 87mm from the bottom of the element and each vertical part was 50mm from the edge of the plate. The horizontal part of the pipe is made by drilling and one end of the horizontal opening is plugged (not shown in detail). Figure 1 also shows the location of temperature measurement points T1 - T7. Figure 2 shows the surface shape of the cooling channels, and Table 1 contains the dimensions of the tested channels of the cooling element and the calculated heat transfer surface per meter as well as the relative heat transfer surface.

Figures 3a - 3d show that the temperatures of cooling elements B, C and D were lower at all cooling water flow rates than the reference measurements taken from cooling element A. However, since the flow cross-sections of the aforementioned test pieces had to be made with different dimensions for technical manufacturing reasons, the heat transfer efficiency cannot be directly compared from the results in Figures 3a - 3d. Therefore, the test results were normalized as follows:

The constant heat transfer between two points can be written:

Q = S xXx (T, - T2), where

Q = amount of heat transferred between two points [W]

S = shape factor (geometry dependent) [m]

A= thermal conductivity of the medium [W/mK]

T, = temperature of point 1 [K]

T2= temperature of point 2 [K]

By applying the above equation to the test results, the following quantities were obtained:

Q = measured heat output transferred to the cooling water

A= thermal conductivity of copper [W/mK]

T = temperature at the bottom of the element as calculated in the tests [K]

T2= temperature of the wall of the channel through which the water flows as calculated in the tests

[K]

S = shape factor for a finite cylinder buried in a semi-infinite medium (length D, diameter P) the shape factor can be determined according to the equation S = 27tD/ln(4z/P) when Z>1.5P,

z = immersion depth measured from the center line of the cylinder [m].

The heat transfer coefficients determined in the aforementioned manner are presented in Figure 4. According to the analyzes with several variables, a very good correlation is obtained between the heat transfer coefficient and the water flow rate, as well as the amount of heat transferred to the water. The heat transfer regression equation coefficients for each cooling element are presented in Table 2 .

Therefore a[W/m<2>K] = c + a x v [m/s] + b x Q [kW].

In order to compare the results, the cross-sectional areas of the flow channels were normalized so that the amount of water flow corresponds to the same flow velocity. The dimensions of the flow channel and the heat transfer area normalized according to the flow rate and its velocity are presented in Table 3. Using the dimensions given in Table 3 for cases A', B', C and D' and the heat transfer coefficients determined as previously stated, the wall-water temperature difference for the normalized cases concerning the flow rate were calculated as a function of the water flow rate for heat rates of 5, 10, 20 and 30kW via Eq.

The results are shown in Figure 5. The figure shows that all cooling elements produced according to the present invention achieve some amount of heat transfer with smaller temperature differences between the water and the wall of the cooling channel illustrating the effectiveness of the process. For example, with a cooling power of 30kW and a water flow velocity of 3m/s, the temperature difference between the wall and the water in different cases is:

When the results are compared with the heat transfer surfaces, it can be seen that the temperature difference between the wall and the water required to transfer the same amount of heat is inversely proportional to the relative heat transfer surface. This means that the surface changes described in this invention can significantly affect the heat transfer efficiency.

Claims (4)

1. A method for producing a cooling plate of a pyrometallurgical reactor, wherein said plate is produced by slip casting of highly thermally conductive copper and has at least one flow channel for cooling water, basically round or oval in cross-section, which is made in the cooling plate during casting, characterized in that, in order to improve the heat transfer capability of the cooling plate, the wall surface of at least one flow channel inside the cooling plate is increased by making one or several grooves inside the surface of the flow channel using a grooved spindles during casting or by machining threads or grooves as in a gun barrel, after casting. 2. The method according to claim 1, characterized in that the grooves as in a rifle barrel are made by means of an expanding spindle. 3. The cooling plate of the pyrometallurgical reactor, produced by sliding casting of copper of high thermal conductivity, and having at least one flow channel for cooling water, basically round or oval in cross-section, indicated by the fact that it is made according to claim 1 and in order to increase the heat transfer capability of the cooling element, the wall surface of at least one flow channel inside the cooling element is increased by grooves, threads or grooves like in a gun barrel or the like. 4. A cooling plate according to claim 3, characterized in that the tube grooves are made with an expanding spindle.