CH683189A5 - Method and apparatus for monitoring and controlling the density and height of unbaked anode blocks, especially for aluminum fusion electrolysis. - Google Patents

Description

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CH 683 189 A5

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description

The invention relates to a method for monitoring and controlling the density and height of unfired anode blocks, in particular for aluminum melt flow electrolysis, the anode blocks being formed from granular raw materials with binder in a vibrating machine with compression. The invention also relates to a device for carrying out the method.

Metallic aluminum is obtained from alumina by melt flow electrolysis in electrolytic cells, in which anodes in the form of carbon blocks are hung, which are produced in anode factories from calcined petroleum coke and pitch as a binder. A mixture is produced from the granular raw materials with certain grain size fractions according to a specific recipe to achieve a dense grain packing, which is preheated to about 130 ° to 150 ° C and intimately mixed with pitch as a binder, usually coal tar pitch. The mixture is then molded in a mold attached to the table of a vibrating machine and compacted into an anode block. The better the binder can penetrate into the pore spaces of the granular raw materials, the higher the anode block density that can be achieved in the vibrating machine and in the final firing stage, and the better and more uniform the current passage through the anode block in the electrolytic cell.

The density of the anode block achieved after shaking is subject to narrow tolerance limits because of its influence on the desired electrical properties of the anode block. In addition, the anode block height after shaking must not exceed or fall below tolerance limits due to the process.

In previous monitoring methods, after the respective compacted anode block has been ejected from the vibrating machine, the final height of the completely vibrated anode block is measured and the density is calculated. With a rotary table vibrating machine, however, at least two further next anode blocks are in the works at this point in time, which can no longer be influenced in terms of density and height when faults occur (in the case of a previous anode block), i.e. if the first anode block is defective in terms of density and / or height, this inevitably results in at least one further defective scrap anode block.

The invention has for its object to ensure in the molding of anode blocks by compression that the molded anode blocks are always within the tolerance limits in terms of their density and final height and the production of faulty reject anodes can be largely avoided even with changing parameters.

This object is achieved in terms of procedure with the measures of the characterizing part of claim 1 and in terms of the device with the measures of the characterizing part of claim 3.

The aim of the invention is to continuously determine the height and density of the anode block to be molded during the compression process in the vibrating machine with the aid of a microprocessor and to control the process so that the anode block density corresponds to the setpoint value and the anode block height is within the tolerance range. When the setpoint of the anode block density is reached, the vibrator is switched off. If the associated anode block height is close to the tolerance limits and there is a risk of the tolerance limits being exceeded, the necessary change in the filling weight and / or change in the binder metering for the next subsequent anode blocks to be molded is calculated and a corresponding control intervention on the filling weight and / or made on the binder metering. For example, the actual height of the anode block just molded is determined and the filling weight of the next anode block is automatically controlled. The anode blocks produced by this method have an almost constant density and height. The risk of producing scrap anodes is minimized. Unnecessarily long shaking in the shaking machine is avoided since the shaking process is automatically stopped after reaching the desired anode block density and the minimum permitted height, which minimizes specific wear, noise development and energy consumption. Production performance is optimized because the cycle times for a rotary table vibrator are reduced on average. The minimum and maximum vibration times can be selected.

The invention and its further features and advantages are explained in more detail with reference to the exemplary embodiment shown schematically in the drawing.

According to the drawing, calcined petroleum coke (10) with a grain size of about 0 to about 30 mm and with a carbon content of about 96% to 99% and possibly residues of used (burned) anode blocks and binders (11), e.g. Coal tar pitch, introduced into a heated mixer (13) via a metering device (12) and intimately mixed there. The hot, free-flowing solid-pitch mixture (14) (green anode mass) leaving the mixer (13) then goes into a container scale (15), which weighs the weight necessary for the anode block to be formed, and the amount thereof into the molding box a vibrating machine (16) is filled. The green anode mass (14) is compressed and molded into a green anode block of high density and homogeneity in the molding box of the vibrating machine (16) with the aid of a covering weight (17), which is then fired in a furnace (not shown). The more uniform the density and height of the green anode blocks to be molded in succession in the vibrating machine (16), in particular the rotary table vibrating machine, the more uniform is the current passage and burning behavior of the finished anode blocks, so that the density and height of the anode blocks are subject to narrow tolerance limits.

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A cover weight rod (18) projecting upwards is connected to the cover weight (17) of the molding box of the vibrating machine (16). A suitable sensor (19) is used to continuously measure the height of the anode block to be molded, e.g. an optoelectronic sensor cooperating with a reflector (20) contacts the position of the cover weight rod (18) of the vibrating machine (16) during the compression process. In this way, the height of the anode block is continuously measured during the molding; the measured value determined by the sensor (19) is fed via the signal line (21) to a microprocessor (22) which, together with the measured quantity (23) of the filling weight of the respective in the mold box of the vibrating machine (16 ) filled anode mass, the respective current density of the anode block to be molded is calculated. Due to the rapid and continuous tracking of the movement sequence of the cover weight rod (18) and thus the cover weight (17), there are always intermediate values and immediately after the shaking process of the shaking machine (16) the exact final values of the anode block height and anode block density are available. When the desired anode block density is reached, a signal is sent from the microprocessor (22) via signal line (24) to the switch (25), which switches off the vibrating machine (16). If the associated anode block height approaches the specified tolerance limits, the necessary change in the filling weight and / or change in the addition of the binder (11) for the next anode blocks to be molded is calculated by the microprocessor (22) and a corresponding control intervention on the filling weight is carried out via the signal line (26) and / or via the signal line (27), a corresponding control intervention is carried out on the metering device (12) of the binder feed (11).

If the density of the anode block to be molded decreases and / or the shaking time increases until the desired anode block density is reached, the control agent (27) on the metering device (12) can be used to increase the metering of binder and vice versa. By constantly monitoring the rate of change in the height of the anode to be molded, the microprocessor (22) determines the consistency of the solid / pitch mixture (14). The microprocessor (22) can thus differentiate the need to act on the filling weight via the signal line (26) and / or on the binder metering device (12) via the signal line (27).

The binder (11) has the following tasks:

a) to glue the grains of the raw materials together;

b) to displace the air in the spaces between the granular raw materials.

Due to changing raw materials, the density of the manufactured anode blocks is subject to constant fluctuations. If the microprocessor (22) detects a trend towards lower densities over a longer production period (e.g. 5 to 10 anode blocks) and the average shaking time approaches the maximum permitted time (approx. 55 seconds), the setpoint of the Binder dosing system can be increased slightly. Falling anode densities are generally the result of increasing porosities in the raw material. To achieve the same density, the additional pore volume must be filled with additional binder.

If a trend towards higher densities is found and the average vibration time approaches the minimum allowed vibration time (approx. 35 seconds), the target value of the binder metering system can be slightly reduced. Too high densities are also not desirable. Such anodes generally contain too much binder, which in the subsequent firing process leads to caking of the packaging material surrounding them, reduces the stability of the anode during the preheating phase, and hinders the diffusion of the gases produced in the anode during firing.

Claims (3)

Claims 1. A method for monitoring and controlling the density and height of unfired anode blocks, in particular for aluminum melt flow electrolysis, the anode blocks made from granular raw materials (10) with binder (11) being molded in a vibrating machine (16) with compression, characterized in that during the molding of the anode block, the height of which is constantly measured, that when the desired density is reached, the vibrator (16) is switched off and that, when the associated anode block height approaches the specified tolerance limits, the necessary change in the filling weight and / or change in the addition of binder for the next following anode blocks to be molded are calculated and a corresponding control intervention is carried out on the filling weight (26) and / or on the binder metering (27). 2. The method according to claim 1, characterized in that with decreasing density of the anode block to be molded and / or with increasing shaking time until the desired anode block density is reached, the binder metering (27, 12) is increased, and vice versa. 3. Device for performing the method according to claim 1, characterized by a microprocessor (22) with a sensor (19) for measuring the anode block height, with a switch (25) for switching off the vibrating machine (16), with a balance (15 ) for measuring and setting the filling weight of the anode block to be molded in each case, and is operatively connected to a metering element (12) for regulating the binder (11) to be added to the granular raw mass (10). 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 3rd