EP1476288A1

EP1476288A1 - A method and equipment for compacting materials

Info

Publication number: EP1476288A1
Application number: EP03703544A
Authority: EP
Inventors: Dag Herman Andersen; Lars Magne Bjorbekk
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2002-02-14
Filing date: 2003-02-07
Publication date: 2004-11-17
Anticipated expiration: 2023-02-07
Also published as: NO316162B1; WO2003068468A1; AU2003206268B2; BR0307353A; RU2004127440A; AR038838A1; CN1633354A; RU2311986C2; ATE366648T1; CA2474878A1; DE60314846D1; IS7397A; CA2474878C; EP1476288B1; NO20020744L; DE60314846T2; CN100415470C; AU2003206268A1; NO20020744D0; ES2289259T3

Abstract

The present invention concerns a method and equipment for compacting a material. In particular, the present invention relates to the vibration of 'green mass' in a moulding process for the creation of mould bodies for the production of electrodes for the melting industry, in particular the aluminium electrolysis industry. The equipment comprises two mould parts (ml, mb) that are mutually integrated with a spring force, which may consist of one or more springs (k3). At least one of the mould parts is equipped with means designed to generate vibration. In one embodiment, the equipment consists of a lower table with mould walls and an upper plumb. In a preferred embodiment, vibration is applied via the plumb. During vibration, the plumb will move downwards towards the table as a consequence of the intermediate mass being compacted. The table may be supported by a base in which at least one spring (k1) and possibly a damper element (d1) are arranged between the table and the base. The springs of the equipment may be designed in such a way that the plumb and the table are allowed to oscillate at one or more given frequencies that produce maximum gain against the mass to be compacted. The damper element (d1) will have a stabilising and insulating function. The compaction may take place at a specific vacuum for the removal of gas.

Description

A Method and Equipment for Compacting Materials

The present invention concerns a method and equipment for compacting materials. More precisely, the present invention concerns vibration of "green mass" in a moulding process for the creation of mould bodies for the production of electrodes for the melting industry, in particular the aluminium electrolysis industry.

Such electrodes, in particular anodes, are created by the "green mass" being subjected to compaction in a vibration device, which may consist of a moulding box with a base and side walls mounted on a table, plus a plumb that is allowed to slide down between the mould walls (the side walls of the moulding box). There are primarily two types of vibration equipment for moulding anodes available on the market, equipment with plumb vibration and equipment with table vibration. The main difference between them is the location of the vibration unit that generates the dynamic vertical input force for the equipment. For equipment with plumb vibration, the vibration unit is fixed to/integrated in the plumb. For equipment with table vibration, the vibration unit is fixed to/integrated in the table.

NO patent no. 132359 concerns vibration equipment with plumb vibration for the compaction of mould bodies for the production of anode and cathode blocks. The specification states that plumb vibration offers many advantages over table vibration, in particular with regard to simplification of the equipment. By moving the vibration unit to the plumb, the claim was that the vibration principle could be simplified, as the base would be stationary, fixed to the floor. In accordance with the reference, the compression effect is achieved by one or more vibration generators being arranged only on the cover weight or the plumb, the base being stationary and the mould walls being fixed to the base during the creation process. In accordance with the solution stated in the reference, the table is to be the stationary base in order to avoid a coupled mechanical system with several vibrating masses, which is also illustrated in the figure attached to the reference. The present invention will be described in further detail in the following by means of examples and figures, where:

Fig. 1 : shows a simplified diagram of improved vibration equipment,

Fig. 2: shows a simplified diagram of a first embodiment of vibration equipment in accordance with the present invention,

Fig. 3: shows a simplified diagram of a second embodiment of vibration equipment in accordance with the present invention,

Fig. 4: shows a simplified diagram of a third embodiment of vibration equipment in accordance with the present invention,

Fig. 5: shows a simplified diagram of a fourth embodiment of vibration equipment in accordance with the present invention,

Fig. 6: shows a diagram of a mechanical realisation of the principle in Figure 4. The figure also shows a proposal for how the anode mass can be vibrated with a vacuum, where a vacuum chamber encloses the anode mass and part of the entire plumb.

The mechanical system as stated in NO 132359 has been tested in experiments, but the experiments showed that vibration equipment containing one vibrating mass did not produce the expected result. The reason for this was propagation of dynamic energy to the environment, and the equipment was also unstable. The table was subsequently improved in the experiments and converted into a mass that could be vibrated by placing a spring ki and a damper d₁ between the table m_t and base U, see Fig. 1. In the figure, the anode mass m_∑ is shown as a complex spring that may also consist of a spring and damper element k₂, d₂. It is expedient for the anode mass or spring damper system between the table and floor to be expressed as complex springs since complex springs have a real spring element and a hysteresis damper element. The anode mass may, of course, have other forms of damping than hysteresis damping, such as friction damping, etc. In the same way, different forms of damping may occur in a real damping element such as rubber dampers mounted between the table and the base. In Figure 1 , the vibrating plumb is shown as rr?_/. The dynamic input force F_dynjn against the equipment is a vertical periodic force. In accordance with the above adjustment, the improved equipment will consist of a coupled mechanical system with two vibrating masses. A coupled system with two vibrating masses can also be established by vibration being applied to the table instead of the plumb.

As a consequence of the above improvement, the noise to the environment was considerably reduced. There were many reasons for this:

• With 2 vibrating masses and by selecting a frequency range in which the table and the plumb will vibrate in virtual phase opposition (towards 180°), the dynamic gain of the compression force against the anode mass increased since the table also accelerated and contributed to the compression force. This meant that the dynamic input force against the equipment could be reduced to achieve the same dynamic compression against the anode mass. In turn, this led to the dynamically transmitted force against the base U being reduced since the dynamic input force was reduced.

• The damper d-i between the table m^and the base U dissipated dynamic energy.

• The base U was protected against shocks from the plumb. A shock contains a range of frequency components. The dynamic energy against the floor could be very high and random if the energy came directly from the plumb. The table was given a protective role so that the floor experienced a continuous sinusoidal force from the table with the same frequency component as the dynamic input force had, rather than shocks from the plumb.

• The equipment was stabilised on account of the damper element αfj. Low- frequency unstable fluctuations of the equipment were damped.

In connection with the development of the equipment in accordance with the present invention, it was decided, on the basis of the above knowledge, that the equipment would not comprise a base or foundation (the large passive mass under the equipment). It was found that optimal equipment should, as far as possible, be able to insulate the dynamic energy itself, so that it is absorbed in the equipment and, as far as possible, in the mass to be compacted/moulded, and so that a minimum quantity of it is emitted to the environment. In the improved equipment, a foundation under the floor on which the vibration equipment may rest has the sole task of damping the rest of the dynamic energy that is emitted from the vibration equipment.

In the aforementioned patent NO 132359, it is also proposed that a "constant compressive force, for example by means of a hydraulic cylinder" (reference number 16 in the figure) be applied to the plumb. The intention was to reduce the weight of the plumb. This is a very unfortunate way of applying external static force to the plumb. Firstly, the hydraulic cylinder was connected in stationary fashion to the base. Dynamic energy will then be propagated via this connection to the base. Secondly, the hydraulic cylinder contains damping and will directly dissipate dynamic energy that was intended for the anode mass. The dynamic gain towards the anode mass was reduced. Experiments with a hydraulic cylinder were carried out, but failed for the above reasons.

The present invention concerns further improvements to the prior art by means of a method and equipment for compacting materials, in particular vibration of "green mass" in a moulding process for the creation of mould bodies for the production of electrodes for the melting industry. The equipment comprises two mould parts, at least one of which has vibration applied to it during the compaction process. Moreover, the mould parts, for example the table and plumb, are mutually physically integrated during vibration by means of a static compressive force, which may consist of at least one spring k₃. The vibration equipment may be designed as a closed system in which the vibration energy is emitted to the environment as little as possible. Four embodiments of the equipment, which are closed systems, are shown in Figures 2-6. A fundamental difference between the embodiment shown in Figure 1 and those shown in Figures 2-6 is that the table in the latter is connected to the plumb via one or more springs fø, or an arrangement equivalent to a spring k₃.

Although the figures show that the plumb is vibrated, the principles of the present invention can also be implemented by the table being vibrated. The principles of the present invention may also be utilised by horizontal vibration of the mould parts. The mould parts may then be mounted in such a way that they can slide across a mainly horizontal base, for example by the mould parts being supported by a support that is able to slide in a horizontal direction (not shown).

With the present invention, materials can be compacted faster and more precisely with a higher degree of compaction and with less loss of energy to the environment than is possible with prior art equipment. Figure 6 also shows how it is possible to realise such equipment mechanically in such a way as also to achieve vibration with a vacuum, where the vacuum chamber is as small as possible and the gas evacuation time is minimal. These and other advantages can be achieved with the present invention as it is defined in the attached claims 1-18.

The attached Figures 2-5 are simplified diagrams of vibration equipment during vibration when a mass m_a is compressed or compacted. Figure 6 is a realisation of the simplified diagram in Figure 4. Figures 2-6 will be explained using the following definitions: Definitions:

U: The base.

m_a: The mass to be compressed by the vibration equipment.

k_∑: The spring constant of the mass m_a.

d_∑: The damping of the mass m_a. The damping may be in the form of hysteresis, viscous damping, friction, etc. (only one symbol is used for damping in the figures although we may have combinations of different forms of damping).

m{. The mass of the plumb or the mass that oscillates between the mass m_a and the body with spring constant k₃. The vibration unit for plumb vibration is included in this mass. In some setups, there may also be a yoke included as the plumb mass, as shown in Figures 3, 4 and 6. As we see in Figure 6, only part of the plumb's total mass is in the vacuum chamber. The yoke and the vibration unit are outside, but are permanently connected with bolts to the part of the plumb that is inside the vacuum chamber.

m,: The mass of the table. The mass that oscillates between the mass m_a and the body with spring constant /c_? or the body with damper element dy.

kf. One or more bodies with a total spring constant k placed between the table with mass m,_b and the base U.

d One or more bodies with a total damping d placed between the table with mass m*, and the base U. The damping may be in the form of hysteresis, viscous damping, friction, etc. (only one symbol is used for damping in the figures although we may have combinations of different forms of damping). k₃: A body with spring constant k₃. The plumb must be connected to the table via equipment with properties equivalent to those of a spring k₃. The equivalent spring must be progressive in the sense that the static force through it must be independent of how much the mass m_a is compressed. With k₃ it must be possible to vary the static force from the table to the plumb or keep it constant regardless of the compression of the mass m_a. At the same time, the equipment that is to represent k₃ must have minimum damping since it takes dynamic energy from the equipment as a whole. One example of such equipment may be air pressure adjustable bellows, as shown in Figure 6, where one set of bellows is placed at each end of the yoke. The bellows "press" the plumb towards the mass m_a during vibration. The bellows receive the compressive force from the table m_>.

As an exception, the body with spring constant k₃ may also have a fixed spring characteristic if the table's "side legs" can be height adjusted during vibration, as shown in Figure 5, so that the static force through k₃ is independent of how much the mass m_a is compressed. Such height adjustment can be implemented by the side legs being telescopic, for example by using screw jacks or hydraulic/pneumatic cylinders.

Fdynjn- The mechanical dynamic input force to the vibration equipment. A periodic force with one or more frequency components. The vibration unit fixed to the plumb for plumb vibration generates the dynamic input force. Fd_ynj_n has the same direction as the mass 77_athat is compressed. In Figure 6, the vibration unit is integrated in the yoke.

The parameters' effect during vibration for plumb vibration:

kι. The vibration equipment was a coupled mechanical system with 2 vibrating masses, an active mass mi and a passive mass m^.

• Increase in dynamic gain of dynamic forces against the mass m_a. The compression force against the mass m_a increases since the mass / 7_ft also contributes to a great extent to the dynamic compression force.

Reduced noise to the environment since the table and plumb in virtual phase opposition cancel each other out to a great extent against the base. The transmitted dynamic force against the base is therefore less.

dr.

• Higher stability since di filters out low-frequency unwanted fluctuations in the vibration equipment on the table and plumb and thus prevents the equipment from becoming unstable.

• Less noise to the environment since di dissipates energy and functions as a buffer against the base.

• However, reduced dynamic gain of dynamic forces against the mass m_a may occur if the damping is too high or the fundamental frequency of the compression force is too low so that it is in the low- frequency range that di has the task to damp.

k₃: Introduction of k₃: with static force from the table to the plumb.

• Further increase in dynamic gain of dynamic forces against the mass m_a on account of a higher compression amplitude of the mass m_a and a higher frequency. Since it is possible to increase the static force against the mass m_a with k₃, the dynamic force against the mass m_a will also increase. The vibration unit is also indirectly connected to the table via k₃ so that this contributes to increased acceleration of the table and thus also to increased dynamic force. Another reason for higher dynamic gain is that the static force through k₃ leads to the dynamic fluctuation of the mass m_a approaching the total dynamic fluctuation of the table and plumb. The plumb's dynamics are thus "pressed" further down in the mass m_a to be compressed. This leads to a higher compression amplitude of m_a. The working frequency of the equipment also increases because the mass m_a accelerates the table and plumb to a greater extent because it is in longer contact with the plumb over an oscillation period. The higher dynamic force will contribute the possibility of a higher degree of compaction of the mass m_a to be compacted. • Reduced vibration time on account of a higher frequency and higher compression amplitude of m_a. This leads to higher capacity. Measurements show that the time can be reduced from a vibration time of approximately 60 seconds to approximately 20 seconds. • Reduced noise since k₃ stores dynamic energy inside the equipment and thus emits less to the environment. Stored dynamic energy in k₃ is emitted to the mass m_a at the time in the vibration when k₃ is extended or m_a is compressed. The equipment becomes more of a closed system. If the spring rigidity in k₃ is increased, the equipment can store more dynamic energy. • Higher stability since di can be increased further without the compression force against the mass m_a being reduced. One of the advantages of the present invention is that it is possible to set the vibration frequency of the equipment with just the vibration unit. The working frequency can thus be moved to a greater extent out of the low-frequency range so that it is easier to damp low-frequency signals with di without damping the compression signal that has a higher frequency (easier to introduce a high pass filter). It is therefore easier to increase the damping di without any negative impact on the dynamic gain against the mass to be compressed. • Flexibility. Since the static force against the mass m_a can be adjusted via k₃, it is possible to adjust the size of the dynamic force against the mass m_a. In Figure 6, the air pressure in the bellows can be optimised to determine the size of the force. With the vibration unit, the amplitude [mmj/frequency ratio [Hz] of the dynamic fluctuation of the table and plumb is set. If the equipment is to function more as a beat oscillator, the frequency is reduced with the vibration unit to increase the amplitude [mm] of the dynamic fluctuation of the table and plumb. If the equipment is to function more as a "vibration press", the frequency is increased with the vibration unit so that the amplitude [mm] of the dynamic fluctuation is reduced. Such a setting also reduces the noise to the environment since impacts are reduced if we keep the same dynamic size of the force with the compressed air in the bellows. The optimal ratio here depends on the mass to be compacted, its dimensions and whether it is vibrated with a vacuum or not.

• Maintenance and robustness. The flexibility stated above can lead to a reduction in impacts. This reduces the wear on the equipment and considerably reduces the noise level.

Reduced dynamic energy to the environment:

The vibration frequency is adjusted as with the modified equipment towards the frequency at which the dynamic gain against the anode mass is greatest. This is also the frequency at which the table and plumb approach phase opposition. Because the plumb and table are connected to each other via the spring k₃, the plumb will contribute to pressing the table up when the table is on its way down towards the floor. Since the transmitted dynamic force against the floor is the sum of the plumb and table forces, where the plumb force acts in the opposite direction to the force from the table, the transmitted dynamic force to the base will be reduced. This results in the vibration equipment emitting less dynamic energy to the environment. In other words, dynamic energy will be stored in the spring k₃ when the table is in the low position and the plumb is in the high position (spring k₃ compressed). The spring then emits energy to the anode mass when it is extended (the anode mass is compressed). Dynamic energy is stored to a greater extent inside the system and less is emitted to the environment. Here it is important for the spring k₃ to have minimal damping so that the energy that is stored in the spring is used to compress the anode mass and is not converted into other forms of energy such as heat, etc.

Increased dynamic gain against the anode mass: With increased dynamic gain against the anode mass, there is an increased possibility of higher dynamic forces against the anode mass and/or lower dynamic input force (eccentric force). This allows for the product to have a higher density and for higher capacity.

Stability:

With stable equipment, there are no random fluctuations of the table and plumb that disturb the steady supply of energy to the mass m_a to be compressed. As the equipment in accordance with the present invention has at least one low resonance frequency in addition to the working frequency chosen, it is important to prevent the equipment from oscillating at these frequencies. It is also important to design the equipment so that dynamic gain in these low frequency ranges is minimised. With the present invention, it is possible, with the vibration unit, to increase the working frequency of the equipment. The damping d-i can thus be increased to minimise low-frequency fluctuations. An upper limit for this damping will be where no significant reduction in dynamic gain against the mass /77_a is achieved at the working frequency of the equipment.

By regulating the static compressive force from the spring k₃, it is possible to adjust the density of the compacted product. The compacting time may also be reduced with an increased static compressive force. This means that the capacity of the equipment can be increased. With the proposed equipment, the compressive force can also be adjusted during the compaction process itself if this is required. For example, it may be effective to vibrate initially at a relatively high compressive force, which subsequently decreases, and to increase it again towards the end of the vibration process.

Vibration equipment built in accordance with the present invention may comprise means that make it possible to vibrate electrodes so that they have the same density or the same physical dimensions. This can be achieved by the equipment being fitted with measuring equipment that registers how far down the plumb goes during vibration. The quantity of material placed in the mould before vibration is predefined, and it is then simple to establish a value that indicates weight/volume. The vibration may be terminated when a specific level has been reached so that the physical external dimensions are identical.

Moreover, the vibration equipment may have equipment that generates a vacuum in the volume that is delimited by the mould parts (the plumb, the table and the mould walls) containing the mass m_a so that any gas can be removed from the moulds (vacuum vibration). This will result in increased density, reduced risk of cracks and vibration at higher temperatures, etc. Figure 6 shows how this can be realised. Some of the complete plumb is inside the vacuum chamber Vr formed by the mould walls Fv1, Fv2 and a vacuum lid Vk. The vacuum lid can be connected via a pipe to equipment that generates a vacuum in the vacuum lid such as a fan or similar (not shown). The rest of the total plumb mass (the yoke A and the vibration unit Ve) is outside, but permanently connected with bolts B1, B2 to the part of the plumb Ld that is inside the vacuum chamber Vr. This results in the smallest possible vacuum chamber, and the evacuation time of the gases can be minimised. The bolts must have the smallest possible overall cross-section so that the vacuum has the least possible "suction effect" on the yoke. At the same time, the bolts must be sufficiently dimensioned and located so that the connection is robust and the torque in the yoke is within reasonable limits. As the mass m_a is compacted during vibration, the yoke A will approach the vacuum lid. There must therefore be a minimum distance to the yoke from the part of the plumb that is inside the vacuum chamber. This distance can be reduced if the vacuum lid also has telescopic properties. The overall height of the vibration equipment can, however, be reduced by integrating the vibration unit in the yoke.

Claims

1. A method for compacting a material, in particular vibration of "green mass" in a moulding process for the creation of mould bodies for the production of electrodes for the melting industry, in particular the aluminium electrolysis industry, comprising equipment with two mould parts, at least one of which has vibration applied to it during the compaction process, characterised in that the mould parts are mutually physically integrated during vibration by means of a static compressive force that may consist of at least one spring {k₃).

2. A method in accordance with claim 1 , characterised in that the static compressive force can be regulated during the vibration process independently of the position of the mould parts.

3. A method in accordance with claim 1 , characterised in that the spring (k₃) has minimal damping and exhibits a progressive spring force so that the static force between the mould parts is kept constant regardless of the position of one mould part in relation to the other during compaction.

4. A method in accordance with claim 1 , characterised in that the vibration is carried out mainly in the vertical direction.

5. A method in accordance with claim 4 in which the mould parts consist of a lower table equipped with mould walls and an upper plumb designed to move downwards towards the table as a consequence of the intermediate mass being compacted and in which the table is supported by a base, characterised in that the table is supported against the base by means of at least one spring (ki) and possibly a damper element (d₇).

6. A method in accordance with claim 5, characterised in that the characteristics of the spring (/j) and the damper element (d_?) are also chosen so that virtually all mechanical energy that is made available by the vibration applied is insulated from the base and is supplied to the material to be compacted.

7. A method in accordance with claims 1-3, characterised in that the vibration is carried out mainly in the horizontal direction.

8. A method in accordance with claims 1-7, characterised in that at least one of the mould parts has vibration applied to it at a frequency such that the mould parts contribute to maximum gain of dynamic force against the mass (/77_a) to be compacted, for example by the mould parts oscillating in virtual phase opposition.

9. A method in accordance with claims 1-8, characterised in that the mass (m_a) to be compacted is subjected to a vacuum.

10. Equipment for compacting a material, in particular vibration of "green mass" in a moulding process for the creation of mould bodies for the production of electrodes for the melting industry, in particular the aluminium electrolysis industry, comprising equipment with two mould parts, at least one of which has vibration applied to it, characterised in that the mould parts are mutually physically integrated by means of a static compressive force, which may consist of at least one spring (k₃).

11. Equipment in accordance with claim 10, characterised in that the spring {k₃) has an adjustable static compressive force.

12. Equipment in accordance with claim 10, characterised in that the spring (k₃) has minimal damping and also has a progressive spring force so that the compressive force is constant regardless of longitudinal changes to the spring.

13. Equipment in accordance with claims 10-12, characterised in that the spring (k₃) consists of one or more elastic, gas-filled bellows in which the gas pressure can be adjusted.

14. Equipment in accordance with claims 10-13 in which the mould parts are vibrated in the vertical direction and also consist of a lower table equipped with mould walls and an upper plumb designed to move downwards towards the table as a consequence of the intermediate mass being compacted and in which the table is supported by a base, characterised in that the spring (k₃) is connected between the table and the plumb via a structure that can be height-adjustable and that also extends up from the table and has a part that can be located above the plumb, which structure is permanently connected to the table.

15. Equipment in accordance with claim 14, characterised in that the table is supported against the base by means of at least one spring (k-i) and possibly a damper element (d-i).

16. Equipment in accordance with claim 14, characterised in that the plumb has vibration applied to it.

17. Equipment in accordance with claims 10-16, characterised in that the mass {m_a) is compacted within a vacuum chamber (Vr) in which a vacuum is provided.

18. Equipment in accordance with claim 17, characterised in that the vacuum chamber [Vr) is delimited by means of mould walls (Fv1, Fv2), the table and a vacuum lid (Vk).