UNIT OF HATSCHEKMACHINE

Technical field

The present invention relates to Hatschek machines and methods for controlling such machines, in particular the liquid level in the premixing unit of a Hatschek machine. State of the art

Hatschek machines are widely known and are operated for the manufacture of fibre cement sheets. In such machines, the flow of various liquids are numerous and the control thereof is complex. Controlling one flow often has a potentially strong influence on the operation of other parts of the machine or on the other liquid flows per se.

Summary of the Invention It is an object of the invention to provide an alternative control of an Hatschek machine, in particular one which guarantees a more stable operation of the settling tank.

According to a first aspect of the invention, a method is provided for controlling the liquid level in the premixing unit of a Hatschek machine comprising a sheet manufacturing machine, a cement slurry preparation mixer, a settling tank, a selectifier and a premixing unit. The method comprises the step of supplying the selectifier with a first portion of the water-loaded cement slurry of the sheet manufacturing machine to the selectifier, wherein a level meter measures the liquid level in the premixing unit and generates a measurement signal, and wherein the measurement signal directly or indirectly controls a control valve, such that the amount of a second portion of the water-loaded cement slurry of the sheet manufacturing machine that is supplied to the selectifier, is controlled.

According to some embodiments, the method may comprise the following steps: o A second portion of the water-loaded cement slurry of the sheet

manufacturing machine is supplied to the settling tank;

o Settled cement sludge from the settling tank is supplied to the selectifier;

o Cement slurry of the cement slurry preparation mixer is supplied to the selectifier;

o The cement mixture of the selectifier, from which the hard particles have been removed, is supplied to the premixing unit;

o Flocculant is supplied to the premixing unit in order to obtain a mixture which is supplied to the sheet manufacturing machine.

According to some embodiments, the generated measurement signal may be processed by a processing unit, which generates a control signal that controls the control valve.

According to some embodiments, the level meter may be a weighing cell.

According to some embodiments, the amount of settled cement sludge from the settling tank which is supplied to the selectifier, may be constant.

A selectifier is a centrifugally operating cleaning device, used to remove solidified particles from a slurry. According to some embodiments, the generated measurement signal may be processed by a processing unit, which generates a control signal that controls the control valve. This processing unit may be a central processing unit, for example, the CPU of a control mechanism or of a computer.

According to some embodiments, the level meter may be a weighing cell. A weighing cell is also known as a pressure box or load cell.

According to some embodiments, the amount of settled cement sludge of the settling tank which is supplied to the selectifier may be constant. According to a second aspect of the invention, there is provided a Hatschek machine for the manufacture of fibre cement sheets, which machine comprises

• a sheet manufacturing machine,

• a cement slurry preparation mixer,

• a settling tank,

· a selectifier,

• a premixing unit,

• a first transport pipe for supplying a first portion of the water-loaded cement slurry of the sheet manufacturing machine to the settling tank;

· a second transport pipe for supplying settled cement sludge of the settling tank to the selectifier;

• a third transport pipe for supplying a second portion of the water- loaded cement slurry of the sheet manufacturing machine to the selectifier; • a fourth transport pipe for supplying the cement slurry of the cement slurry preparation mixer to the selectifier;

• a fifth transport pipe for supplying the cement mixture, from which the hard particles have been removed, of the selectifier to the premixing unit;

• a means for adding flocculant to the premixing unit;

• a sixth transport pipe for supplying a mixture of the premixing unit to the sheet manufacturing machine;

• a first level meter for measuring the liquid level of the premixing unit and generating a first measurement signal;

• a first control valve for controlling the second portion of the water- loaded cement slurry of the sheet manufacturing machine to the selectifier;

• a processing unit for converting the first measurement signal to a first control signal for controlling the first control valve.

A control valve is a valve whose opening is controllable by means of a control signal. The control valves can be any valves, but are, for example butterfly valves.

A transport pipe is to be understood to mean any duct, tube or pipe, whether or not fixed or flexible, which is suitable for transporting a liquid from one location to another.

According to some embodiments, the machine may further comprise at least a second control valve for controlling the amount of the mixture of the premixing unit, which is supplied to the sheet manufacturing machine via the sixth transport pipe. According to some embodiments, the sheet manufacturing machine may comprise at least a first container in which at least one cylindrical screen is ratably suspended for the manufacture of a fibre cement web, wherein the sixth transport pipe supplies the mixture of the premixing unit to said first container of the sheet manufacturing machine. According to some embodiments, the first container may comprise a second level meter for measuring the liquid level in said first container and for generating a second measurement signal, and wherein the processing unit converts the second measuring signal into a second control signal for controlling the second control valve.

According to some embodiments, the sheet manufacturing machine may comprise at least a second container in which at least one cylindrical screen is ratably suspended for the manufacture of a fibre cement web, wherein a seventh transport pipe supplies the mixture of the premixing unit to said second container of the sheet manufacturing machine. According to some embodiments, the machine may further comprise at least a third control valve for controlling the amount of the mixture of the premixing unit, which is supplied to the sheet manufacturing machine via the seventh transport pipe. According to some embodiments, the second container may comprise a third level meter for measuring the liquid level in said second container and for generating a third measuring signal, and wherein the processing unit converts the third measurement signal into a third control signal for controlling the third control valve.

The independent and dependent claims provide specific and preferred features of the embodiments of the invention. Features of the dependent claims may be combined with features of the independent and dependent claims, and this in any suitable manner as would be evident for a skilled person.

The above mentioned and other features, characteristics, and advantages of the present invention will be elucidated with the help of following exemplary embodiments, optionally in combination with the drawings. The description of these exemplary embodiments is given as clarification, without the intention to limit the scope of the invention. The reference numerals in the following description refer to the drawings.

Brief Description of the Figures

Figure 1 is a schematic representation of a Hatschek machine according to the invention.

Description of exemplary embodiments

The present invention is hereinafter described using specific embodiments. It should be noted that the term "comprising", such as, for example, used in the claims, should not be construed in a limiting sense, limited to the subsequent elements, features and/or steps. The term "comprising" does not exclude the presence of other elements, features, or steps. Hence, the scope of a wording "an object comprising the elements A and B", is not limited to an object which contains only the elements A and B. The scope of a wording " a method comprising the steps A and B", is not limited to a method which only includes the steps A and B. In the light of the present invention, these wordings only mean that the relevant elements and steps of the invention respectively, are the elements and steps A and B, respectively.

In the following specification reference is made to "an embodiment," or "the embodiment". Such reference means that a specific element or feature, described based on this embodiment, is comprised in at least this one embodiment. The occurrence of the terms "in an embodiment" or "in one embodiment" on various locations in this description, however, does not necessarily refer to the same embodiment, although it may refer to the same embodiment.

Furthermore, the properties or features may be combined in any suitable way in one or more embodiments, as would be apparent to the skilled person. A Hatschek machine 1000 for the manufacture of fibre cement sheets is shown in Figure 1 . The machine 1000, comprises

• A sheet manufacturing machine 100,

• a fibre cement slurry preparation mixer 200,

· a settling tank 300,

• a selectifier 400, and

• a premixing unit 500.

A first transport pipe 601 is provided for supplying a first portion of the water- loaded cement slurry of the sheet manufacturing machine 100 to the settling tank 300.

A second transporting pipe 602 is provided for supplying settled cement sludge of the settling tank 300 to the selectifier 400.

A third transport pipe 603 is provided for supplying a second portion of the wa- ter-loaded cement slurry of the sheet manufacturing machine 100 to the selectifier 400.

These first and third transport pipes 601 and 603 can partially proceed via mutual pipelines 604, which spit, in turn, into the first and third transport pipe 601 and 603.

A fourth transport pipe 605 is provided for supplying cement slurry of the fibre cement slurry preparation mixer 200 to the selectifier 400. The flow rate of fibre cement slurry between the mixer 200 and the selectifier 400 is determined by means of a valve (not shown on Figure 1 ). Said opening of said valve is determined by the thickness of the formed fibre cement sheet, and, if necessary, can be adjusted.

A fifth transport pipe 606 is provided for supplying the cement mixture, from which the hard particles have been removed, of the selectifier 400 to the premixing unit 500. A means is provided, being an inlet 607, for adding flocculant to the premixing unit 500.

A sixth transport pipe 608 is provided for supplying a mixture of the premixing unit 500 to the first container 101 of the sheet manufacturing machine 100. The sheet manufacturing machine 100 also comprises a second container 102 and a seventh transport pipe 609 which supplies a mixture of the premixing unit 500 to said second container 102 of the sheet manufacturing machine 100. Each of the containers 101 and 102 has at least one, and as can be seen in this embodiment, in each case two rotary screens 103, which provide in a known way a fine fibre cement web to a conveyor belt 104, which conveys these webs to a forming drum 105. The Hatschek machine 1000 further comprises some level meters. It comprises a first level meter 501 , being a weighing cell, for measuring the liquid level in the premixing unit 500 and for generating a first measurement signal 701 .

The machine 1000 further comprises a second level meter 106 and a third level meter 107 for measuring the liquid level in the first container 101 and the second container 102, respectively. Each of these level meters generates a measurement signal 702 and 703.

A first control valve 801 is provided for controlling the second portion of the wa- ter-loaded cement slurry of the sheet manufacturing machine 100 to the selecti- fier 400. A second (802) and third (803) control valve is provided for controlling the supply of the quantity of the mixture of the premixing unit 500 to the sheet manufacturing machine 100 via the sixth and seventh transport pipe. A processing unit 900 is provided for converting the first, second and third measurement signal (701 , 702 and 703) to a first, second and third control signal (704, 705 and 706) for controlling the three control valves. The first control signal 704, which controls the control valve 801 , will, of course, predominantly, or even only be generated on the basis of the measurement signal 701 of the weighing unit 501 . The second control signal 705, which controls the control valve 802, will, of course, predominantly, or even only be generated on the basis of the measurement signal 702 of the level meter 106 in the first container 101 . The third control signal 706, which controls the control valve 803, will, of course, predominantly, or even only be generated on the basis of the measurement signal 703 of the level meter 107 in the second container 102. The level meters 106 and 107 are preferably bubble tubes (also called bubble pipes).

The machine 1000 further comprises a pilot valve 804 which controls the permanent supply of a quantity of sludge of the settling tank 300, also called decanter unit or cone, to the selectifier. A pilot valve is a valve the opening of which can be controlled, but the opening of which is normally not changed in function of a control signal.

When the machine 1000 is in operation, the level in the premixing will unit 500 will fluctuate depending on the requested amount of fibre cement slurry via the valves 802 and 803 by the sheet manufacturing machine 100. These fluctua- tions are absorbed by supplying more or less water-loaded cement slurry via control valve 801 to the selectifier 400, which in turn flows into the premixing unit 500. This control of the amount takes place via control signal 704.

Due to this control, the amount of water-loaded cement slurry which is supplied via transport pipe 601 to the settling tank 300, will vary in time, during which a same amount of sludge from said container 300 is constantly supplied via valve 804 to the selectifier. However, the fluctuations in the supply of the water, loaded with cement slurry are hardly going to disturb nor impede the stable operation of the settling tank 300.

If, as in the past, the level of the premixing unit 500 was controlled by the optional supply of sludge (and, consequently, placing the valve 804 either in an open position or a closed position), at a constant supply of water-loaded cement slurry via the transport pipe 603, the operation of the settling tank will be disturbed. Such on/off control has a disturbing effect on the currents in the container 300.

It is clear that, although the embodiments and/or materials for providing embodiments according to the present invention have been discussed, several modifications or changes may be made without departing from the scope and/or gist of this invention.