EP0115957A1

EP0115957A1 - Method and apparatus for the manufacture of cementitious building product

Info

Publication number: EP0115957A1
Application number: EP84300608A
Authority: EP
Inventors: Howard Anthony Barker; Stephen Holt; Denise Margaret Hoys
Original assignee: Redland Technology Ltd; Redland Technologies Ltd
Current assignee: Redland Technology Ltd; Monier Braas Research and Development Ltd
Priority date: 1983-01-31
Filing date: 1984-01-31
Publication date: 1984-08-15
Also published as: GB8302584D0; AU2390484A; AU2390384A

Abstract

In a method of manufacturing a cementitious building product; mixing cement, water and a processing aid are first mixed. The mixture so formed is then subjected to high shear mixing in a twin-roll mill, the rolls of which are rotated at unequal speeds. The mixture is thereby caused to plasticise and then to adhere to only one of the rolls to form a band of material thereon. The band of material is then removed from the said roll, forming into blanks, and cured to form a building product.

Description

The invention is concerned with a method and apparatus adapted for use in the manufacture of cementitious building products. The invention is particularly concerned with high shear mixing and processing apparatus utilising such mixing for the manufacture of cementitious products. The term "cementitious building products", where used herein, is intended to include roofing slates,tiles, panels or the like and particularly such cementitious products of relatively thin cross-section and high flexural strength.
One of the major drawbacks of cementitious materials, which prevents their use for thin cross-section products, is the inherent lack of flexural strength in the finished products. Obviously, one may add reinforcing materials to improve the flexural strength of such products; however, the addition of reinforcing materials to cementitious products escalates their cost or, in the case of asbestos or other fibres of a carcinogenic nature, they increase the health risk of those operatives involved in the manufacture and use of the products made therefrom. In addition,it is difficult to process cementitious compositions of low water cement ratios, e.g. approximately 0.15 and with economically acceptable amounts of polymer additives at high production rates. It is an object of the invention to provide a method and apparatus which are suitable for the processing of oemntitious compositions having low water/cenent ratio and relatively small amounts of polymer additives as processing aids.
The invention provides a method of manufacturing a cementitious building product comprising the steps of mixing together cement, water and a processing aid, subjecting the mixture so formed to high shear mixing in a twin-roll mill, the rolls of which are rotated at controlled speeds and thereby causing the mixture to plasticise and then to adhere to only one of the rolls to form a band of material thereon; removing the band of material from the said roll; forming the material into blanks, and curing the blanks to form a building product.
The invention also provides apparatus for manufacturing a cementitious building product comprising means for mixing together cement, water and a processing aid; a twin-roll mill comprising a pair of contra-rotating rolls; means for rotating the rolls at controlled speeds to effect high-shear mixing of the cementitious mixture and thereby form a band of material on one of the rolls; means for removing the band of material from the said roll, and means for forming and curing the band of material to form a building product.
Preferably the distance between the rolls of the twin-roll mill is varied between a first position in which the distance is small to prevent the mixture from passing through the rolls before it plasticises and a second position in which the remainder of the mixing takes place.
Preferably the material is formed into blanks by cutting the material into separate pieces and pressing the pieces to form the blanks.
The rolls of the mill are preferably rotated at unequal speeds, for example, in the ratio 10:11.
The distance between the rolls of the mill may be varied by rams (for example, hydraulic) extended and retracted by an appropriate control circuit.
A doctor blade may be provided for running the band of material from the faster-moving roll, the band falling onto a conveyor which may transfer the band to cutting, pressing and curing stations. Edge trimming means may also be provided for trimming the band of material to a predetermined width
One aspect of the invention is the provision of high shear mixing apparatus comprising

a) a twin roll mill comprising a pair of horizontally arranging contra-rotating rolls;
b) means for rotating the rolls at differential speeds to create friction therebetween to cause, when the apparatus is in use, a cementitious composition fed thereto to be mixed under high shear;
c) means for effecting relative movement between the rolls to vary the size of the nip whereby; when the apparatus is in use, the nip may be opened in a controlled manner to prevent the composition being lost from the mill before it plasticises and firstly adheres to the surfaces of both rolls and ultimately due to the differential roll speeds to adhere to the surface of only the faster moving roll to form a band of material about the roll;
d) means for removing the band of material thus formed from the roll to which it has adhered.

The said apparatus may comprise part of a processing plant for producing, when the plant is in use, cured cementitious products from the band of material removed from the said roll, the plant also comprising:-

a) mixing means for intimately mixing the constituents of a cementitious composition;
b) feeding means for feeding the cementitious composition to an upper side of a nip of the rolls of the said apparatus at a controlled rate;
c) means for receiving the band of material from the said apparatus onto a cutting surface;
d) cutting means for cutting the band of material into green state blanks;
e) pressing means for compacting and moulding the blanks under low pressure;
f) transfer means for transferring the blanks from the cutting means to the pressing means and thereafter to a curing station; and
g) a curing station where the green state blanks are cured.

Preferably the relative speeds of the two rolls of the apparatus are in a ratio of at least 10:11 however, in other convenient arrangement the speeds of the rolls may be in a ratio of up to 1:4.
In operating the plant the distance between the rolls (the nip), may be opened at a controlled rate, as the cementitious composition fed thereto becomes plastised, from a zero nip to that distance apart which defines the final thickness of the band of material to be produced.thereby.
In an alternative arrangement the size of the nip may be altered in stages with a short dwell time between each stage in order to optimise the high shear mixing action of the contra-rotating rolls.
The present invention also conveniently provides an improved method of manufacturing roofing slates comprising the steps of:-

i) procuring an intimate mixture of cement, water and a processing aid;
ii) subjecting the said mixture to high shear mixing in a twin roll mill wherein the high shear is applied to the mixture by the action of the nip between contra-rotating rolls of the mill while rotating the rolls at unequal speeds;
iii) causing the mixture to adhere to only one of the rolls to form a band thereon;
iv) removing the band of cementitious material from the roll;
v) cutting the material into separate blanks;
vi) pressing the blanks under substantially low pressure; and,
vii) curing the blanks to form roofing slates.

There now follows, by way of example only, a detailed description, to be read with reference to the accompanying drawings,of an apparatus provided by the present invention together with a description of a method of operation of the apparatus to produce cured cementitious products, for example, roofing slates.
The description is to be read with reference to the accompanying drawings, in which:-

Figure 1 is a schematic drawing showingthe general arrangement of processing apparatus including a twin-roll mill for use in the invention;
Figure 2 is a circuit diagram of a hydraulic circuit for adjusting the gap between the rolls of the twin-roll mill of the apparatus of Figure 1;
Figure 3 is a diagrammatic front elevation of the twin-roll mill showing an edge trimming arrangement; and
Figure 4 is a diagrammatic side elevation of part of the twin-roll mill showing a doctor blade arrangement.

Artificial slates have in the past been produced from reinforced cementitious materials and apart from the well known disadvantages of utilising reinforcing material of a carcinogenic nature there are many other reasons why cemsntitious materials cannot easily be utilised in thin sections. For example; low flexural strength due to inadequate properties of fibres substituted for asbestos and the excessive porosity in the finished product leads to excessive water absorption and poor durability in adverse conditions.
The apparatus described below overcomes or mitigates these drawbacks by providing apparatus with which cementitious products of acceptable flexural strength and durability can be readily produced from low water/cement ratio compositions.
Referring first to Figure I, the apparatus or plant comprises a mixer 10, a feed conveyor 12, a twin-roll mill 14, a transfer conveyor 16, a cutting device 18,a press 20 and a curing chamber 22.
The mixture 10 is of conventional design and is utilized, when the plant is in use, to form a homogeneous mixture from the constituents placed therein. While the mixer shown may be tipped about a fulcrum 24 to deposit its contents onto an upper run 26 of the conveyor 12, any other suitable transfer arrangement may be used.
The conveyor 12 is arranged with its upstream end 28 adjacent the mixer 10 and a downstream end 30 overlying the nip of the twin-roll mill 14. The mill 14 has a pair of contra-rotating rolls 32 and 34 the distance between adjacent surface portions of which is defined as the nip between the rolls.
The rolls 32 and 34 are arranged in side-by-side relationship and have an operative width defined by edge cutters not shown in Figure 1 but which are described below with reference to Figure 3. The rolls are of equal diameter and are arranged with their axes parallel and horizontal.
The rolls 32 and 34 are arranged to run at unequal speeds and a suitable motor and gears are provided for this purpose. The slower moving roll 32 has a peripheral speed of 20m per minute in one example, while a differential gear ratio of 10:11 ensures that the faster moving roll 34 has a peripheral speed of approximately 22m per minute.
The rolls 32 and 34 rotate in the direction of the arrows 32a and 34a respectively and the roll 32 is arranged to be moved relative to, the roll 34 to vary the size of the nip formed therebetween. The means for effecting the said movement of the roll 32, comprises a pair of hydraulic rams 35(one of which is shown in Figure l) and an appropriate control circuit which is shown in Figure 2. Each ram 35 is connected between an end of the roller 32 and a fixed part of a frame 37 of the twin-roll mill 14. The control circuit allows an operative to cause a progressive spacing of the rolls 32 and 34 or a stage by stage spacing thereof with appropriate dwell periods between each stage.
Turning now to Figure 2, the hydraulic circuit 50 for operating the rams 35 comprises a pump 51 for supplying pressure fluid via a directional control valve 52 to supply lines 53, 54 for the respective rams. Each supply line 53, 54 includes, connected in series to the supply side of the ram, a flow control valve 56 and two check valves 57, 58 arranged to operate in opposite directions. A preset relief valve 59 and a pressure transducer 49 is also provided in each supply line. The return side of the rams is connected via return line 4₃ to the directional control valve 52 and thence to a tank 45. Finally a main circuit relief valve 41 and on/off control ₄₄ are connected between the pump 51 and tank 45.
The circuit 50 is designed to allow the rams 35 to be extended very quickly to a position in which the nip between the rolls 32, 34 is a zero gap and to retract the rams at a controlled rate, keeping the rolls parallel, to a position in which the gap is approximately 4mm. The flow control valves 56 control this basic movement. The relief valves 59 ensure that neither piston is overloaded at any time while the pressure in the hydraulic circuit is controlled by valve ₄₁. The solenoid operated check valves 57, 58 allow both rams to be extended or retracted independently.
As illustrated in Figure 2, it will be seen that the hydraulic circuit 50 is partly automated but it will be appreciated that an appropriate control system may readily be added so that the extension and contraction of the rams 35 is fully automatic and follows a predetermined pattern to allow continuous operation of the plant.
Returning now to Figure I the apparatus 14 also includes a return conveyor 36 which is located beneath the nip of the rolls 32 and 34 and which, when the apparatus is in use, returns cementitious material dropping through the nip of the rolls 32 and 34, and not being processed thereby, to the feed conveyor 12.
It will be appreciated that, as well as the conveyor 36, there is also a requirement to trim and collect surplus material from the edge of the rolls 32, 34. The apparatus is intended, in use, to produce a band of cementitious material of a predetermined width and an edge trimming arrangement is therefore required. Such an arrangement is illustrated in Figure 3.
As will be described below, the cementitious material, when processed, adheres to the faster moving roll 34 and the edge trimming means 60 is therefore associated with the roll 34 (see figure 1). Referring now to Figures 1 and 3, the edge trimming means 60 comprises two circular blades 61, arranged one adjacent each end of the roll 34 and in contact therewith. To ensure that the blades 61 remain in contact with roll 34, each blade 61 is mounted on an adjustment screw 63 and is spring urged towards the roll 34. The screw 63 are in turn fixed to a bar 64 attached to the support frame 37 of the mill. The cutting blades 61 are effective to cut a band of material on the roll 34 and scrapers 65,then remove the excess material from the roll 34. Each scaper 65 comprises a curved blade which is fixed to a bracket 66 fixed to the frame 37 and one end of which is in close proximity to the roll 34. The material thus removed from the roll 34 falls into a collecting trough 68 arranged below the blades 61 where it is collected. The excess material thus collected"may simply be removed or may be recycled to the mill by suitable means.
The trimmed band of material remaining on the roll 34 is removed by a cut-off device 70 in the nature of a doctor-blade.
Referring now to Figure 4, the cut-off device 70 comprises a doctor blade 72 extending the full width of roll 34 and slidable in a housing 73 into and out of engagement with the roll 34. A pair of double-acting pneumatic rams 75 are connected between a bracket 76 fixed to the edge of the doctor blade 72 remote from the roll 34 and brackets 77 fixed to the frame 37 and are operable to move the doctor-blade 72.
The housing 73 comprises two metal plates 78 having brackets 79 at their ends by means of which the housing is fixed to the support frame 37. Clamped between the plates 78 are two pairs of retaining strips 81 and two felt pads 82 therebetween, the blade 72 slides between the pads 82 which are effective to clean the blade as it is retracted.
In operation, the blade 72 is effective to slit a band of cementitious material from the faster moving roll 34, and allow the band to fall by gravity onto the transfer conveyor 16.
The transfer conveyor 16 of the processing plant comprises an upstream end 38 located beneath the roll 34 and a downstream end 40 located adjacent to the press 20. The cutting device 18 overlies an upper run 42 of the conveyor 16 and comprises a die-sett which, when the plant is in use, cuts a series of green state blanks from the laid out band of cementitious material.
The plant includes a transfer device, not shown, but of known type, for transferring the green state blanks to the press 20. The press 20 is a conventional press used in the manufacture of building products capable of exerting a pressure of 6Mpa on the blanks to compact them.
The curing chamber 22 of the plant is one which operates under 100% relative humidity and at temperatures between 30° and 90°C. The blanks are transferred from the press to the curing chamber in known manner. Appropriate packaging means (not shown) may also be provided in the processing plant for stacking and packaging the finished products.
In carrying out the manufacture of cementitious building products for example, artificial roofing slates on the apparatus described above, the following method is used.
An operative mixes a cementitious composition in the mixture 10 the composition being a, cementitious composition of low water/cement ratio and containing a processing aid.
Examples of such compositions are described in European Patent Application 81305630.6 (Publication No.0055035).
The composition is mixed until a high degree of homogeneity is achieved. The mixer 10 is then rotated about its fulcrum 24 to deposit the composition, which at this stage has a crumbly quality, onto the upper run 26 of the conveyor 12. The composition is carried by the upper run of the conveyor 12 and thence falls into the nip of the rolls 32 and 34.
The conveyor speed is 15m per minute and the composition is fed at a constant rate over 10 to 20 seconds.
The crumbly cementitious composition is thus carried by the upper run of the conveyor and passes from the end 30 of the conveyor into the nip of the rolls 32 and 34. The nip between the rolls 32 and 34 is initially set by rams 35 to zero gap and the peripheral spee of the rolls 32 and 34 are, as stated above, twenty and twenty-two meters per minute respectively.
As the cementitious composition falls into the nip of the rolls they are progressively spaced apart by operation o rams 35 to widen the nip. The rate of movement of the roll 32 as it wiè the nip is controlled to ensure that as little as possible of the composition falls straight through onto the conveyor As it passes through the nip, the composition is subjected to high shear mixing and, initially, the composition sometimes adheres to both rolls. Ideally, the nip opening rate and speed of rotation of the rolls should ensure that the cementitious material adheres to only the faster moving roll; however, under any circumstances the unequal speed of the rolls ensures tha the degree of high shear mixing reduces the composition eventually to a state approaching high plasticity and any material on the slower moving roll 32 will then transfer to the faster moving roll 34 and forms a single band thereon with a thickness defined by the final spacing of the nip between the rolls, for example 4mm.
The width of the band is determined by the edge cutters 6 defining the operative width of the rolls 32 and 34.
The operative then operates the cut-off device to slit the band of highly plastic cementitious material which then passes onto a steel platen (not shown) which is carried by the upper run 43 of the conveyor 16.
The laid out band of cementitious material from a typical mix is about 152cm x 86cm x 0.4cm and has a consistency similar to putty.
The conveyor 16 is arrested with the platen and material under the cutting device 18 which is operated to cut the material into green state blanks.
The platen and blanks are thereafter conveyed to the press 20 where they are arranged in a stack for pressing at a pressure of 6MPa for two hours.
After pressing, the stacks of blanks are conveyed to a curing chamber where they are subjected to 100% relative humidity at a temperature of 60%C for 8 hours. The roofing slates may then be ready for use or may be subjected to further ageing prior to use.
In a modified plant provision may be made for applying an organic polymer or other suitable coating material to at least one surface of the cememtitious products prior to or after the curing operation.
The operation of the apparatus is not limited to the process discribed above. For example, the relative speeds of the rolls of the mill may be varied and one or other of the rolls may be heated or cooled to assist in plasticising the cementitious composition as described above.
It will be appreciated that the invention is not limited to the preferred embodiment described above and may process cementitious materials other than those of the type described. However, cementitious products manufactured in a plant utilising a high shear mixing apparatus as hereinbefore disclosed and having a composition as described have been found to have an enhanced flexural strength and modulus of elasticity.
It is believed that the intimate mixing imparted by the mixer 10 followed by high shear mixing in the apparatus 24 creates a dense material of low porosity which only requires pressing at a substantially low pressure prior to curing in the chamber 22.
It will further be appreciated that the apparatus is not limited to that described above and may be modified. For example a motorised system may replace the rams 35 for adjusting the nip between the rolls 32, 34 and such a system may be advantageous for continuous operation of the plant.

Claims

1. A method of manufacturing a cementitious building product characterized by the steps of; mixing together cement, water and a processing aid; subjecting the mixture so formed to high shear mixing in a twin-roll mill; the rolls of which are rotated at controlled speeds and thereby causing the mixture to plasticise and then to adhere to only one of the rolls to form a band of material thereon; removing the band of material from the said roll; forming the material into blanks, and curing the blanks to form a building product.

2. A method as claimed in claim I characterized in that the distance between the rolls of the twin-roll mill is varied between a first position in which the distance is small to prevent the mixture from passing through the rolls before it plasticises and a second position in which the remainder of the mixing takes place.

3. A method as claimed in claim 1 or claim 2 characterized in that the material is formed into blanks by cutting the material into separate pieces and pressing the pieces to form the blanks.

4. A method as claimed in any one of the preceding claims characterized in that the rolls of the mill are rotated at unequal speeds.

5. A method as claimed in claim 4 characterized in that the ratio of the speeds of rotation of the rolls of the twin-roll is 10:11.

6. Apparatus for manufacturing a cememtitious building product characterized by the combination of means for mixing together cement, water and a processing aid; a twin-roll mill comprising a pair of contra-rotating rolls; means for rotating the rolls at controlled speeds to effect high-shear mixing of the cementitious mixture and thereby form a band of material on one of the rolls; means for removing the band of material from the said roll; and means for forming and curing the band'of material to form a building product.

7. Apparatus as claimed in claim 6 characterized in that means are provided for varying the distance between the rolls of the twin-roll mill.

8. Apparatus as claimed in claim 7 characterized in that the distance varying means comprise at least one ram operable to move one of the rolls of the mill and control circuit means for extending and retracting the ram.

9. Apparatus as claimed in any one of claims 6 to 8 further characterized by conveyor means for transferring the cementitious mixture from the mixing means to the twin-roll mill.

10. Apparatus as claimed in any one of claims 6 to 9 characterized in that the rolls of the mill are of generally equal diameter and arranged with their axes parallel and horizontal.

11. Apparatus as claimed in any one of claims 6 to 10 characterized in that the forming means comprises cutting means and pressing means.

12. Apparatus as claimed in any one of claims 6 to 11 characterized in that the removing means comprises a blade for slitting the band of material from the said roll and conveyor means onto which the band falls by gravity.

13. Apparatus as claimed in any one of claims 6 to 12 further characterized by edge trimming means for trimming the band of material to a predetermined width.

14. A cementitious building product manufactured by method of claims 1 to 5.

15. A cementitious building product manufactured by the apparatus of claims 6 to 13.

16. A cementitious building product as claimed in claim 14 or claim 15 characterized in that it is a roofing tile.