WO2000008402A1

WO2000008402A1 - Continuous rotary kiln for calcination, especially suited for the calcination of inorganic pigments

Info

Publication number: WO2000008402A1
Application number: PCT/EP1999/005530
Authority: WO
Inventors: Angelo Lami; Colombo Spaccini
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-08-04
Filing date: 1999-08-02
Publication date: 2000-02-17
Anticipated expiration: 2001-02-04
Also published as: ITVI980146A1

Abstract

The invention produces a continuous rotary kiln (1) especially suited for the calcination of inorganic pigments including: a combustion chamber (2) developed mainly in a lengthways direction; means (5) for heating said chamber according to temperature gradients variable along pre-set zones; a series of chambers suited for the calcination treatment, where said chambers consist of two or more tubes (6) set at the same angle, placed inside the combustion chamber and which hold the material undergoing the treatment, said tubes being made to rotate synchronously by means of a drive transmission system.

Description

CONTINUOUS ROTARY KILN FOR CALCINATION, ESPECIALLY SUITED FOR THE CALCINATION OF INORGANIC PIGMENTS This invention concerns a continuous rotary kiln especially suited for the calcination of inorganic pigments. As is known inorganic colours, or inorganic pigments, are obtained from inorganic oxides that are combined together by means of a solid-solid reaction in stoichiometric quantities to obtain a crystallographically stable compound. This is achieved by means of the so-called calcination operation that is effected at temperatures starting from approximately 700° up to 1300°C. Depending on the type of colours that have to be obtained, the calcination is also carried out in modified atmosphere, for example neutral reducing or oxidising.

One of the known methods of calcinating inorganic pigments is by achieving this roasting treatment in a muffle furnace where the inorganic compounds involved in the calcination treatment are placed in containers of refractory material which in turn are placed inside a furnace and each constitute a calcination chamber. The temperature reached inside the container enables the chemical reaction that produces the desired pigment, that is a crystallographically stable compound. Static kilns however produce pigments that are not perfectly uniform since the layers of powder that are put inside said containers, even though they are first mixed thoroughly, reach markedly different temperatures and thereby give irregular results from a colour pigment viewpoint. Another method of calcinating inorganic pigments is to use a rotary kiln. Document DE-A 4202749 describes a rotary kiln for treating granular powdery materials that has a single rotating cylinder made with a ceramic lining, more specifically in alumina. The cylinder is set practically level, inclined at a slight angle to allow the material inside the cylinder to move. The powder material to be treated is introduced in the higher part of the cylinder and exits out of the lower end.

For the rotary kiln described above to have a useful application in industrial production, the single cylinder constituting the calcination chamber has to have a sufficiently large diameter, around 350-400 mm., for it to be able to treat a significant quantity of product at an industrial level. One of the limits of the device described above is the poor flexibility of the plant in changing the type of pigment to be produced. What's more the aforementioned plant is not suited to treating small industrial quantities. Then, it has to be noted that a tube lined in alumina having a diameter of around 300- 400 mm. is very expensive. This means that the whole rotary kiln becomes very costly.

With a rotary kiln having a single cylinder it is also equally difficult to change production from one colour to another, unless it does not have a very large series of tubes for the treating, so that the tube of one colour can be replaced with the tube of another. In view of the extremely high cost of the actual tubes, because of the characteristics of the material they are made of and their large diameter, it is quite improbable that the rotary kiln may have more than one tube to make changeovers related to the various colours. The scope set for the invention is to overcome the technical limits illustrated above. The intention is to above all produce a rotary kiln, by which variable quantities of inorganic pigments can be treated, ranging from a few kilos to industrial quantities of several kilograms/hour.

Another intention is to achieve a change in type of colour in the shortest time possible, even replacing components of the kiln that are not costly. Another scope set for the finding is to make it extremely easy to treat a vast range of colours with treating temperatures that vary up to around 1350°C. The intention is to also be able to easily modify the atmosphere in the calcination chamber. What's more the intention is to produce a kiln that only requires easy, cheap maintenance.

Last but not least, a scope is to produce a rotary kiln that can run with low energy costs.

All these scopes and others that shall be better illustrated below, are achieved by a continuous rotary kiln especially suited for calcination of inorganic pigments that, in accordance with the contents of the first claim includes:

- a combustion chamber developed mainly in a lengthways direction;

- means for heating said chamber according to temperature gradients variable along pre-set zones;

- a series of chambers suited for the calcination treatment said kiln being characterised in that said chambers for the calcination treatment consist of two or more tubes set at the same angle, placed inside the combustion chamber for most of their length, which hold the material undergoing the treatment, said tubes being made to rotate synchronously by means of a drive transmission system. According to a preferred form of execution of the invention the tubes that make up the rotary kiln invention are coplanar and stand side by side, set parallel to one another at a uniform distance. The rotating motion is transmitted to these tubes by a single motor. The uniform angle of the tubes is obtained by raising the whole kiln in its entry upstream, with respect to its exit downstream, by a given height that can vary according to the desired hourly production, and in other words depending on how long the material should stay inside the treatment chamber.

The tubes making up the kiln's treatment chambers have a small diameter, preferably around 100 to 200 mm. and are made of special steel, or all alumina. The length of the tubes will preferably be a standard trade length, with the scope of using tubes that can be found on the market. In this way the overall cost of the kiln is significantly lowered.

An important characteristic that has to be underlined, that is specific to the kiln invention, is that lots of different coloured pigment can be treated at the same time in different tubes without this creating any kind of problem whatsoever. It is also important to underline that if the tubes in the rotary kiln require maintenance and/or replacement for any reason, this is achieved very rapidly by just sliding the tubes out of the kiln and then returning them in place by inserting them in their seats and hooking up the drive transmission system to the replaced tubes.

An advantage of the kiln invention is that it provides maximum operating flexibility since it can produce large quantities of pigment, as well as being able to produce small lots of pigment, without losing in economic efficiency. This will depend on the number of tubes used during the production cycle, which can even be lower than the actual number of tubes mounted on the kiln. If only some of the tubes are used, it may be worth sliding the unused tubes out of the combustion chamber, so that the kiln is heated quicker and thereby saving on the calories required to reach the required temperature by avoiding to heat up unused treatment chambers. It is also evident that the kiln invention is extremely cheap since its construction basically involves a combustion chamber and several treatment chambers, being composed of tubes that are extremely cheap and easily replaced.

Additional characteristics and details of the invention shall be better illustrated in the description of a preferred form of execution of the invention given as a guideline but not a limitation, and shown in the attached diagrams where:

- fig. 1 shows a line diagram of the prospective view of the kiln invention;

- fig. 2 is a lengthways section of the kiln invention;

- fig. 3 is a cross section of the kiln invention; - fig. 4 shows a detail of the drive transmission for the series of tubes in the invention.

With reference to the aforementioned figures the kiln, generally indicated by 1 , has a combustion chamber, indicated by 2, made in refractory material and basically forming a parallelepiped raised from its support floor and its position being adjustable by means of two jacks 3 and 4 so that the angle α of the combustion chamber 2 over horizontal can be varied according to the treatment speed that is required, since the dwell of the materials introduced in the treatment chambers depends on the angle of the combustion chamber, as will be seen below. The combustion chamber 2 is heated by two or more burners, indicated by 5, that are generally burners fuelled by methane or nevertheless with fuel suited to create a temperature inside the combustion chamber 2 of at least up to 1350°C. A series of treatment chambers are arranged inside the combustion chamber, composed of tubes, indicated by 6. These tubes, that in the example count 5, but can be any number whatsoever, are arranged in a line, parallel to each other, and are made of ceramic material or special steels. It should be noted that these tubes should preferably not exceed a 200 mm. diameter and this complies with one of the invention's scopes to use materials that can be commonly sourced in the trade. Standard tubes measuring 6 metres long and up to 200 mm. in diameter are easily found on the market. These tubes can be found in special steels, or in a special steel that resists to kiln temperatures of up to 1300°C, being the maximum treatment temperature for the calcination of inorganic pigments, since the calcination treatment of inorganic pigments can vary from approximately 750° to approx. 1300°C. This type of tube is preferred in this invention. The inorganic material to be treated, consisting of a mixture of several components to obtain various pigments is put inside one or all the tubes 6. The products are fed into the hopper 6 and enter the interior 61 of the tube 6 and move in a continuous turning over motion, due to the rotation of the tube 6, until the outlet 62 of the tube that constitutes the treatment chamber. The tube reaches a temperature that depends on the power and quantity of burners 5 that radiate heat to the interior 21 of the combustion chamber 2. Known means of regulation and stabilisation of the temperature inside the kiln are provided to control the operation of the burners 5 according to a pre-set temperature. In the example in fig. 2, the inside 21 of the combustion chamber 2 is divided into three zones 210, 211 , and 212 that are kept at different temperatures. It should be noted that the kiln's productivity depends on how many tubes are installed on the kiln, the tube length, its diameter, the tube's rotation speed and also the angle of the kiln. In fact, the greater the angle, the faster the material reaches the outlet.

What's more, since the diameter of each tube is small enough, it is reasonably easy to create a modified atmosphere in the interior 61 of each treatment chamber 6 that may be neutral, reducing, or oxidising. The production quality and quantity are therefore controlled by adjusting the aforementioned parameters.

The tubes 6 that constitute the treatment chambers are made to rotate, in the example, by means of a single electric motor 7 that, as seen in fig. 1 , is connected by means of a belt or chain to a sprocket wheel 65 being a solid part of tube 6. Another sprocket wheel 66 is mounted on the outside of the tube 6 connected to the motor and said sprocket wheel engages with an identical sprocket wheel 66 belonging to the adjacent tube. The same happens for the other 6 adjacent tubes. This shows how each tube 6 is rotated by the sprocket wheels engaging together. In this way a pair of adjacent sprocket wheels are rotated in opposite directions, which is totally unimportant since the direction of rotation of the tube has not influence whatsoever on the calcination treatment.

The tubes can also rotate in the same direction: in this case, not shown in the example, a closed loop chain connects all the sprocket wheels 66 in this case not engaging together. The tubes made of alumina, shown in the example, can also be produced in refractory steel or special steel suited to support the working temperatures required when calcinating inorganic pigments.

The kiln invention can also have several rows of tubes arranged on parallel planes or set in any way, all inside a single combustion chamber. It is evident that for calcinating inorganic pigments not requiring high temperatures, for instance 970-980°C it will be sufficient a normal type of steel and therefore an even cheaper steel tube, always available in the trade.

The versatility created by the number of treatment chambers, represented by the series of tubes, allows the kiln invention to satisfy customer requests in real-time without having to use back-up stocks. In this way stocks can be reduced to practically nil and a production based on requests in real-time can be offered.

Claims

1 ) Continuous rotary kiln (1 ) especially suited for the calcination of inorganic pigments including:

- a combustion chamber (2) developed mainly in a lengthways direction; - means (5) for heating said chamber according to temperature gradients variable along pre-set zones;

- a series of chambers suited for the calcination treatment; characterised in that said chambers consist of two or more tubes (6) set at the same angle placed inside the combustion chamber and which hold the material undergoing the treatment, said tubes being made to rotate synchronously by means of a drive transmission system.

2) Rotary kiln according to claim 1 ) characterised in that said tubes (6) are set in one or more parallel rows.

3) Rotary kiln according to claim 1 ) characterised in that said tubes (6) have a variable angle, lifting means of the combustion chamber (2) being provided at the inlet of said tubes.

4) Rotary kiln according to claim 3) characterised in that the lifting means are hydraulic jacks (3, 4) that act on the front of the combustion chamber. 5) Rotary kiln according to claim 1 ) characterised in that the rotary drive to the tubes is transmitted by a motor (7) to the first tube (6) directly connected to the others by means of sprocket wheels (66) engaging together, each of which is shrunk onto each tube, so that each tube rotates in the opposite direction to the adjacent tube. 6) Rotary kiln according to claim 1 ) characterised in that the rotary drive to the tubes is transmitted by a motor (7) to the first tube (6) connected to the others by means of sprocket wheels moved by a single closed loop chain on said sprocket wheels.

7) Rotary kiln according to claim 1 ) characterised in that the diameter of each tube is less or equal to 200 mm.

8) Kiln according to any of the previous claims characterised in that each tube is up to approx. 6 metres long.

9) Kiln according to any of the previous claims characterised in that each tube is made of refractory material. 10) Kiln according to claim 9) characterised in that the refractory material is alumina.

11 ) Kiln according to any of claims 1 ) to 8) characterised in that each tube is in refractory steel.

12) Kiln according to any of claims 1 ) to 8) characterised in that each tube is in special steel.