GB2056640A - Heat treatment of uncalcined, carbon-containing fireclay - Google Patents

Abstract

Particles of uncalcined, carbon- containing fireclay are heat treated in a kiln through which an air draft is maintained. The air draft or the kiln atmosphere is enriched in oxygen by adding to it a gas containing more oxygen than air. This makes possible maintenance of an oxidising atmosphere in the kiln thereby ensuring complete oxidisation of the carbon. The kiln may be a vertical shaft (as shown) or a rotary kiln. The vertical shaft kiln 2 has three sets 4, 6, 8 of burners used to fire the raw fireclay. Air is supplied through calcined fireclay outlet 10 and ports (not shown). The raw fireclay is fed into the top of the kiln 2 from a hopper 16. Heat treated fireclay is continuously discharged from the outlet at the bottom of the kiln so that there is a progressive movement of the charge down the kiln. Each set of burners raises the temperature of the fireclay. Pure oxygen is introduced into the kiln through lances 12 located just above the intermediate set 6 of burners. <IMAGE>

Description

SPECIFICATION Heat treatment method This invention relates to a heat treatment method. In particular it relates to a method of heat treating particulate material to yield a product comprising particles of calcined fireclay.

It is known to produce a fireclay product, eg refractory bricks, from a starting material consisting of scrap material and a precalcined raw material. The scrap material may be taken from one or more of a number of sources. For example, in the production of fireclay bricks, some bricks may be physically damaged and others may be unsuitable or undesirable. In another example, bricks which may have been worn or damaged in use, for example in the lining of a furnace, may be used as the source of scrap.

This scrap (known as "grog") is mixed with newly calcined fireclay in the desired proportions. It is ground up, and used together with fresh clay in the production of products such as firebricks and refractory bricks.

The newly calcined fireclay is produced from raw fireclay by a heat treatment method. In the heat treatment, the raw fireclay is fired so as to bring about two major changes which take place at progressively higher temperatures. First, moisture and water of crystallisation are driven off. Second, carbon contained in the raw fireclay is oxidised. At this stage also other undesirable constituents may be driven off including organic and volatile materials and sulphur. In addition any ferrous oxide contained in the material would be oxidised to ferric oxide.

These changes are brought about by firing the mixture of raw fire clay and calcined fireclay.

It has been found that certain problems arise in maintaining an adequate oxidising atmosphere during this heat treatment method. If the atmosphere becomes insufficiently oxidising there tends to be incomplete oxidation of carbon which results in a deterioration in the quality of the finished product.

An insufficiently oxidising atmosphere may also limit the rate of production and or increase the fuel requirement of the kiln.

The invention aims at providing a method of improving the above mentioned heat treatment method.

Accordingly, the invention provides a method of heat treating particles of uncalcined carboncontaining fireclay by firing the particles in a kiln.

wherein an air draft is maintained through the kiln and the air draft (or kiln atmosphere) enriched in oxygen by adding to it a gas containing more oxygen than air lin or upstream of) a region where carbon in the raw fire clay is oxidised.

The method according to the present invention is particularly applicable to shaft kilns.

It has been found that the preferred means of adding the enriching gas (ie. the gas containing more oxygen than air) depends upon the design of the shaft kiln and the nature of the fireclay being treated. For a fireclay having a relatively high vitrification temperature, say 1 300'C or above, it is preferred to add the enriching gas at or near to the base of the kiln where the temperatures are high and the rate of carbon and organic impurity removal is at its highest. In this case the enriching gas may be added to the air draft as it enters the base of the kiln; this draft may be forced or natural, or in cases where it is present, added to the forced cooling air at the kiln base.

lfthefireclay has a lower vitrification temperature the enriching gas is preferably injected directly into a region nearer the top of the kiln. For example, the enriching gas may be added through lances situated above burners which raise the temperature of the fireclay sufficiently for oxidation of carbon to be initiated, or it may be added to air entering the kiln through excess air ports in such burners or it may be added through such burners.

The method according to the invention may also be performed on a rotary kiln. In such a kiln the enriching gas may be added to the air draft as it enters the kiln. Alternatively, it may be introduced into the kiln atmosphere through one or more lances below the flame emanating from the burner or one or more of the burners used to fire the fireclay.

Typically, the or each lance is mounted below and a small distance (eg. 10cm) to one side of its associated burner. Here each lance is preferably water-cooled.

The enriching gas is preferably substantially pure oxygen. Alternatively, it may be oxygen-enriched air.

If the source of enriching gas is substantially pure oxygen, it may typically be supplied to the kiln at a flow rate in the order of 50 to 100 or 50 to 150 cubic meters per hour.

We believe the method according to the invention makes possible the achieven7ent of one or more of the following advantages. First, by increasing the rate of oxidation of carbon it is possible to increase the rate of production of the kiln since it is often this phase of the heat treatment cycle which is a constraint to increased production. Second, as a result of such reduction in the residence time of the material in the kiln, it is possible to make a consequential reduction in the total fuel consumption per unit weight of material produced. This reduction in specific fuel consumption results from the increased production rate, the reduction of the kiln draft which is normally above the preferred rate to provide oxygen for carbon removal, or both.

Third, it is possible to ensure that there is no residual carbon and other oxidisable materials in the final product. Thisis sometimes a problem with existing methods, which leads to a substantially reduced quality of the final product.

The calcined fireclay produced as a product of the method according to the invention may be used in the manufacture of bricks or other fireclay products and is included within the scope of this invention.

The method according to the invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 7 its a schematic sectional view of a vertical shaft kiln adapted to the method according to the invention: Figure 2 is a cross-section taken through the line A-A Figure 1; Figure 3 is a schematic drawing illustrating a rotary kiln adapted to perform the method according to the invention, and Figure 4 is a schematic cross-section through the line B-B in Figure 3.

Referring to Figures 1 and 2 of the drawings, the illustrated vertical shaft kiln hasthree sets of burners, 4, 6 and 8, each set being at a different height.

The burners of set 8 are shown in Figure 2, and it will be seen there are four burners iri the set, the burners being equally spaced circumferentially. Analogously there are four burners in each of the sets 6 and 4.

The kiln 2 has a restricted outlet 10 at the bottom through which calcined fireclay leaves the kiln and through which a draft of excess air enters it This draft may be natural, forced or induced. Air may also be added through ports (not shown) intermediate of the burners 8 and the outlet 10. This air helps to prevent an excessive temperature being created in the bottom ofthe kiln.

Located above each burner in the set 6 are lances 12 connected to a source of pure oxygen (the source is not shown). Typically, these lances are provided through existing and viewing or rodding ports immediately above the burners. Although not shown, each burner in each set will be associated with an excess air port through which air can be drawn into the kiln so astro supplementtheair entering through the outlet for the calcined fireclay.

The kiln 2 has an open top 14. Particles of raw fireclay are fed into the top of the kiln 2 from a hopper 16 located thereabove. If desired, the particles can be ground to a suitable size in a bali mill or like machine before being loaded into the hopper 16.

In operation,the-fireclay progressively descends the shaft of the kiln, being raised in temperature by the burners as it falls. The upper burners raise the temperature of the material passing through the shaft kiln under gravity to a temperature of 450'C which is sufficient to drive off any moisture and water of crystallisation. The second set of burners 6 raise the temperature of the particulate material to 950 C. At this temperature burn-out or oxidation of carbon from the rawfireclayhas normally been completed. However, in some cases the heat treatment of the material is not completed until substan- tially higher temperatures are attained.In these instances a third set of burners are used to raise the temperature of the particulate material to about 11 50"C, so as to effect completion of the burning out of carbon or other oxidisable materials.

The introduction of oxygen into the kiln through the lances 12 helps to increasethe local concentra- tion of oxygen in the airdraft passing.through the material in the kiln and therebyincreasesthe rate of carbon burn-out.

Typically, the burners may be fuelled by oil ar natural gas. The kiln may be about 25 metres high with an external diameter of 2.5 metres. The total heat input from the burners may be arranged to be 44therms per tonne of charge.

Referring to Figures3 and 4 of the drawings, the illustrated rotary kiln has a cylindrical chamber 32 mounted for rotation. The axis of the chamber 32 is inclined at a small angle to the horizontal. In operation, the charge of particles of raw fireclay is fed into the upper end of the kiln. Rotation of the kiln causes the particulate material to tumble under gravity until it falls out of the lower end. The angle of inclination of the chamber 32 and the speed of -rotation may be chosen so as to provide a suitable residence time for the particulate material in the kiln.

At its outlet end, the kiln is fitted with a hodd 34 which is not mounted for rotation but which is fitted with a burner 36. An air draft into the chamber 32 is induced between its lower end and hood 34. The burner 36 is coaxial with chamber 32 for a considerable-length as shown in Figure 3 of the drawings.

Located below the burner 36 and a few centimetres to one side thereof is a water cooled oxygen lance 38. In operation the lance directs a jet of pure oxygen below the burner flame.

In operation, the charge is fed into the upper end of the chamber 32, and chamber 32 is rotated at chosen speed. The burner 36 is lit and oxygen jetted into the chamber 32 through the lance 38.

In operation, the charge of particulate material entering the chamber 32 at its upper end gradually travels downwards under gravity along the length of the chamber to its outlet. The heat generated by the burnerenables moisture and water during its crys tallisation to be driven off from the raw fireclay, carbon to be burned out of the raw fireclay and, vftrifieation of the raw fireclay to take place. Introduction of the oxygen in addition to the airdraft helps to increase the concentration of oxygen in the kiln atmosphere and thereby to accelerate the rate of carbon and other organic material burn out. If desired, the oxygen may be directed at a part of the interior of the kiln chamber 32 immediately above a region where carbon burn out takes place.

The invention is further illustrated by the following examples.

Example 1 A shaft kiln 55 metres high and 2.5 metres in external diameter was fed with raw fireclay. The burners were fired with natural gas, and supplied heat at the rate of 45 therms per tonne of product.

The rate of charging the kiln was 2.6 tonnes per hour over a twelve hour period, charging being discontinued overnight. The kiln was provided with three sets offourburners, located atheights of approxi-4 mately 15m, 25m, and 35m from the base of the shaft.

Oxygen was supplied continuously at a rate of 140 m3/hrthrough lances located 25 cm above the lower set of burners. No additional fuel was added. The charge-to the kiln was increased to 2.9-tonnes/hr, an increase of 11.5% with a decrease in specific fuel consumption to 41 therms per tonne or 9.75%. No traces of residual carbon or organic material were discernible in the material after heat treatment.

Example 2 The procedure of Example 1 was followed except that oxygen was added at a rate of 120 m3/hr to the cooling air supply at the base of the kiln intermediate to the outlet 10 and the burners 8. The charge was increased to 3.0 tonnes/hour and the specific fuel consumption reduced to 39 therms per tonne. Again no residual carbon or organic material was present in the final product.

Claims (2)

1. A method of heat treating particles of uncalcined carbon-containing fireclay by firing the particles in a kiln, wherein an air draft is maintained through the kiln and the air draft is enriched in oxygen by adding to it a gas whqse concentration of oxygen is greater than that of air in or upstream of a region where carbon in the raw fireclay is oxidised.

1. A method of heat treating particles of uncalcined carbon-containing fireclay by firing the particles in a kiln, wherein an air draft is maintained through the kiln and the air draft is enriched in oxygen by adding to it a gas containing more oxygen than air in or upstream of a region where carbon in the raw fireclay is oxidised.

2. A method as claimed in claim 1, in which the kiln is a vertical shaft kiln.

3. A method as claimed in claim 2, in which the enriching gas is added to the air draft at or near the base of the kiln.

4. A method as claimed in claim 2, in which the enriching gas is added to the air draft through lances situated above burners which raise the temperature of the fireclay sufficiently for oxidation of carbon to be initiated.

5. A method as claimed in claim 2, in which the enriching gas is supplied through at least one of a number of burners which raise the temperature of the fireclay sufficiently for oxidation of carbon to be initiated.

6. A method as claimed in claim 1, in which the -kiln is a rotary kiln.

7. A method as claimed in claim 6, in which the enriching gas is added to the air draft as it enters the kiln.

8. A method as claimed in claim 6, in which the enriching gas is added to the air draft through one or more lances at a region below the flame emanating from the burner or one of the burners usd to fire the fireclay.

9. A method as claimed in any one of the preceding claims, in which the enriching gas is.

substantially pure oxygen.

10. A method of heat treating particles of uncalcined carbon-containing fireclay substantially as described herein with reference to Figure 1 or Figure 2 of the accompanying drawings.

11. A method of heat treating particles of uncalcined carbon-containing fireclay substantially as described in Example 1 or Example

2.

12. Fireclaywhich has been heat treated buy a method as claimed in any one of the preceding claims, or an article made from such fireclay.

New claims or amendments to claims filed on 9th December 1980 Superseded claims 1 New or amended claims: