WO1997012842A1

WO1997012842A1 - Reduction of hazardous emissions during firing

Info

Publication number: WO1997012842A1
Application number: PCT/GB1996/002386
Authority: WO
Inventors: John Stephen Bratt; John Joseph Cooper
Original assignee: Jesse Shirley & Son Limited
Priority date: 1995-09-30
Filing date: 1996-09-27
Publication date: 1997-04-10
Also published as: EP0853598A1; GB9520007D0

Abstract

A method of reducing hazardous emissions during firing, and particularly a method of reducing fluoride emissions during firing of bricks. The method comprises adding to the material to be fired a substance which is apatitic or phosphatic, and particularly calcium hydroxyapatite.

Description

REDUCTION OF HAZARDOUS EMISSIONS DURING FIRING

This invention concerns a method of reducing hazardous emissions during firing, and particularly but not exclusively a method of reducing fluoride emissions during firing, and especially when firing bricks.

Many countries now have regulatory requirements for industry to limit the emissions of Hazardous Air Pollutants (HAP). One HAP which is of particular concern in many industries, including the ceramic industry, is fluorine as hydrogen fluoride (HF). HF is a toxic, acid gas which can inflict damage on both plant and animal life, and is considered to be a particularly damaging emission from the point of view of its environmental impact.

In for example the firing of bricks, the source of the fluoride emissions is the presence of fluorine substituted for hydroxyl groups in some hydrous minerals, such as the clays and the micas. In addition, some materials used in the heavy clay and whitewares industries contain fluorspar CaF. as an impurity mineral. Cornish stone falls into this group even in its beneficiated form.

The amount of fluorine in the products which is evolved as hydrogen fluoride depends upon a number of factors including the initial quantity present and its site, the firing regime and the water vapour content of the kiln atmosphere. Fluoride emissions generally increase with increasing firing temperatures above about 700X and increasing firing times.

The evolved HF is very reactive and other fluorides such as silicon tetrafluoride and fluorosilicic acid can also be formed. The effects of fluoride emissions on plant and animal life have been documented and include fluorosis in cattle grazing on pastures near to brick producing plants and damage to plants including vines.

Various options exist for the control and reduction of fluoride emissions and these include:- a) Dry scrubbers utilizing limestone (CaCO.) beds. b) Hydrated lime (Ca(OH)_) iiyection. c) Wet scrubbing with alkali.

All of these options have the disadvantages of high capital costs, appreciable running and maintenance costs and the problem of disposal of fluoride contaminated waste.

According to the present invention there is provided a method of reducing hazardous emissions during firing, the method comprising, prior to firing, adding to a material to be fired a substance which is apatitic or phosphatic.

Preferably less than 5%, and desirably 296, by weight of the substance is added to the material.

The substance preferably contains calcium hydroxyapatite, which desirably has the formula:-

Ca_ιo(PO₄)₆(OH).

The calcium hydroxyapatite is preferably derived from any of a phosphate rock, bone ash, bone char, or other bone containing materials such as bone meal. Alternatively synthetic calcium hydroxyapatite may be used.

The substance is preferably in the form of a finely divided powder.

The substance preferably also comprises a diluent, which desirably comprises a naturaUy occurring mineral component of heavy clay or whiteware bodies, such as for example silica sand, clays or feldspars.

The invention also provides a method of reducing fluoride emissions during firing, the method being according to any of the preceding six paragraphs. The invention further provides a method of firing bricks, the method being according to any of said preceding six paragraphs.

An embodiment of the present invention will now be described by way of example only.

A 196 weight addition of a 300's mesh bone ash was made to the raw materials of a brick body. The bricks were fired in a natural gas intermittent kiln according to the following schedule:-

Ambient to 600°C in approximately 18 hours

600X to 900°C at 20°C per hour

5 hours dwell at 900°C

900 - 1015^*C at 20'C per hour

5 hours dwell at 1015°C

Sampling for fluoride emissions was performed according to standard practice by passing measured volumes of flue gases through bubblers containing a 296 sodium hydroxide solution and then analysing for fluoride content by the use of an Ion-Selective Electrode. The results obtained, normahsed to 1896 oxygen and compared to a similar firing with no additions, were as follows:-

Timp Wpighted Average Peak

No addition 51.0 mg/m³ HF 257 mg/m³ HF

196 bone ash addition 26.1 mg/m³ HF 87 mg/m³ HF

The appearance and properties of the bricks were unaffected by the additive.

Figs. 1 and 2 show graphs illustrating the time weighted average results, with Fig. 1 showing no addition, and Fig. 2 with a 196 bone ash addition. In the graphs the x-axis shows time in hours from the start of monitoring. The left hand y-axis shows temperature (°C) whilst the right hand y-axis shows the fluoride emissions in mg/m³ HF. The open diamonds show kiln temperature whilst the filled in diamonds show fluoride emission at 1896 oxygen. The open circles show the PG3/2 (91) limit, and the hnes show the time weighted average. This limit refers to the limit of hydrogen fluoride permissible according to the U.K. Secretary of State's Guidance Note "Manufacture of Heavy Clay Goods and Refractory Goods (PG3/2(91))^*'.

During firing of the product, the apatitic material component in the body reacts with and immobilises some of the evolved fluorine according to the following reaction to give fluorapatite (FAP):-

Ca₁₀(PO₄ )₆(OH)₂ + 2HF

There is thus described a method of reducing hazardous emissions, and particularly, fluoride emissions, during firing. This method is particularly suited for use in the brick industry. The method does not affect the resultant bricks produced. Moreover, this method does not require any expensive equipment such as previous arrangements, and which also require subsequent maintenance. Furthermore, this method does not involve the problem of disposing of fluoride contaminated waste. If the substance comprises bone char or bone meal, this comprises in addition to hydroxyapatite, carbon or carbonaceous matter. This matter has a calorific value which can help in fuelling the firing of the bricks.

Various modifications may be made without departing from the scope of the invention. For example, other apatitic or phosphatic materials could be used, and in different quantities. A diluent could be provided in the material to aid uniform and consistent dispersion of the apatitic component in the material being fired. The diluent material may comprise a naturally occurring mineral component of heavy clay or whitewares bodies, such as silica sand, clay or feldspar. If hydroxyapatite is used this could alternatively be obtained from an appropriate phosphate rock, bone char or other bone containing materials such as bone meal.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

Qa_ms:

1. A method of reducing hazardous emissions during firing, characterised in that the method comprises, prior to firing, adding to a material to be fired a substance which is apatitic or phosphatic.

2. A method according to claim 1, characterised in that less than 596 by weight of the substance is added to the material.

3. A method according to claim 2, characterised in that 296 by weight of the substance is added to the material.

4. A method according to any of claims 1 to 3, characterised in that the substance contains calcium hydroxyapatite.

5. A method according to claim 4, characterised in that the calcium hydroxyapatite has the formula:-

Ca_ιo(PO₄)₆(OH)₂

6. A method according to claims 4 or 5, characterised in that the calcium hydroxyapatite is derived from any of a phosphate rock, bone ash, bone char, or other bone containing materials such as bone meal.

7. A method according to claims 4 or 5, characterised in that synthetic calcium hydroxyapatite is used.

8. A method according to any of the preceding claims, characterised in that the substance is in the form of a finely divided powder.

9. A method according to any of the preceding claims, characterised in that the substance comprises a diluent.

10. A method according to claim 9, characterised in that the diluent comprises a naturaUy occurring mineral component of heavy clay or whiteware bodies, such as for example silica sand, clays or feldspars.

11. A method of reducing fluoride emissions during firing, characterised in that the method is according to any of the preceding claims.

12. A method of reducing fluoride emissions during the firing of bricks characterised in that the method is according to any of claims 1 to 10.