WO1994014719A1

WO1994014719A1 - A process for the manufacture of cement and use of used flue gas cleaning mass containing calciumfluoride

Info

Publication number: WO1994014719A1
Application number: PCT/DK1993/000433
Authority: WO
Inventors: Niels Lang Mathiesen; Hans Erik Borgholm
Original assignee: Niels Lang Mathiesen; Hans Erik Borgholm
Priority date: 1992-12-23
Filing date: 1993-12-21
Publication date: 1994-07-07
Also published as: DK155892A; DK155892D0; AU5809294A; DK170877B1

Abstract

A process for the manufacture of cement, in which process a raw material comprising finely ground lime, sand or clay or raw materials having a similar composition and a content or admixture of flux containing ferric oxides or calcium fluoride is burnt in a rotating high-pressure furnace into cement clinkers, which are re-made into cement powder. With a view to utilizing a most economic source for producing calcium fluoride, the process is characteristic in that at least part of the fluxing agent is a calcium-based, used flue gas purification mass derived from purification of a fluorine-containing flue gas originating from a heating or acidic composition process based on fluorine-containing minerals. The other substances present in form of unreacted basic mass as well as calcium sulphate, calcium sulphite and calcium chloride, silicon oxide, and process dust follow as common raw materials. Thus used flue gas purification mass derived from purification of fluorine-containing flue gas is used as combined calcium admixture and fluxing agent source in the raw material in the production of white Portland cement.

Description

A PROCESS FOR THE MANUFACTURE OF CEMENT AND USE OF USED FLUE GAS CLEANING MASS CONTAINING CALCIUMFLUORIDE

The present invention relates to a process for the manufacture of cement, in which process a raw material com¬ prising finely ground lime, sand or clay or raw materials having a similar composition and a content or admixture of fluxing agents containing ferric oxide or calcium fluoride is burnt in a rotating high-pressure furnace into cement clinkers, which are re-made into cement powder. The inven¬ tion also relates to the use of used flue gas purification mass for the purification of fluorine-containing flue gas from a heating or acidic decomposition process.

In the manufacture^'of white Portland cement which is based on raw materials without iron, calcium fluoride can be used as fluxing agent in replacement of the iron flux which is normally used in Portland cement, but which is the main cause for the grey colour of the cement. Calcium fluoride melts at 1423°C and does not release gaseous fluorine com- pounds. In a Portland process calcium fluoride can be used with calcium carbonate and calcium sulphate present therein, as these substances already are substantial components of the raw material used in the process. Calcium fluoride is available as deposits in the nature and can be won by min- ing. Direct purchase of calcium fluoride is too expensive in the manufacture of cement.

The present invention provides a hitherto unused source for procuring calcium fluoride-containing material to be used in the manufacture of cement, in which the raw mate- rials are present in a favourable form, in which they can straight away take part in the preparation of the raw mate¬ rial, and in which the other substances present also are valuable as raw materials for the cement production.

Quite a number of mineral raw materials contains fluorine compounds which escape as gas during production ^' processes, in which these mineral products are subjected to a thermal influence, for instance clay burning into tiles and ceramic products or melting of minerals to glass, or clinkers - or acidic decomposition - for instance in the production of superphosphate and phosphoric acid.

Fluorine escapes as hydrogen fluoride HF or as sili¬ con fluoride SiF.. In many countries a scrubber plant is re- quired or dry gas filters for intercepting these fluorine compounds when letting out flue gases from these produc¬ tions, also in cases where the fluorine compounds are pre¬ sent in low concentrations like for instance in flue gas from tile burning. In such air purification plants lime, CaCO-, burnt lime, CaO, or hydrate lime, Ca(OH) are the compounds most often used as basic substance for reaction with the hydrogen fluoride. These reactions are:

(1) CaC0₃ + 2HF —> CaF₂ + C0₂ + H₂0

(2) Ca(OH)₂ + 2HF --> CaF₂ + H₂0

(3) SiF₄ + 2Ca(0H)₂ —> Si0₂ + CaF₂ + 2H₂0

Other acidic gases liberated in the abovementioned types of processes are hydrogen chloride, HCl, sulphur dioxide, SO and sulphur trioxide SO . These acidic gases are also re¬ tained in the same lime filters in form of calcium salts.

Gas purification is normally performed by letting the gas pass through layers of porous lime stone. Depending on the concentration, in which hydrogen fluoride, hydrochloric acid, S0₃ and SO are present in the flue gas, and a number of other process and lime properties, a varying part of the calcium carbonate is decomposed. Some processes use a socalled peeling of the outer layer of the lime granulate, in which the most heavy congestion of the pores are present, and re-use the peeled off granulate for attaining a high utilization of the lime.

The flue gas purification mass used has hitherto been deposited as waste, for instance in mine galleries and in controlled dumping grounds, as it is considered detrimental to the environment on account of the substances contained therein, in particular the salts which can be washed out, CaCl and CaSO.. According to the invention the used flue gas purification mass is utilized in the manufacture of cement, in which both the content of fluorine and sulphur compounds as well as non-reacted calcium carbonate can be utilized. The process for the manufacture of cement is characterstic by the subject matter of the characterizing clause of claim 1.

By utilizing flue gas purification mass from the tile industry, a considerable waste problem is solved for this industry and at the same time an effective, alternative cal¬ cium fluoride source is provided for the cement production. This source is economically so advantageous that it may even replace part of the ferric oxide which constitutes the major part of the addition of fluxing agent which is used in the manufacture of the common, grey Portland cement.

The used flue gas purification masses come out partly as meal from plants using peeling, partly as granulate. The granulate is with a view to functioning effectively at the purification of the flue gas high-porous, and it is therefore easy to grind prior to the suspension or the admixture to dry raw meal depending on the process used for the preparation of the raw material for the clinker burning furnace.

In cement products of the Portland type hydrated cal- cium sulphate, CaSO , is a raw material and consequently not an undesired substance present, but immediately usable.

Calcium sulphite, CaSO , is not a harmful substance in the small amounts in question here, and it will besides be possible for part of it to be oxidized during the clinker burning. According to an aspect of the invention the flue gas purification mass with the additional substances present therein is used as it comes out.

Calcium chloride is an undesirable substance, as the chlorine compounds may accumulate in the tube furnace on ac- count of volatility conditions. The cement process is not impeded by the chloride amounts normally present in flue gas purification masses from the tile industriy and the ceramic industry. If the amount of CaCl is higher than desired - for instance because others of the cement raw materials also contain chlorides - it is easy to wash out the purification masses prior to the admixture to the raw meal or the suspension in the clinker furnace, as CaCl^ easily dissolves and the other compounds present have low solubility products and consequently very low solubility in Ca containing, aqueous media.

Other substances which may be present in the used flue gas purification masses are ash particles from a possible direct firing, for instance coal ash or product dust, for instance clay dust. Also these substances are natural raw materials in cement.

Claims

C L A I M S

1. A process for the manufacture of cement, in which process a raw material comprising finely ground lime, sand or clay or raw materials having a similar composition and a content or admixture of fluxing agents containing ferric oxides or calcium fluoride is burnt in a rotating high-pres¬ sure furnace into cement clinkers, which are re-made into cement powder, c h a r a c t e r i z e d in that at least part of the fluxing agent is a calcium-based, used flue gas purification mass derived from purification of a fluorine- containing flue gas originating from a heating or acidic decomposition process of fluorine-containing minerals.

2. A process according to claim 1, c h a r a c t e r- i z e d in that the fluorine-containing flue gas together with simultaneously formed salts from other acidic residues, in particular sulphate, sulphite, chloride, and silicon oxide, originates from a direct-fired tile industry or cera¬ mic industry.

3. A process according to claims 1 or 2, c h a r a c t¬ e r i z e d in that the flourine compounds of the flue gas are absorbed on or have reacted with a flue gas purification mass consisting substantially of one or more of the com- pounds: CaCO 3, CaO or Ca(OH)2.

4. A process according to any of the claims 1 - 3, c h a r a c t e r i z e d in that the purification mass is used with the other substances present therein as available after use.

5. A process according to claims 1, 2, or 3, c h a- r a c t e r i z e d in that the flue gas purification mass before being admixed to the cement raw materials is washed out for removal of chlorides.

6. Use of used flue gas purification mass derived from purification of a fluorine-containing flue gas as a combined calcium and fluxing agent source in the raw material in the manufacture of Portland cement.

7. Use according to claim 6, c h a r a c t e r i z e d in that the manufactured cement is white Portland cement.