GB2234237A

GB2234237A - Production and storage of constituent materials for the manufacture of Portland Cements containing pfa (fly ash)

Info

Publication number: GB2234237A
Application number: GB8928615A
Authority: GB
Inventors: Philip Leonard Owens
Original assignee: EVANS NICHOLAS PAUL
Current assignee: EVANS NICHOLAS PAUL
Priority date: 1989-07-29
Filing date: 1989-12-19
Publication date: 1991-01-30
Anticipated expiration: 2009-12-19
Also published as: GB8928615D0; GB8917410D0; GB2234237B

Abstract

Constituent materials for use in the manufacture of Portland cements comprise selected pfa (fly ash) and selected gypsum, both obtained as by-products from thermal power stations, which are blended together in selected proportions and under controlled conditions, and the intimate mixture, either in dry powder or granular form, is stored as a bulk constituent ready for use in the manufacture of said cements.

Description

Title: Production and Storage of Constituent Materials for the Manufacture of Portland Cements containing pfa (fly ash).

DESCRIPTION Field of Invention This invention relates to the production and storage of constituent materials for the manufacture of Portland cements containing pfa (fly ash).

Background to the Invention In the mixing of concrete or mortar the setting and early rate of hardening of every type of Portland cement is conventionally controlled by gypsum. This control is achieved by intergrinding a predetermined proportion of gypsum with Portland cement clinker at the time of manufacture. Some types of Portland cement contain fly ash as a constituent to increase the strength and durability performance of concrete in which it is incorporated. Gypsum, therefore, is essential in the manufacture of Portland cement and fly ash is increasingly recognised as a valuable constituent for enhancing the performance of the finished product.

Environmental legislation for preventing atmospheric pollution involves not only prevention of particulate contamination by such by-product materials as fly ash but also the desulphurization of flue emissions from thermal power stations generating electricity. Flue emissions are predominantly complex mixtures of various gases arising from the combustion of fossil fuels, such as coals containing minor amounts of organicsulphur, which volatilises when fired to become sulphur dioxide (SO2), which when released into the atmosphere combines with the water vapour present and is later precipitated as acid rain.

LL1me (CaO) is produced from chalk or limestone (calcium carbonate CaCO3) which has been decarbonated by calcination at more than 8300C.

Lime and/or limestone can be used to desulphurize the flue gas emissions at thermal power stations by various methods. The type of equipment and its mode of operation depend on. the size of the power station, the larger favouring "gas scrubbing with mixtures of water and lime producing a slurry of calcium hydroxide, Ca(OH)2, which re acts with the SO2 in the gas to produce various forms of calcium sulphate (gypsum).

Fly ash, or pulverised fuel ash (pfa) is produced at power stations operating on pulverised coal, resulting from the presence of impurities in the coal.

In theory both the gypsum and the pfa (fly ash) produced as byproducts of the above-described processes could be used for the manufacture of Portland cement. However, neither of these constituent materials is in itself well suited to bulk storage and transport whilst awaiting use in cement manufacture.

The Invention According to the invention, pfa (fly ash) and gypsum are blended, either in powder form or with added water (if necessary), in predetermined proportions and under controlled conditions, and dry stored or formed into granules for use in the manufacture of Portland cements containing fly ash.

Explanation of the Invention As mentioned above, thermal power stations operating on pulverized coal as fuel have to remove solid particles of ash carried by the exhaust gases prior to gas scrubbing. The ash known as fly ash or pulverised-fuel ash (pfa) is present because of impurities in the coal. Where lignitic through to anthracitic coals are used the chemical composition of the ash is mainly present as silica (SiO2) and alumina (Al203). Where combustion temperatures exceed 125O0C, and fusion takes place the physical shape of the pfa (fly ash) particle is mainly spherical. Whereas at temperatures below 1250 C -and where fusion does not take place particles are more crystalline.

Removal of pfa from the exhaust gases before desulphurization is either by electrostatic precipitation or-bag house filtration where some of the SO2 condenses onto the surface of the pfa (fly ash) particles. Where the amount of impurities (ash) in the coal exceed 15% and the size of furnace is large, i.e. exceeds 100MW, the preferred method of pfa (fly ash) removal is by electrostatic precipitation. The amount of pfa (fly ash) removed from the gas stream is typically more than 98.5%. The quantity of ash, however, depends on impurity or ash content of the coal which is dependent on the source. Different sources known to be in the use, range in ash content from as little as 8% to more than 36%.

Typically 95% of pfa (fly ash) is found to have a particle size less than 200 um, of which on average about 65% is less than 45 pm. The burning efficiency of power stations is affected by many constraints but mainly by the demand for power, which in turn is affected not only by the time of year but also by the time of day. If the demand for electricity is steady, i.e. the variations are smaller than normal, then coals with lower sulphur may be selected. However, because sulphur is an aid to both combustion and precipitation, when power demand fluctuates greater than normal, coals of higher sulphur content can help to accommodate changes in furnace operations. Burning also depends on the volatility of the coal, which decreases as it ranges from lignitic through to anthracitic.A further major constraint is the prevention of chemical corrosion of the metal boiler tubes; hence burning conditions in furnaces are controlled to prevent oxidation.

Thus, most burning conditions in furnaces are managed for oxygen reduction, and this inevitably leaves an amount, normally less than 5% loss on ignition (LOI) mainly present as unburnt carbon (volatilisation has been completed) in the pfa (fly ash). LOI is used as an index and guide to furnace efficiency as related to the type of coal burnt-. The greater the amount of carbon in pfa (fly ash) the greater the effect on reducing the performance of air entraining agents when used in concrete. and mortars; therefore LOI either must be uniform or controlled, depending on the final cement to be produced.

Typically, as the coal rating increases from lignitic through to anthracitic, so the LOI in pfa (fly ash) increases typically from about 1 or 2% to up to 15%. Pfa (fly ash) with a defined LOI of less than 7% and with a fineness or sieve residue at 45 ,um of less than 12.5% is currently specified in British Standard (BS) 3892 Part 1 : 1982 "Pulverised-fuel ash for use as cementitious component in structural concrete'-'. Pfa (fly ash) of this quality can be used up to 35% by mass of cement used in structural concrete, provided all. other conditions are in compliance with BS 5328 : 1981 "Methods for specifying concrete, including ready mixed concrete" and BS 8110 "Structural use of concrete: Part 1 : 1985 Code of Practice for design and construction".

Alternatively, pfa (fly ash) with LOI less than 7% can be used to manufacture cements as defined in BS 6588 1985 "Specification for Portland pulverised-fuel ash cement" or BS 6610 : 1985 "Specification for pozzolanic cement with pulverised-fuel ash as the pozzolana".

Alternative methods are allowed in these specifications for the introduction of pfa (fly ash) of different fineness gradings during the manufacture of these cements. If the pfa (fly ash) is interground there is no restriction to the fineness requirement. However, if the cement is manufactured by dry blending the two components, that is, the Portland cement and pfa (fly ash), they must comply with BS 12 1989 "Specification for Portland cement", and BS 3892 Part 1 : 1982 respectively.

In both cements (BS 6588 and BS 6610) Portland cement clinker, as a calcined product from a rotary kiln, is interground with a proportion of gypsum, with the pfa (fly ash) being either added simultaneously at the mill or dry interblended after the Portland cement has been manufactured. Cement produced by dry interblending prepared Portland cement and pfa (fly ash) originating from the harder coals imparts a water reducing benefit in concrete and mortars because the finer spherical particle of pfa (fly ash) remain uncrushed, i.e. there is no pulverising action from milling.

Conversely, any pfa (fly ash) that is interground has increased chemical reactivity as the fractured surfaces exposed by milling are more available to the hydroxyl ions that enter the aqueous solution when mixed in concrete or mortar. However, milling preferentially reduces the "coke" carbon particles that are also present as LOI in pfa (fly ash), and this increases the influence that carbon has on i) air entraining agents due to "soap" flocculation and ii) colour, particularly as it affects the appearance of the finished colour.

Both these aspects are serious limitations to the use of pfa (fly ash) in cement production and the European CEN-prestandard pr ENV 197 Portland cements conditionally, at present, imposes arbitrary limits on pfa (fly ash) to an LOI of 5% maximum when incorporated as a main constituent. This constraint poses serious problems to both industries (power and cement) for, while the majority of pfa (fly ash) produced by power stations on average is significantly less than 5% LOI, there are occasions when LOI can increase to greater than 5%. This thus presents both industries with serious logistic problems which are mainly storage and transport orientated to handling pfa (fly ash) of the correct quality.

Obviously power stations that are designed to produce more than 3000 tonnes of pfa (fly ash) in any one day have to make capital provision for pfa (fly ash) dumping sites for the period of the power stations life which may extend more than 40 years. Therefore, there is very little incentive for adaptation of the handling and storage of bulk quantities of pfa (fly ash) if additional large capital investment is required to store the "correct" quality of pfa (fly ash). -An added disincentive, particularly in the United Kingdom, is that the majority of electrical power (and thus pfa (fly ash) is generated in the winter, whereas the majority of construction takes place in the summer, and therefore supply and demand cannot be reconciled on the basis of sufficient pfa (fly ash) of the correct quality being immediately available during the peak periods of construction.

The problem of storing the appropriate quality of pfa (fly ash) in an alternative form as well as a powder could in theory be achieved by producing free flowing pellets or granules that can be stock piled on open sites to await transportation, for instance in open bodied tipper lorries. In immediate-response to surges in demand th-is removes one of the principal restraints to the use of pfa (fly ash) as it would not be necessarily supplied in powdered form.

Major changes due to European legislation and the harmonisation of cement and concrete standards are planned well in advance of an anticipated 1992 implementation date BS 12 : 1989 "Specification for Portland cements", plus BS 6588 and BS 6610 are currently under revision by the British Standards*Institution to align them with prestandard ENV 197, which has been prepared during the past decade by CEN (Comite European de Normalisation or European Committee for Standardisation). Eventual adoption of the final draft pr EN(V) 197 "Cement: Composition, specification and conformity criteria" is anticipated before the Autumn of 1991 by all the eighteen participating countries to CEN.

In 1992 it will be mandatory to adopt this or a variation of this specification and all National standards for cement that are in conflict with the cements defined in ENV 197 will be withdrawn. At present cements described in ENV 197 are defined as types CEI, CEII, CEII-S, CEII-Z, CEII-C, CE-F, CEIII and CEIV, all of which permit the inclusion of fly ash. Cement types CEII, CEII-C and CEIV allow the incorporation of 0-23%, 6-28% and up to -40% fly ash, respectively, but the LOI of the fly ash should not exceed 5% by mass. The remaining cements allow up to 5% fly ash with no restriction on its LOI. However, for cement types CEl and CEIII the finished cement has a maximum LOI requirement of 5%, which includes the LOI of the fly ash.Further restrictions for finished cements include either a 3.5 or 4.0% maximum SO, content and, in the case of CEIV, the fly ash must satisfy the pozzolanicity test. It is the LOI limitation which can be regarded as restrictive to the wider adoption and use of pfa (fly ash) in powder form in cement manufacture. The present invention results from consideration of many of the technical and logistic problems that surround and prevent capitalisation, not only the availability at present of pfa (fly ash), but in prospect the industrial gypsum that will arise from the desulphurization of flue gases.As stated, the invention provides that, by combining the pfa (fly ash) and gypsum in appropriate proportions by dry blending and storing or preparation into a granular form, a suitable low cost storage and handling material can be prepared as the second bulk raw material for use in cement manufacture.

Further features of the Invention First, the raw materials will be briefly reviewed.

Power stations fitted with flue gas desulphurization (FGD) units will produce industrial gypsum as a by-product of electricity generation, resulting from the need to reduce the potentially harmful emissions from the exhaust gases. The gypsum will be produced in either slurry, damp (conditioned), or dry form from the FGD units creating an additional disposal problem to that already present, i.e. pfa (fly ash).

Pfa (fly ash) is in the UK usually collected in hoppers beneath each of three banks of electrostatic precipitators. It is usual practice for the pfa (fly ash) to be automatically and continuously removed from hoppers with no attempt being made to reconstitute the various fractions to the same grading as produced in the furnace. Also while LOI is used as a measure of burning efficiency it generally increases with particle size therefore coarser pfa's-(fly ashes) which are the majority precipitated from the exhaust gases tend to have higher LOI.

However, power stations that are operated under base load or steady state conditions, i.e. where the furnaces are fired at the theoretical rated capacity, invariably produce pfa (fly ash) with the least variation in LOI across the full range of particle sizes. Therefore provided the LOI of the coarsest fraction is below 5%, then any combination of the other fractions can be reconstituted to less than 5%, which would be in compliance with the current ENV 197 and its requirements for all cement types.

For the purpose of this invention, any pfa (fly ash) may be considered suitable, provided the LOI is controlled to the requirements of the finished cement. Also while it is not vitally important, the particle size distribution may need to be synthesized to obtain the most appropriate particle packing to accommodate leaner mixtures of the gypsum/pfa (fly ash) granulate, when applicable.

The present invention provides for the manufacture of a dry blended or granulate, of controlled composition and preparation, made of industrial gypsum, selected pfa (fly ash) as produced at coal fired power stations or similar and/or suitable additions.

The granulate form is of suitable strength and durability to sustain prolonged storage and handling so that it may be used as the secondary major bulk material in the manufacture of Portland cement as may be described in the next revision of BS 12, and the Portland fly ash (pfa) cements as currently described in BS 6588 or BS 6610 or as may be revised eventually to comply with EN 197.

The invention dispenses with the necessity to import separately, into the cement producing units, rock gypsum as a granulate and pfa (fly ash) as a powder, the prepared powder or granulate from this invention may be interground or blended on its own, with or without including the 1.0% additives permitted and described in ENV 197, with Portland cement clinker to produce a cement of required properties.

The invention is also concerned with: i) the selection, preparation, and/or. treatment of suitable dry, slurried or conditioned pfa (fly ash) of appropriate composition and physical properties; ii) proportioning the pfa (fly ash), as related to its chemical composition; iii) the selection, preparation and/or treatment of suitable industrial gypsum either in slurry, conditioned or dry form; iv) proportioning the gypsum as related to its chemical composition; v) receipt of a combination of pfa (fly ash) and gypsum direct from the power station in slurry, conditioned or dry form for further processing and/or treatment to form the dry blend powder or granulate;; vi) mixing the two main constituents pfa (fly ash) and gypsum in the form (detailed in i) and iii) above, or any combination of forms) with or without water and/or additive (if appropriate) and the preparation of a powdered or granular form of finished product.

The addition of a minor constituent can be made to assist the manufacture (if appropriate).

Thus, by mixing and homogenizing together the constituents as v) and vi) above, a material which is either mouldable and formable into granules that harden into a form suitable for bulk handling and storage, or can be stored in a dry blended powder form, is produced for use in the manufacture of Portland cements containing pfa (fly ash).

The intended use of the powder or granulate is for adding or grinding together with Portland cement clinker so that the finished product complies in every respect with EN 1-97 its revisions or adaptations for Portland, Portland fly ash or Pozzolanic cements containing fly ash as either a main or minor constituent.

Selection of the pfa (fly ash) is an important consideration. On power stations where there is sufficient pfa (fly ash) produced, and where flue gas desulphurization equipment is installed either on the power station in question or one reasonably adjacent to it, pfa (fly ash) is selected and homogenized so that: i) the loss in ignition complies with the requirement of ENV 197 or its revisions för the finished cement product; ii) the gradation of the pfa (fly ash) particles, are conducive to optimise the densification of particle packing; iii) the solubles and S03 contents can be determined.

Selection of the gypsum is also important. While existing power stations are having retrospective flue gas desulphurization (FGD) plant installed future pulverised-fuel thermal power stations will have constructed integrated FGD plant. The form in which the finished gypsum may be produced from any power station could be either slurried, conditioned or a dry particulate. In all cases any form can be-accommodated by this invention.The gypsum can either be reconstituted or homogenized so that i) the purity can be determined as SO3, for it is improbable that limestone or chalk used will be totally free from impurities such as silica (SiO2); ii) the form in which the gypsum is present if dihydrate, hemihydrate or anhydrite, which relates to the presence of water of crystallisation; iii) the fineness, if required, can be reduced or prepared so that it is compatible with the gradation of pfa (fly ash) particles to optimize densification and the use of the least amount of water for preparation of the composite into a mouldable granulate; iv) the inclusion of compatible powdered, liquid or water soluble additives is possible.

The preparation of the powder or granules is in a number of stages which depend on the chemical composition and form that both or either the pfa (fly ash) or gypsum are presented as raw materials, wet, conditioned, or dry. The proportions by mass on a dry basis are approximately between 1% and 90% pfa (fly ash) content in the final formulation which is constrained by the limits beyond which not more than'l0%~fracties shall deviate from the finished cements as detailed in ENV 197 or its revisions.

The stages involved in the process of preparing the granulate are as follows: i) treat and/or homogenize the constituent materials (gypsum, pfa (fly ash), additions and/or additives according to the conditions previously outlined, to give the appropriate powdered or granulate form required. The treatment of the constituents includes the drying of and/or heating to a temperature conducive to the formation of granules or aid to dry blending; ii) strictly monitor the chemical composition of both components and proportion to the requirements of the required finished cement.

These proportions may have to change from time to time as the composition of the constituents vary; iii) blend the main constituents with sufficient water and/or minor constituents plus additives if used or required and mix to the required consistency for forming granules. This may be one of many means, appropriate to the ability of the constituents to form into "green granules". The granules may be produced by any means such as inclined pelletizer, screw feeder or auger mixer feeding an extruder fitted with appropriate sized dies and a means of cutting the "green" granules to size, or "moulded" in appropriately sized and shaped moulds; iv) the "green" granules can then be air, partially or "roast" dried to the most appropriate granulate form for cement manufacture.

Then depending on the finished granulate' size, strength, remaining evaporable- water content and the resistance to weathering, the finished material is stored or stock piled to await transportation to the cement works; v) blend and homogenize the main constituents and/or minor constituents plus additives, if required to form an intimate mixture of the constituent prior to dry storage in silos or similar. Alternatively the intimate mixture may be densified (compacted) by silo aeration to form agglomerated particles.

Claims

1. A method of preparing constituent materials for use in the manufacture of Portland cements containing pfa (fly ash), according to which pfa (fly ash) and gypsum are blended, either in powder form or if necessary with added water, in predetermined proportions and under controlled conditions, and the pfa (fly ash)/gypsum blend is dry stored or formed into granules for use in the manufacture of the said cements.

2. A method as claimed in claim 1, according to which both the pfa (fly ash) and the gypsum are initially obtained as by-products of power stations.

3. A method as claimed in claim 2, wherein the pfa (fly ash) is obtained by removal of particulate solids from the exhaust gas emissions of coal-operating power stations and the gypsum is obtained by scrubbing the exhaust gas emissions of fossil fuel-burning power stations.

4. A method as claimed in any of claims 1 to 3, according to which, prior to blending with the gypsum, the pfa (fly ash) is selected in amount and quality for one of: the manufacture of Portland cements containing up to 40 per cent pfa (fly ash) with a loss-on-ignition index (LOI) not exceeding 5 per cent by mass; the manufacture of Portland cements containing not more than 5 per cent pfa (fly ash) with an unrestricted LOI; and the manufacture of Portland cements with an LOI not exceeding 5 per cent inclusive of the LOI of the incorporated pfa (fly ash).

5. A method as claimed in any of claims 1 to 4, according to which, prior to blending, the relative proportions of the pfa (fly ash) and gypsum to be blended are selected in accordance with the physical and chemical compositions thereof having regard to physical and chemical requirements for Portland cement subsequently to be manufactured.

6. A method as claimed in any of claims 1 to 5, wherein the pfa (fly ash) is blended with gypsum in slurry or dry form, with or without pre-conditioning or pre-treatment of said pfa (fly ash) or gypsum.

7. A method as claimed in any of claims 1 to 6, wherein the pfa (fly ash) and gypsum are blended together with the addition of a minor constituent to assist manufacture.

8. A method as claimed in any of claims 1 to 7, according to which, prior to blending, the pfa (fly ash) particulate is graded to optimise densification of particle packing.

9. A method as claimed in any of claims 1 to 8, according to which, prior to blending, the solubles and SO3 contents of the pfa (fly ash) are determined.

10. A method as claimed in any of claims 1 to 9, according to which, prior to blending, the gypsum is tested and/or treated and/or prepared for SO3 content, the presence of water or crystallisation, and particulate fineness.

11. A method as claimed in any of claims 1 to 10, according to which the pfa (fly ash) content in the finished pfa (fly ash)/gypsum blend lies In the range 1 to 90 per cent.

12. A method as claimed in claim 11, according to which, prior to blending, either one or both of the pfa (fly ash) and gypsum are treated by drying and/or heating.

13. A method as claimed in any of claims 1 to 12, according to which the blend is formed to a consistency suitable for forming "green granules" which are subsequently dried ready for storage and/or transportation.

14. A method as claimed in any of claims 1 to 12, according to which, after blending, the mixture is densified by silo aeration.

15. A method of preparing constituent materials for the manufacture of Portland cements substantially as hereinbefore described.

16. A pfa (fly ash)/gypsum blend constituting a bulk constituent for the manufacture of Portland cements, produced by the method of any of claims 1 to 15.

17. A bulk constituent for the manufacture of Portland cements which comprises an intimate mixture of pfa (fly ash) and gypsum which have been blended in predetermined proportions and under controlled conditions.

18. A bulk constituent according to claim 17, wherein the intimate mixture is in the form of granules.