CA1146983A - Process for producing portland and other hydraulic cements - Google Patents

Abstract

ABSTRACT
Portland and other hydraulic cements are produced by introducing nodular, pulverulent, or molten feed materials, properly proportioned, into a melt that is contained within an electric furnace. The furnace may be arc, induction, resistance or other type using electric current as the source of energy. This current not only heats the melt, but further serves to produce turbulence which mixes the feed materials within the melt so that those feed materials chemically combine within the melt, providing the melt with the desired chemical constituency. The size, form of electrical energy input, and other design features of the furnace are related to required feed rate and the material melting rate. The melt is thereupon withdrawn and cooled to solidify it into a substance that has the chemical constituency and properties of the desired hydraulic cement.

Description

This invention relates to portland and other hydraulic cements, and more particularly to a process for the making of such cements.
The hydraulic cements represent an important group of cementing materials which are used principally in the con-struction industry. These cements have the special property of setting and hardening under water. The essential compon-ents of the cements are lime (CaO), silica (SiO2) and components derived from them. In the presence of water, ~hese components react to form, ultimately, a hardened product con-taining hydrated calcium silicate. The hydraulic cements include portland cement as well as high alumina cement, hydraulic lime, and other lesser known cements.
Of all the hydraullc cements, portland cement is by far the most important, fox this cement is a major construction material that is utilized in practically all concrete as well as in most of the masonry mortars. The principal components of portland cement are tricalcium silicate (3CaO-SiO2), ; dicalcium silicate (2CaO.SiO2), and tricalcium aluminate (3CaO-A12 O3), all of which, when in a ground or powdered condition, will react with water to form a hard, stone-like substance held together with intermeshed crystaLsO Other com-pounds, such as magnesium oxide (MgO) and tetracalcium alumino-ferrite (4CaO A12O3-Fe2O3), do not exhibit any cementitious properties. The exact composition of portland cement is defin-ed in A.S.T.M. Standard Specifications which are accepted by the industry.
Generally speaking, portland cement is currently obtained by finely lntergrinding lime and silica containing materials and heating the mixture within a rotary kiln to the point of fusion. Fusion occurs at or about 1280C, the precise temperature depending upon the chemical composition of the feed materials and the type and amount of ~luxes that are present in the mixture. The principal fluxes are alumina (A1~03) and iron oxide (Fe2 03), and these fluxes enable the chemical reactions to occur at relatively lower temperatures.
Normally the lime is obtained from natural calcareous deposits such as limestone, marl, and aragonite. Under certain condi-tions r lime may be derived from industrial by-products such as phospho-gypsum, a pulverulent calcium sulfate from phos-phoric acid manufacture. The silica and fluxes, on the otherhand, are normally derived from natural argillaceous deposits such as clay~ shale, and/or sand.
Other materials may be present in the feed matQrials or are derived from the fuel used to heat the kiln. Alkalies, primarily compounds of sodium and potassium, are usually found in small quantities in all the raw materials and in the ash of coal. Sulfur compounds are found in the clay or shale and in the fuel. These materials generally volatilize in the hotter region of the kiln and are carried with the co~bustion gases and dust ~rom the other raw materials, out of the kiln as alkali sulfates or alkali oxides. ~ime particles in the dust may also form calcium sulfate and exi-t the kiln. Therefcre, the kiln dust (or stack or pr0cipitator dust) is usually hiyher in alkali compounds and/or gypsum than the raw feed materials.
Although the kiln dust is sometimes partially returned to the kiln, eventually it builds up a level of alkalies and/or sulfates that is unacceptable and must be discaxded, presenting a waste disposal and/or an environmental problem.
More specifically, to manufacture portland cement, an argillaceous material and a calcareous material are crushed, mixed, and interground to a fine powder, with the proportions

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of the two materials and the composition of each being main-tained within narrow limits. The mixing and intergrinding may be done in the dry condition (the dry process) or it may be done in water (wet process).
In either case, the mixture passes into the upper end of a rotary kiln where it i5 heated eventually to the fusion point. However, before this point, water and carbon dioxide are driven off As the hottest region is approached, a part of the interground mixture of materials melts and chem-ical reactions take place between the constituents of th~ rawmixture. In the course of these reactions new compounds are ormed. After passing the hottest region, the compounds fuse and form a clinker. The clinker then is discharged into some form of a cooler, or is conveyed to a clinker pile where it is cooled, sometimes with a spray of water. When cool, the clinker is mixed with a carefully controlled quantity of gypsum, and the mixture is ground to a very fine powder. That finely ground powder is the portland cement of commerce.
Rotary kilns vary in length and diameter. They revolve slowly (one turn in every 1 to 2 minutes or more~ and, as they are slightly inclined, the charge slowly travels down-wardly toward the hot end of the kiln. Being heated from its lower end, a rotary kiln develops its hottest temperatures in a rather narrow zone of the ki}n, with the tem~erature becom-ing progressive]y less toward the upper end. At no time does the entire mixture in the rotary kiln in the hottest zone become molten. Special reractories are required, es~ecially for the hot zone at the lower end. Attempting to operate a rotary kiln above its normal operating temperature range will result in a high percentage o the eed mixture becoming liquid at one time and running uncontrollably out of the kiln.

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It will also cause severe damage to the refractories and to the kiln shell.
Generally, rotary kilns are heated by burning a fossil fuel at the lower end, with the hot combustion gases traveling up the kiln. Heat energy ls transferred to the downwardly moving raw feed by direct contact and indirectly by heating the refractory lining. As the raw materials become dried, heated, and partly calcined by the hot gases 9 some of the finer particles are picked up and transportecl out of the kiln. Sulfur in the gas stream usually combines with lime and/or alkalies to form sulfates. In the hotter end of the kiln the alkalies vaporize and join the gas stream, con-den~ing into dust particles at the cooler end of the kiln and exiting with the other dust particles. That part oE the sulfur compounds in the gases that does not combine with alkalies ox lime exits with the flue gases. If sulfur is sufficie.ntly high in quantity, the flue gas stream may become environmentally unacceptable and require treatment to meet emission standards.
One of the principal objects of the present invention is to produce portland and other hydraulic cements in an elec-tric furnace, thereby eliminating the need for a rotary kiln.
Another object is to provide a process of the type sta~ed which can utilize a wide variety of feed materials includiny naturally occurring calcareous and argillaceous materials, as well as by-products from industrial processes, irrespective of whether those materials are in a molten or a pulverulent or non-p~lverulent solid state. A further object is to provide a process of the type stated, the e~uipment for which is con-siderably less expensive than conventional kilns. These and other objects and advantages will become apparent hereinafter.

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'' ' ~, The present invention provides a process f.or pro-ducing a hydraulic cement such as portland cement, said process comprising: introducing appropriate feed materials into a vessel with the feed materials containing compounds suitably proportion~d for a desired hydraulic cement; heating the feed materials sufficiently within the vessel to produce a melt in which the feed materials are molten and further mixing the molten feed materials by thermal currents so ~hat they react and chemically combine within the melt~ the heating and mixing lO being effected by electrical energy; permitting a portion of the melt in the region of the vessel walls to solidify to pro-tect those walls from the molten material of the melt; with-drawing the melt from the vessel; and cooling the withdrawn melt to solidiEy it into a solid substance that has the chemi-cal constituency and properties of the desired hydraulic cement.
Some of the gases evolved from the cons~ituents of the feed materials can be cooled and efficiently retrie~ed as solid particles are absorbed into other solids, permitting nearly total recovery without creation of a pollution prob-20 .lem.
The preferred process carrying out ~he present in-vention will now be described in greater detail4 The preferred process involves introducing raw feed materials that contain compounds necessary for the pro-duction of portland or other hydraulic cements into a melt in proper proportions. Within the melt the compounds react with each other and with the melt, all such that the melt acquires the chemical consituency necessary for portland cement or whatever hydraulic cement that is desired. The mel~ is contain- :
30 ed within an electric furnace which supplies enough heat to the .
meld to maintain it in a molten cordition and at least at the . ~

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temperature at which the chemical reactions occur. The fur-nace further produces turhulence by thermal currents withln the melt to effect necessary mixing of the feed materials.
The furnace is tapped either intermittently or continuously to withdraw the portion of the melt that has acquired the chemistry of the desired cement, and this withdrawn portion is permitted to solidify.

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Since portland cement is by far the most significant of all the hydraulic cements, the process will be described as it applies to the production of portland cement, but it should be recognized that by vaxying raw materials and proportions, other hydraulic cements may be produced.
Once the melt is established in the furnace, the raw feed materials are added to the surface of the melt in the proper proportions. Those raw materials may be molten or solid, and when in the solid form, they may be pulverulent or nodular. Also, they may be at ambient temperature, or they may be pre-heated or even pre-calcined. When nodular, the practical upper size lim t is determined by design so that the nodules will not interfere w:ith the operation of the furnace and its feed equipment. In this regard, the meit reaches high temperatures so that all materials introduced into it eventually reach a molten state.
One of the raw feed materials should include a source of lime, that is calcium oxide (CaO), and perhaps the most common source of lime is limestone which contains primarily calcium carbonate (CaCO3). When heated to about 900C, this compound decomposes into lime (CaO) and carbon dioxide (CO2), the latter of which, being a gas~ normally escapes. Usually, the limestone is preheated prior to its introduction into the furnace, not only to drive off the carbon dioxide, but to also place lesser electrical energy demands on the furnace as well. While limestone is quite common, other naturally occurring calcareous materials also contain high quantities of calcium carbonate and are equally suitable for use as a raw feed material in the process. Such materials may be aragonite, chalk, marl, cement rock, or marine shells. Normally, naturally occurring calcareous ' materials are crushed to a nodular form for use in the process, but any fines produced in the crushing operation are likewise suitable as a feed material and are introduced into the melt along with the nodules, preferably after undergoing preheat-ing and calcining so that lime is es~ential:Ly the feed material that is introduced.
Even tha by-products of certain industrial processes are suitable for use as the source of lime. For example, certain tailings from flue gas scrubbers contain a considerable amount of lime. Also suitable is kiln dust, which is normally collected from conventional cement kilns, and is high in alkali and sulfur content.
Phospho-gypsum, or naturally occurring gypsum, which is essentially calcium sulfate (CaSO~), may also be used as a source o the lime, but suitable means for disposing of the sulfur trioxide (SO3) gas should be available, e.g. sulEuric acid or elemental sulfur production.
The raw feed.materials should also include a source of silica (SiO2~, and an excellent source for silica is certain naturally occurring argillaceous materials such as clay, shale, sla~e, and sandO These materials, of course, are solids and are easily reduced to fines or nodules, if not already in that stat~. They may be introduced into the melt as such, whether at ambient temperature or preferably preheated. Certain fly ash and also coal ashes have a high proportion of sili.ca, and may likewise be introduced into the melt as is or preheated.
Another excellent source of both lime and silica is calcium silicate (CaO-SiO2~ which is found in slag derived from many industrial processes. For example, the blast furnaces used in producing steel produce a laxge arnount.of slag. The same is true of certain processes used to extract phosphorous _~.

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from calcium phosphate rocks and other processes used to melt-chromium ore. The slag is, of course, initially in a molten state, and as such is usually poured out to produce a slag heap which grows larger and larger. Heretofore, little use has been found or the slag, and as a consequence it merely accumulates. The slag may be broken up into a granular or nodular consistency and introduced as such into the melt within the furnace.
However, the energy requirements of the cement making process may be reduced substantially if the sla~ is introduced into the electric furnace in its initially molten condition. This, of course, requires locating the cement making facility in close proximity to the industrial process from which the slag is derived. In this regard, calcium silicate melts at about 1300C and a considerable amount o heat is required to elevate it to that temperature.
The raw feed materials, in addition to including a source of lime and a source of silica, may also include a ; source of alumina, that is aluminum oxide (A12O3), although the amount of alumina that is useful is considerably less than the amount of lime or silica. Silica and alumina are often found together in nature as well as in the slag-type by-products of many industrial processes, so more often than ; not the source o silica will likewise serve as the source of alumina.
E'inally, the feed materials may contain a flux -to lower the temperature at which the desired chemical reactions will occur within the melt of the ~urnace. To a measure, the alumina functions as a flux. Another common flux is iron oxide ~Fe2O3), which like alumina is found in many argillaceous substances as well as slag-type industrial by-products.

_ y ~i3 Other materials often appear in minor quanti-ties in the various feed materials. These include compounds of the alkalies, sodium and potassium~ and of sulfur, ti-tanium, magnesium, manganese, phosphorus, barium, and strontium. If present in excessive quantities, they may be harmful to the cement product. This is particularly true of the alkali and phosphorus compounds.
Within the melt of the electric furnace the feed materials combine chemically so that the melt when withdrawn and cooled will have adequa~e proportions of ~ricalcium silicate (3CaO-SiO2), dicalcium silicate (2CaO.SiO2~ and tricalcium aluminate (3CaO-A12O3), and whatever other compounds are necessary for the desired cement. In this regard, the chemical constituency of the feed materials must be known and the feed materials must be proportioned such that the melt within the electric furnace acc~uires the chemistry necessary for the desired portland or other cements. Normally, the proportioning is achieved by passing the feed materials over scales and weighing them before they are introduced into the fuxnace. Although the feed materials may contain compounds in addition to the lime, silica, and alumina, many of these evolve as gases during preheating or under the intense heat of the furnace. For example, when calcium carbonate (CaC03) is the source of lime, the carbonate is driven off in the form of carbon dioxide ~CQ2), leaving lime (CaO). The electric furnace is particularly effective in volatilizing alkali compounds and phosphorous compounds in the feed materials because of the concentrated high heat in the melt, usually leaving only trace amounts of these compounds in the inal product and these amounts are acceptable. Other non-ce~ntitious compouncls may also appear as merely traces in the final product ancl they do .~

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not significantly affect its ability to serve as a cement.
Free lime should be maintained at a minimum and within specifications.
The electric furnace itself is conventional, and as such includes the usual vessel in which the melt is contained.
This vessel has a shell which i6 lined with a suitable refrac-tory material. Moreover, the melt can be solidified adjacent to the refract.ory lining where it will serve to protect that lining. The vessel may be a fixed structure, in which case it is tapped through holes in its refractory lining and .shell, or it may be supported on trunnions, in which case the melt is poured from it.
Not only does ths electrical current supply heat to the melt and thereby maintain it at a temperature sufficient for the chemical reactions to occur, but it further churns the melt by thermal currents to insure that the eed materials are ~horoughly mixed in the melt. Indeed, the turbulence produced provides all of the mixing that is necessary to effect the necessary chemical reactions between the various raw materials, even when those raw materials are introduced into the melt separately and at different locations over the surface of the melt. .
The electric furnace will generate thermal currents within the melt, and the particular type of electric urnace is not critical as long as it has the desired capability.
For example, an induction urnace will provide the necessary thermal currents as will some resistance-type furnaces. Also, electric arc furnaces are highly suitable. In an electric arc furnace, the melt is maintained in a molten state by energy supplied to it through electrodes that project downwardly toward, but do not normally contact the upper surface of the ' :

melt. The electrodes, which may be arranged in a triangular or an in-line pattern, are positioned above the melt such that an arc will span the space between the lower end of each electrode and the upper surface of the melt when an electric potential of sufficient magnitude is applied across the electrodes. As a consequence, a current flows between the electrodes, the path of that current being through the arcs and the molten material in the region between the arcs. The temperature of the molten material in this region is substan-tially above the temperature of incipient fusion or the feedmaterials within the furnace. Sufficient electrical energy is applied at the electrodes to maintain the materials in a molten state.
Any solidified materials which are introduced to the melt become molten even when they are essentially in a nodular state. They further combine chemically wi-th the other components, so that the melt becomes homogeneous in character.
The homogeneous melt possesses the chemical constituency which provides the desired portland or other cement when the homo-geneous melt is allowed to cool~ In fact, it is the uniquecapability of the electric furnace to respond quickly to variations ln individual feed materials that permits a level of chemical quality control that cannot even remotely be attained in the conventional rotary kiln, where considerable time elapses between feeding and formation of the clinkers.
If one or more of the feed materials are pulverulent, it will be necessary to control the feed rate and/or the size of the electric furnace to the melting rate of the material to prevent a massive heat sink forming in the melt because of the fine particle sizes involved.

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, Whenever the chemical constituency of the feed materials is such that excessive amounts of unvaporized alkalies, sulfates, or phosphates are present, the furnace is equipped with a roof and an exhaust system so that those gases evolving from the melt can be collected. Products such as alkali oxides or sulfates can be condensed into a solid and recovered. Excess sulfur compounds can be absorbed by other compounds for recovery or disposal simultaneously. This removes any environmentally unacceptable materials from the evolved gases before they are vented to the atmosphere.
It is within the region of the homogeneou~ melt that the furnace is tapped, and the melt which is withdrawn in a controlled manner and is allowed to cool into a clinker. The clinker then passes to an air quenc-h; cooler where its temper-ature is reduced in the conventional manner. Th~ cooling must be rapid enough to prevent the dicalcium silicate in the beta phase from changing to the gamma phase. The latter crumbles or "dusts" and is not cementitiousO
The heat that is extracted from the clinker at the air quench cooler may be transferred to the preheater where it serves to preheat the feed materials that pass through the preheater, This is best achieved by directing heated air discharged from the cooler into the preheater. Or, it may be used as pre-heated combustion air if a fossil fuel energy source is used to pre-heat the feed materials.
The clinker upon leaving the cooler is either shipped to a purchaser or stored. Ultimately the clinker is crushed and ground to a fine powder which is suitable for use as cement.

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a hydraulic cement such as portland cement, said process comprising: introducing ap-propriate feed materials into a vessel with the feed materials containing compounds suitably proportioned for a desired hy-draulic cement; heating the feed materials sufficiently with-in the vessel to produce a melt in which the feed materials are molten and further mixing the molten feed materials by thermal currents so that they react and chemically combine within the melt, the heating and mixing being effected by elec-trical energy; permitting a portion of the melt in the region of the vessel walls so solidify to protect those walls from the molten material of the melt; withdrawing the melt from the vessel; and cooling the withdrawn melt to solidify it into a solid substance that has the chemical constituency and prop-erties of the desired hydraulic cement.

2. The process according to claim 1, wherein the melt is maintained within the vessel and the feed materials are introduced into the melt at the surface of the melt.

3. The process according to claim 1, wherein at least one of the feed materials is a solid.

4. The process according to claim 3, wherein the solid feed material contains a lime-yielding substance.

5. The process according to claim 2, wherein at least one of the feed materials is in a molten state.

6. The process according to claim 5, wherein the molten feed material contains calcium silicate.

7. The process according to claim 1, wherein the heating and mixing are effected by producing an electric arc between the upper surface of the melt and an electrode.

8. The process according to claim 7, wherein at least two arcs are maintained between the surface of the melt and separate electrodes, and an electrical current is conducted through the melt, the arcs, and the electrodes.