CA1160647A - Method for producing cement - Google Patents

Abstract

METHOD OF PRODUCING CEMENT

ABSTRACT OF DISCLOSURE
Tricalcium silicate materials useful in the production of portland and other hydraulic cements are prepared by adding a mixture of silica and lime containing materials to an electric furnace and melting the mateials. The materials may be added in lump form and need not be pulverized. The voltage between the electrodes and the conductivity of the melt are controlled so that a predetermined separation will be maintained between the lower end of the carbon electrodes and the melt.

Description

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BACKGROUND OF THE INVENTION
This invention relates to the production of tricalcium silicate useful in the manufacture of Portland and other cements and in particular to the manufacture of such materials in electrie arc furnaces.
Portland cement is commonly prepared from certain indefinite compounds of lime, silica, alumina, iron oxide, magnesium oxide and small quantities of other ozides. The raw materials useful in the preparation of Portland cement inelude cement rock, limestone, marl, clay and shale, blast-furnace slag, and gypsum sand. One eommon method for preparing Portland eement eomprises erushing or grinding the raw materials into a powder whieh is then proeessed in dry form or as a slurry. The mixture of raw materials is placed in a rotary kiln for caleinization and burning into elinker.
Temperatures of about 28006F are typieally used at the hottest heating zone near the diseharge end of the kiln. Following heating, the clinker is air-cooled, pulverized and blended with various chemicals added to modify performance or setting time and then bagged or otherwise stored for use. This method is not wholly satisfactory beeause it requires a large amount of energy and eontrolling the eomposition of the final product is relatively difficult.
OBJECTS OF THE INVENTION
It is a primary objeet of the present invention to provide a new and improved method of making Portland eement elinker.
Another object of the present invention is to provide a method of manufacturing Portland cement clinker in an electric arc furnace.
Yet another object of the invention is to provide a method of manufaeturing Portland eement elinker in an eleetrie arc ,.

furnace wherein electrical parameters are controlled to prevent contamination of the melt with carbon from the electrodes.
A still further object of the present invention is to provide a method of manufacturing Portland cement clinker in an electric arc furnace wherein raw materials added to the melt are in lump form.
Another object of the present invention is to provide a method of manufacturing Portland cement clinker in an electric arc furnace wherein limestone lumps are calcined to lime before addition to the melt.
How these and other objects and advantages of the present invention are accomplished wil] be described in the following specification taken in conjunction with the drawing.
In general, the invéntion comprises a process which includes the steps of charging a pre-reduced slag into an electric arc furnace having at least two carbon electrode and energizing the electrodes to maintain a temperature in the furnace of about 3200F to maintain the conductivity of the melt at about 175 mho/meter. Further, raw materials are added during the process to insure the desired composi-tion for the final metl. Preferably, the process is con-tinuous with the molten product being tapped for cooling and the feed rate of material to the furnace being at a rate which does not lower the temperature below about 3200F or extinguish the arc.
BRIEF DESCRIPTION OF THE DRAWING
The single figure of the drawing schematically illus-trates an electric arc furnace in which the method of the invention may be practiced.

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~ ~ 6~47 DESCRIPTION OF THE PREFERRED EMBODIMENTS
The single figure of the drawing schematically illus-trates an electric arc furnace 10 in which cement clinker may be produced according to the method of the present invention. In general terms, electric furnace 10 includes a metallic shell 11 and a refractory lining 12. A plurality of electrodes 14, which are essentially carbon may be of the prebaked or self-baking type, extend through suitable openings 15 in the arched roof 16 of furnace 10. While two electrodes 14 are shown, any suitable number may be employed.
The electrodes 14 may be energlzed from any conventional source, symbolized by the single phase~ alternating current transformer 17 which is coupled by a pair of conductors 18 to the electrode clamps 19 of each electrode. Those skilled in the art will appreciate that each clamp 19 will include conductive members for engaging the electrode surface whereby electric current may be transferred readily therebetween.
Each electrode 14 is supported for vertical movement relative to the furnace 10 in any suitable manner such as by means of a schematically illustrated positioning mechanism 20 which includes a control 21, a positioning motor 22 and a cable assembly 23. As will be discussed more fully below, the control mechanism 21 is operative to sense electrode current and voltage and to provide control signals to motor 22 which, in turn, adjusts cable assembly 23 so that its associated electrode 14 is adjusted vertically. In this manner, the electrodes 14 are positioned at the desired distance from molten bath 24 within the furnace 10.
while only a single control 21, motor 22 and cable mechanism 23 is shown to be connected to one of the electrodes 14, it ~ 1 6(~7 will be appreciated that there will be an identical control system for the other electrode 14 as well.
The cable mechanism 23 includes a cable 26 coupled at one end to its associated electrode 14 and extending upwardly therefrom and over sheaves 28. The opposite end of cable 26 is connected to a drum 29 which may be reversibly driven by motor 22.
The control 21 is generally conventional and is coupled to conductor 18 for receiving a first signal functionally related to electrode voltage and a second signal functionally related to electrode current. Assuming the conductivity of the bath 24 remains the same, those skilled in the art will appreciate that the electrode current will be inversely related to the distance between the electrode 14 and bath 24 while electrode voltage will be directly related. More specifically, control 21 includes an isolation transformer 30 whose primary is connected to conductor 18 through resistor 32 so that a signal appears in the secondary of transformer 30 which is functionally related to the potential between electrode 14 and ground. This signal is rectified and provided to control 21 through conductors 33. Also coupled to conductor 18 is a current transformer 34 which generates a current signal frunctionally related to electrode current.
This signal is rectified and applied across resistor 36 so that a voltage signal functionally related to electrode current is applied to control 21 through conductors 38.
Control 21 is operative to compare the voltage signals delivered through conductors 33 and 38 and to provide an output signal to motor 22 when the relationship between the input signals deviates from a preselected value. The ~ ~a6~

output signal from control 21 will vary in magnitude and sense depending upon the manner and degree that the input signal relation deviates from the preselected value.
Should the gap between electrode 14 and melt 24 increase from a desired amount, electrode voltage will increase and electrode current will decrease and there will be corres-ponding changes in the signals at conductors 33 and 38.
When these changes are sensed by control 21, the latter will then provide an error signal to motor 22 which becomes opera-tive to lower the electrode 14 toward the bath 24 until thedesired electrical conditions are again achieved. Conversely, should the electrode move closer to the bath than the pre-selected desired position, the electrode current will increase and electrode voltage will decrease. These parameter changes will be sensed by control 21 which will signal motor 22 to raise the electrode 14 until it is elevated to the desired position.
Another parameter affecting electrode voltage and current is the conductivity of the melt. Normally, the conductivity of tricalcium silicate, the principle con-stituent of the melt, will be about 175 mho/meter at 3200F. However, it will be appreciated that in a continuous process, as molten material is withdrawn through tap 39, additional material 40 to be melted will be provided to the furnace from hoppers 41. However, the conductivity of the material varies from a relatively low value at its melting point to the value indicated above at the operating temperature of 3200F. It will also be appreciated that the temperature of the melt is sensitive to the feed rate and particularly in the vicinity of the electrode tips. Should 4'~

the material be fed too fast, therefore, so that there is a decrease in conductivity, the electrode current will decrease even though the gap between the electrode and the bath may be relatively short. This condition would be sensed by the control 21 which then attempts to lower the electrode 14 further so that the graphite electrode might tend to become immersed into the bath 30. The tricalcium silicate bath, however, is very reactive with carbon elec-trodes at temperatures of 2900F-3200F which is the normal operating range of the furnace. This reaction of lime and carbon forms calcium carbide in the melt. The formation of even very small amounts of this carbide renders the sub-sequent clinker useless as a cement making material because of deleterious effects it has upon the crystalline proper-ties of solidified material. For this reason, the chargematerial 40 which is fed into furnace 10 from hopper 41 must be fed at a controlled rate through valve 42 so as not to lower the temperature of the melt below about 3200F. For this purpose a control 43 is connected to receive electrode voltage and current signals. These signals are employed to determine the power delivered to the furnace 10 and to provide an output control signal functionally related thereto.
The output signal is used to control the valve 42 which may take the form of a vibratory or screw feeder. This control is based upon the finding that the power consumed by the furnace is related to the material feed rate. If the material is fed too fast, the power drops off sharply and conversely increases from a preselected value if material is fed too slowly. Thus by measuring power delivered to the furnace, the material feed rate can be controlled to ~, 4 ~
insure that the power is maintained within preselected limits and further to insure that the electrode tips do not make contact with the melt.
As the charge material 40 falls into the furnace, it will be heated within the melt by the electric currents in the furnace and will be influenced by the thermal convection current generally flowing away from the electrodes toward the furnace walls. The location of the feed chutes may be used to provide layering of the charge on the top of the bath near the electrodes so that it is placed where it can advantageously use the superheat in the top layer of the melt for melting as it moves from the arc contact zone toward the furnace walls. The ideally sized furnace bath crucible is one in which the superheat is completely trans-ferred to the feed on the surface by the time the convectioncurrent has transported the melt to the refractory lining.
When this condition is maintained, a portion of the charge 45 solidifies on the inner surface of the refractory lining 12 thereby providing a protective layer or self-lining.
Because the melt 24 is highly corrosive when in contact with most known refractory materials, the self-lining 45 helps to prolong substantially the life of the furnace refractory 12.
In accordance with the method of one embodiment of the invention, a mixture of clay, which is principally silicon dioxide, and the waste product produced when phosphate rock is treated with sulfuric acid for the extraction of phos-phorous are stored in hopper 41. This waste product consists principally of gypsum (caS0-2~ O). The mixture is fed into the furnace 10 at a rate controlled by valve 42. The '~

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electrodes are energized to provide a bath temperature of about 2900F-3200F. At this temperature, the following chemical reactions occur:

3CaSO 2H O+Si0 CaO-SiO + 3SO + H20 This reaction is rapid and virtually complete by the time the mixture of sulfate and silicous materials is melted. After the materials charged from hopper 41 into furnace 10 melt, the molten material 24 may be tapped through tap hole 39.

Accordingly, this process can be continuous and in this par-ticular embodiment permits the recovery of SO by means of a smoke hood 46 which is connected at one end to an opening 47 in the roof 16 of the vessel 10 and whose other end is connected to a storage facility 48. Means may also be provided to produce a slight suction at the lower end of hood 46 to facilitate the collection of SO gas. After recovery in facility 48, the SO gas may subsequently be used for the production of sulfuric acid which may then be used in the phosphate rock treating process.

In this example of the method according to the invention, a rectangular furnace 1' by 2' in plan v;ew and 1' deep was employed. Two 3 inch diameter electrodes were connected to a 150kVA power source with the furnace power requirements being 5OkW. The furnace bath temperature was maintained at about 3200F and the voltage between each electrode and ground was maintained at about 45 volts. By-product gypsum and clay were fed into the furnace in the following propor-tions at the rate of 50 pounds per hour:
CaO 73%

S O 24%

Fe O 3%

6~1 A satisfactory portland cement composition shown in Table 1 was produced with the electrodes being maintained a slight distance above bath level.
Table 1 Melt 2 Melt 3 SiO 26.9 26.6 A123 7.1 10.3 Fe O 3.2 3.4

2 3 CaO 59.1 57.2 MgO 4.3 2.4 SO 0.23 0.21 L.O.T. 0.11 0.01 The importance of maintaining the electrodes in a spaced relationship with respect to the melt may be illustrated by reference to a series of tests conducted by the inventors.
Initially, limestone was utilized as the calcium containing material and furnace slag was employed as the silica con-taining material.
A first series of tests was conducted. In the first test of this series~ a limestone to slag ratio of 56:44 was employed and in the remaining two tests of this series a limestone to slag ratio of l.S:l was used. In each of the tests, the raw materials were employed in lump or nonpul- t verulent form. In prior cement manufacturing processes, it was considered impermissible to use particles larger than finely ground particles.
The tests w~re initiated by adding a portion of the slag material to the furnace and stricking an arc to form a molten 11 ~ B~6~ 7 pool. The electrodes were then immersed in the pool and limestone and additional slag added by gravity at intervals~
The added materials were melted into the existing melt by resistance heating. The limestone disintegrated and rapidly went into solution.
Electrode consumption and energy requirements were measured. The results indicated that a tricalcium silicate clinker could be produced using the process, but energy requirements were extremely high and electrode consump-tion was unacceptable. Tests conducted of the clinker todetermine if it had usefully hydraulic cement properties indicated an inordinate amount of magnesia (mgo).
A second series of tests was then conducted, the primary differences being that burnt lime was employed instead of limestone and a carbon lined crucible was employed. The burnt lime was also employed in a lump or nonpulverulent form. The burnt lime was prepared by the calcinization of limestone to convert carbonate to calcium oxide or lime.
These tests led to several important discoveries. First, all clinker produced in this second series of tests disintegrated into the gamma form upon cooling, even when various cooling techniques were attempted to control decrepitation of the clinker. Second, carbinde formation was noted by the evolu-tion of a gas having an odor like acetylene. Finally, the melt had reacted with the carbon lining at the temperatures employed to produce the melt.
A third series of tests was then conducted. The slag material used in this series contained approximately 44.61%
calcium oxide and 45.71% silicon dioxide with the remainder being relatively smaller amounts of the oxides of iron, aluminum, magnesium, potassium, sodium, manganese, phosphorus~
and approximately 2.1% iron. The burnt lime used in the final series of tests contained 94.38% clacium oxide with minor amounts of silicon dioxide, iron oxide, and magnesium oxide. The slag was received in large chunks and was crushed into lumps of less than one inch in size.
The furnace itself was the same one employed in the second series of tests except the lining was removed and a new rammed Magnesite lining was installed. Sufficient lining was installed to assure that the furnace feed material would form its own refractory crucible under the power load anticipated. The third series of tests was also carried out in a manner which would prevent any significant contact of the electrodes with the melt.
Various batch mixtures of the crushed slag and lump burnt lime feed materials were prepared to produce various tricalcium silicate and dicalcium silicate ratios for the final clinker. An initial feed was added to the crucible to form a melt and portions of the batch mixtures were added to the melt at approximately 45 minute intervals. The furnace operation was stable at 100 volts on the trans-former secondary which produced 100 volts at the electrodes and a power input of 75 kw. The electrode position under these conditions was just slightly above the melt and, therefore, produced a short arc from the electrode to the melt. The bath was heated both by radiation from the arc and as a result of current f~ow through the bath. A constant power input of 1 kw/lb. was maintained and the tap temperatures were maintained in the vicinity of 3040 to 3200F., a temperaure sufficient to insure chemical combination of the slag and lime into the desired hydraulic cement.

1 ~ 6~6~7 On conclusion of the third series of tests, it was noted that the furnace refxactory did not experience erosion as has been seen in previous tests. Electrode consumption was determined to be substantially reduced and carbide forma-tion was effectively controlled by maintaining sufficientvoltage between the electrode tip and the bath to prevent contact of the carbon with the melt. Testing of the various clinker materials produced in the third series of tests confirmed that they had the desired hydraulic proper-ties for use as a hydraulic cement.
The charge constituents may be selected from a varietyof calcium and silica bearing materials such as lime, lime-yielding materials, aragonite, blast and phosphorus furnace slags, etc. and the teachings of the present invention may be adapted for these and other materials by one skilled in the art after reading the present specification.
While several embodiments of the invention have been illustrated and described, the invention is not to be limited thereby it is to be limited only by the scope of the appendant claims.

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Claims (15)

Claims

1. A process for producing hydraulic cements including portland cement, said process comprising: maintaining a melt within an electric furnace, with the melt having the chemical constituency of the desired hydraulic cement; heating the melt within the electric furnace and contemporaneously mixing the melt with thermal currents generated within the melt by the electric furnace; feeding appropriate materials by gravity into the melt at the surface of the melt and in proportions suitable for providing the melt with the chemical constituency of the desired hydraulic cement; allowing the feed materials to chemically combine within the melt; withdrawing the melt from the electric furnace; and cooling the withdrawn melt to solidify it into a solid substance that has the chemical constituency and properties of the desired hydraulic cement.

2. A process according to claim 1 wherein at least one of the feed materials is a solid and contains calcium carbonate and said one solid feed material is heated prior to introduc-tion into the electric furnace with the heating being suffi-cient to either partially or completely decompose the calcium carbonate.

3. A process according to claim 1 wherein at least one of the feed materials is a solid and is composed of larger particle sizes than is permissible under any current cement manufac-turing technology.

4. A process according to claim 1 wherein all or part of the feed materials can be composed of larger particle sizes than is permissible under any current cement manufacturing technology.

5. A process for producing portland cement or other hydraulic cements, said process comprising: introducing raw materials containing compounds suitable for the production of the desired cement into a melt that is contained within an electric furnace and is hot enough to cause the compounds to chemically combine with each other and the melt such that the melt acquires the chemical constituency of the desired cement, at least one of the materials being a solid in a nonpulverulent condition.

6. The process according to claim 5 and further comprising heating the melt to maintain it at a temperature sufficient to chemically combine the compounds of the raw materials.

7. The process according to claim 6 wherein the melt is heated from within the melt by an electric heating apparatus.

8. The process according to claim 5 wherein said one raw material is lime or a lime-yielding substance.

9. The process according to claim 8 when another of the raw materials is a solid that is rich in calcium silicate and is in a nonpulverant condition.

10. The process according to claim 5 wherein said one material is derived by heating calcium carbonate that is in a nonpulverulent condition to a temperature sufficient to at least partially decompose the calcium carbonate.

11. The invention set forth in any of claims 1, 2, 3 or 4 wherein said heating is carried out by employing electrodes in proximity with but space apart from the surface of the melt.

12. The invention set forth in any of claims 5, 6, 7, 8, 9, or 10 wherein the melt within said furance is maintained at the desired temperatures by employing electrodes in proximity with but spaced apart from the surface of the melt.

13. A process for producing hyraulic cements comprising:
providing an electric arc furnace having electrode heating means; adding a slag material to said furnace; energizing said electrode heating means to create a pool of melt within said furnace; adding burnt lime and additional slag to said furnace in proportions required for providing a melt having the chemical composition of the desired hydraulic cement; and heating the melt with said electrode heating means to a tem-perature sufficient to cause said slag and burnt lime to chemically combine within the melt while maintaining the electrodes in proximity with but spaced apart from said melt.

14. The process set forth in claim 13 wherein said burnt lime and said slag are added to the melt in lump form.

15. The process set forth in claims 13 or 14 wherein said burnt lime and said slag are added periodically to said fur-nace and wherein said process further comprises the step of periodically removing melt of the desired chemical consti-tuency from the furnace whereby the process may be continuous.