US3722867A - Method of calcining limestone - Google Patents

Method of calcining limestone Download PDF

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Publication number
US3722867A
US3722867A US00148342A US3722867DA US3722867A US 3722867 A US3722867 A US 3722867A US 00148342 A US00148342 A US 00148342A US 3722867D A US3722867D A US 3722867DA US 3722867 A US3722867 A US 3722867A
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Prior art keywords
limestone
hearth
chamber
flame
kiln
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US00148342A
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W Butler
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    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2/00—Lime, magnesia or dolomite
    • C04B2/02—Lime
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2/00—Lime, magnesia or dolomite
    • C04B2/10—Preheating, burning calcining or cooling

Definitions

  • Calcination of limestone is the conversion of limestone Ca CO into lime CaO by heating the limestone to a temperature of at least l600 F to disassociate the CO therefrom, leaving the desired CaO.
  • limestone is fed into the top of a vertically disposed refractory-lined shaft of substantial height.
  • the shaft of the kiln may have various crosssectional shapes as desired.
  • the limestone is calcined, by virtue of combustion attained through the use of gas or other suitable fuel, the limestone travels downwardly through the shaft, passing successively through a storage zone, a preheating zone, a burning zone, and finally through a cooling zone.
  • the lime produced may be drawnofi from the bottom of the shaft in a continuous or in an intermittent manner.
  • limestone normally less than 2 inches is fed into one end of an elongated refractory lined tube which revolves continuously around its longitudinal axis which axis is tilted at a slight angle with respect to the horizontal.
  • the tube is heated by gases.
  • the limestone is calcined and moves in a progressive manner to discharge at the lower outlet end of the tube.
  • a burning zone is arranged at the discharge end of the tube and combustion gases pass up the tube toward its charging end and thus approximate in their travel the burning and pre-heating zones found in the vertical kiln.
  • the chamber. employed for calcination is heated by means of fuel burners which fire into the space between the limestone charge and the refractory enclosure, and heat transfer to the limestone charge is mainly by direct radiation from the chamber.
  • This method of heating the CO released by the limestone during disassociation remains essentially in a stratified blanketing layer of gas around the limestone. This gas is released at a temperature of approximately l600 F, and exerts a cooling influence upon the bed of limestone. In the case of the moving hearth kiln, this cooling effect is sufficient to prevent the top surface of the hearth refractory from exceeding a temperature of 1700 F.
  • S0 gas and other contaminant gases present either as a constituent of the limestone charge or as a constituent of the fuel and which are of a high density, may also settle into the voids of the limestone bed and can, in fact, combine with the lime formed by disassociation.
  • limestone is calcinated in a chamber having refractory walls and a refractory hearth that is movable through the chamber, and the invention comprises the steps of depositing the limestone in a layer on the hearth, heating the limestone above l600 F as the hearth moves through the chamber by directing burning gases against the surface of the layer of limestone and downwardly into the limestone and against the hearth, evacuating CO and other gases released from the limestone and gas combustion from said chamber and removing the limestone from the hearth.
  • FIG. 1 is a plan view of a rotary hearth type kiln in which part of the roof has been cut away to show the chamber and the flame pattern on the limestone bed;
  • FIG. 2 is a radial cross-section of the chamber taken at section 22 of FIG. 1;
  • FIG. 3 is a cross-section of the chamber taken at section 3--3 of FIG. 2.
  • my apparatus for calcining limestone consists of an annular refractory hearth 10 supported by a steel base 12 carried on wheels 14 which move on a circular track 16.
  • the hearth 10 is enclosed by a stationary annular refractory chamber comprised of inner wall 18, outer wall 20, and roof 22 which are supported by steel framework 24, 26 and 28.
  • Raw limestone 1 in a cold or preheated state is deposited on the hearth by the charging mechanism, generally indicated by numeral 30, and is carried through the kiln chamber as the hearth rotates in a counter-clockwise direction, calcined and discharged from the hearth by a discharge mechanism, generally referred to by the numeral 32.
  • heat is transferred to a bed of limestone primarily by radiation
  • heat is transferred to a layer of limestone on the rotating hearth by directing burning gases against the surface of the limestone and downwardly into the limestone and against the hearth so that the burning takes place all around the limestone and subjects it to a rapid and intense heating to-achieve an efficient calcination.
  • the movement of the hot burning gases of the flame within the bed of limestone disperses CO blanket created by the disassociation of the limestone.
  • the evacuation of these gases which are released at the relatively low temperature of 1600 F., greatly increases calcinating efficiency because of the higher flame temperatures (in excess of 2400 F.) which surround the lime-stone pieces.
  • Heat is introduced to the chamber from the burners 34 located in the roof in the form of the open flames 36 which penetrate the limestone bed 1 right through to the hearth.
  • Products of combustion from the flames as well as CO released by disassociation of the limestone travel clockwise through the kiln chamber to flues 38 for removal from the film.
  • These waste gases are carried from the flue to an exhaust fan and stack for discharge to atmosphere.
  • a damper is provided in the waste gas duct to regulate the exhaust fan suction.
  • Stone preheat equipment, combustion air preheat equipment and gas cleaning equipment can be installed in the waste gas system if desired and the exhaust fan size adjusted to suit the additional static pressure requirement.
  • the basic rotary movable hearth kiln and its associated equipment (apart from the means for introducing heat to a bed of limestone) is well known in the art.
  • the burners 34 supplying heat to the kiln chamber are arranged in stations or zones for ease in controlling (a) flame length, (b) flame position, (c) chamber temperature.
  • the factors which will dictate the exact number of zones and the number of burners per zone are (a stone type, (b) degree of calcination required, (c) total kiln production, (d) whether charging preheated or non-preheated stone, and (e) the width of hearth employed.
  • FIGS. 1, 2 and 3 show a kiln having 9 zones of burner control in which the last zone contains six burners and the others each contain four burners.
  • the last or finishing zone serves a special function, in
  • the flames of this zone cover the whole width of the limestone charge in order to (a) act as a screen to prevent air infiltration from the discharge area, and (b) saturate the lime bed with flame of a reducing nature (deficiency of air) to prevent sulphur pickup by the lime if the fuel contains sulphur or if sulphur was present from some other source.
  • Burners are staggered transversely of the chamber from zone to zone so that there is complete coverage of limestone with flame in each two adjacent zones. This division of heat input and burner position is desirable for four reasons: first, areas of limestone charge and hearth which are not directly in the path of the flame provide a continuous source of lower temperature into which heat can flow from those areas which are in the path of the flame, and thereby create a better condition for greater heat transfer from the flame than if the full width of hearth was in the path of the flame simultaneously. Secondly, means are provided for transferring products of combustion and CO from disassociation through the kiln chamber to the flue area.
  • the open areas between the flame provide an adequate path through which the gases can flow, and the area can be regulated so that the height of the chamber is proportional to the volume of gases in transit.
  • the disposition of the flames of zones having four burners provides each and every flame with an open channel directly in front of it between the flames of the zone toward which the spent products of combustion of the flame must flow. This will help to avert a traditional fault of rotary hearth type furnaces, wherein kiln gases tend to follow the path of least resistance and travel to the inside wall of the annular chamber.
  • the location and direction of the flames will tend to discourage the horizontal stratiflcation of kiln gases flowing through the chamber without developing turbulent flow conditions and the related increase in pressure drop.
  • the velocity of the flames and the flame envelope at the point of contact with the flame and the limestone charge on the hearth is sufficient to insure that the flame will penetrate through the bed of limestone and come in contact with the refractory surface of the hearth which supports the limestone bed.
  • the burners are specifically selected so that the flame from the burner is sufflciently long to accomplish this purpose and to have a s'plattering or boiling effect for some distance in front of the point where the flame first makes contact with the charge, which insures a large area of contact between the flame and charge.
  • the combined effect of flame velocity and splattering will also serve to disturb the limestone bed in a manner which will insure penetration of the burning gases of the flame and prevent any stratiflcation of gases within the limestone bed.
  • the burners in any one zone are connected to a common air supply 38 (which may be cold air or preheated air) and a common fuel supply 40, each of which are equipped with metering and flow control devices so that the chamber temperature within that zone can be automatically controlled by thermocouple 42, and a suitable control instrument.
  • a common air supply 38 which may be cold air or preheated air
  • a common fuel supply 40 each of which are equipped with metering and flow control devices so that the chamber temperature within that zone can be automatically controlled by thermocouple 42, and a suitable control instrument.
  • Each zone is controlled independently to provide the proper temperature in that zone for calcination of the limestone bed.
  • the individual burners in any one zone are sized so that the heat input from that burner in relation to the heat input from the complete zone is in like proportion to the quantity of stone heated by that burner in relation to the quantity of stone heated by the whole zone.
  • the burners are designed so that the flame length and width may be adjusted without changing the heat input of the burner.
  • the burners burn propane or any other suitable gaseous or liquid fuel.
  • the number of zones, burners per zone, length and width of flame, angle of incidence of flame to hearth, degree of flame penetration into limestone bed, temperature of individual zones and other aspects of the process and apparatus can be increased or decreased as required to suit the calcining requirements.
  • the calcination time for limestone pieces contained in a bed supported on a refractory hearth is reduced by a minimum of 50 percent by directing the flame into the bed of limestone and against the hearth as opposed to directing it into the atmosphere above the bed of limestone.
  • This is a remarkable difference in calcinating efficiency.
  • the calcinating efficiency with this invention is not materially affected by the depth of the bed providing that the flame penetrates the bed to the refractory surface of the hearth.
  • Bed depth will depend on the manner in which the limestone packs.
  • a uniformly crushed limestone of 2 inches diameter can be deposited to about a 6 inch depth.
  • the flame will penetrate.
  • a nonuniformly crushed material of slabby nature cannot be deposited as deeply because the flame cannot penetrate as well. Flame penetration is the limiting factor.
  • This process and apparatus enables me to subject the limestone charge to the direct flame of the heating source, thereby permitting the use of a greater depth of limestone bed than hitherto employed, while at the same time creating a higher temperature environment for the limestone charge without increasing the temperature of the calcining chamber.
  • the invention provides a means for quickly removing the CO of disas- -sociation as well as other gaseous contaminants from the limestone bed, which in turn also increases the temperature environment of the limestone pieces.
  • one is readily able to control the heat input in like proportion to the heat demand for calcining and simultaneously provide means for controlling the flow of spent products of combustion and gas of disassociation within the heating chamber. I am also enabled to achieve underburning or overburning of the limestone pieces without affecting the increased productivity.
  • a continuous method of calcinating limestone in a chamber having refractory walls, spaced apart fuel burners and a refractory hearth movable through said chamber comprising the steps of depositing limestone pieces in a layer on the hearth, heating the limestone pieces to calcining temperature by moving the hearth through a series of spaced burner zones in said chamber and by directing burning gases from said fuel burners through the surface of the layer and downwardly into the layer to burn around the limestone pieces and against the hearth, evacuating CO, and other gases released from the limestone and gas combustion from said chamber and removing the limestone pieces from the hearth.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Ceramic Engineering (AREA)
  • Materials Engineering (AREA)
  • Structural Engineering (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Vertical, Hearth, Or Arc Furnaces (AREA)
  • Tunnel Furnaces (AREA)
US00148342A 1971-06-01 1971-06-01 Method of calcining limestone Expired - Lifetime US3722867A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US14834271A 1971-06-01 1971-06-01

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US3722867A true US3722867A (en) 1973-03-27

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US (1) US3722867A (2)
AU (1) AU466389B2 (2)
CA (1) CA958421A (2)
DE (1) DE2225782A1 (2)
FR (1) FR2139592A5 (2)
GB (1) GB1381787A (2)
IT (1) IT958145B (2)
SE (1) SE378013B (2)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4220631A (en) * 1977-04-30 1980-09-02 Metallgesellschaft Aktiengesellschaft Process of calcining limestone or hydrated lime in a rotary kiln
US4670228A (en) * 1983-09-21 1987-06-02 A/S Megon & Co. Process for the recovery of valuable metals, particularly rare earths and similar metals, from a carbonate-containing raw material
US5989019A (en) * 1996-08-15 1999-11-23 Kabushiki Kaisha Kobe Seiko Sho Direct reduction method and rotary hearth furnace
US20060280907A1 (en) * 2005-06-08 2006-12-14 Whitaker Robert H Novel mineral composition
US20070104923A1 (en) * 2005-11-04 2007-05-10 Whitaker Robert H Novel mineral composition
US20070261337A1 (en) * 2006-04-18 2007-11-15 Whitaker Robert H Novel mineral filler composition
US20080173212A1 (en) * 2005-11-04 2008-07-24 Whitaker Robert H Novel mineral composition
US20120214118A1 (en) * 2009-11-30 2012-08-23 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Rotary hearth furnace
EP1695013B2 (de) † 2003-12-16 2014-06-18 Rheinkalk GmbH Verfahren zum brennen von kalkstein

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4220631A (en) * 1977-04-30 1980-09-02 Metallgesellschaft Aktiengesellschaft Process of calcining limestone or hydrated lime in a rotary kiln
US4670228A (en) * 1983-09-21 1987-06-02 A/S Megon & Co. Process for the recovery of valuable metals, particularly rare earths and similar metals, from a carbonate-containing raw material
US5989019A (en) * 1996-08-15 1999-11-23 Kabushiki Kaisha Kobe Seiko Sho Direct reduction method and rotary hearth furnace
EP1695013B2 (de) † 2003-12-16 2014-06-18 Rheinkalk GmbH Verfahren zum brennen von kalkstein
US20060280907A1 (en) * 2005-06-08 2006-12-14 Whitaker Robert H Novel mineral composition
US20070104923A1 (en) * 2005-11-04 2007-05-10 Whitaker Robert H Novel mineral composition
US20080173212A1 (en) * 2005-11-04 2008-07-24 Whitaker Robert H Novel mineral composition
US7651559B2 (en) 2005-11-04 2010-01-26 Franklin Industrial Minerals Mineral composition
US20070261337A1 (en) * 2006-04-18 2007-11-15 Whitaker Robert H Novel mineral filler composition
US7833339B2 (en) 2006-04-18 2010-11-16 Franklin Industrial Minerals Mineral filler composition
US20120214118A1 (en) * 2009-11-30 2012-08-23 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Rotary hearth furnace

Also Published As

Publication number Publication date
FR2139592A5 (2) 1973-01-05
DE2225782A1 (de) 1972-12-14
IT958145B (it) 1973-10-20
CA958421A (en) 1974-11-26
SE378013B (2) 1975-08-11
GB1381787A (en) 1975-01-29
AU466389B2 (en) 1975-10-30
AU4251272A (en) 1973-11-22

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