DE102008031293A1 - Pure quicklime production from calcium carbonate-carbon mixture, e.g. black lime, by granulating, oxidizing contained carbon with oxygen and calcining - Google Patents

Abstract

Production of quicklime (CaO) comprises (a) granulating a homogenized, fine particulate mixture of (by weight) 80-95% calcium carbonate (CaCO 3) and 5-20% carbon (C) with 1-10% CaO and/or calcium hydroxide (Ca(OH) 2) and (b) drying the granules, oxidizing the C in the granules to carbon dioxide (CO 2) using atmospheric oxygen and calcining the granules to release of CO 2.

Description

The present invention relates to a method for producing quicklime (CaO) starting from very finely divided calcium carbonate (CaCO ₃ ).

at the large-scale Production of cyanamide and dicyandiamide from lime nitrogen by acidic Hydrolysis with carbon dioxide is a large by-product to so-called black lime, consisting of 80 to 95 wt .-% calcium carbonate and 5 to 20 wt .-% carbon (in graphite form).

These Black limes are commonly found in Agriculture as cost effective fertilizer and used in the cement industry as a lime replacement.

adversely when using this Schwarzkalke are the application technology Properties. So the black limestones have a relatively high moisture content from 18 to 22 wt .-%, whereby the corresponding material very bad to promote, dosing and storing is.

Besides, they are Black limes usually very finely divided and possess a middle one Particle size of 10 up to 50 μm on. Due to this fineness arise in the usual firings size Quantities of dust, so that the resulting exhaust gases at high temperatures not with technically usual Separators are to be cleaned.

It has not been lacking in attempts to process this black calf, to improve their performance characteristics and at the same time in this way, new uses to open up.

So For example, attempts were made to burn black lime in the usual lime shaft kilns. which is not due to the fineness of the starting material or only very badly possible was.

Of Further calcination attempts have been made in the past carried out in a rotary kiln, in turn, the fineness of the black lime used the main problem represented. Quite apart from that, the separators stuck by the fine material, the rotary kiln process is very energy-intensive and in the case of black lime uneconomical.

Also corresponding experiments in the fluidized bed process did not lead to the desired Success, as this procedure is also very energy intensive is. In addition, the residence times and product qualities with the common ones Fluidized bed reactors can not be controlled.

The present invention therefore an object of the invention to develop a technically simple process for the production of quicklime starting from finely divided CaCO ₃ starting material, wherein the quicklime has good performance properties.

• a) eine feinteilige Mischung aus 80 bis 95 Gew.-% Calciumcarbonat (CaCO₃) und 5 bis 20 Gew.-% Kohlenstoff nach dessen Homogenisierung mit 1 bis 10 Gew.-% CaO und/oder Ca(OH)₂ granuliert und anschließend
• b) das in Stufe a) erhaltene CaCO₃-Granulat trocknet, den im Granulat enthaltenen Kohlenstoff mit Hilfe von Luftsauerstoff zu CO₂ oxidiert und anschließend das CaCO₃-Granulat unter CO₂-Abspaltung zum Branntkalk (CaO) calciniert.

This object has been achieved in that one

• a) granulated a finely divided mixture of 80 to 95 wt .-% calcium carbonate (CaCO ₃ ) and 5 to 20 wt .-% carbon after homogenization with 1 to 10 wt .-% CaO and / or Ca (OH) ₂ and then
• b) the CaCO ₃ granules obtained in step a), oxidizes the carbon contained in the granules with the aid of atmospheric oxygen to CO ₂ and then the CaCO ₃ granules with CO ₂ cleavage to quicklime (CaO) calcined.

It has surprisingly been found here that the finely divided CaCO ₃ starting material can be calcined very well and, above all, inexpensively by the granulation (stage a)) and by the presence of the carbon, since the amount of carbon present in the calcium carbonate starting material is sufficient, to bring the lime to deacidification temperature and carry out a cost-effective calcination.

In the method according to the present invention, therefore, a finely divided mixture of 80 to 95 wt .-% calcium carbonate (CaCO ₃ ) and 5 to 20 wt .-% carbon in step a) granulated and then the obtained in step a) CaCO ₃ granules in step b) calcined.

The finely divided mixture of calcium carbonate and carbon has this a preferred average particle size of 10 to 50 microns, and in particular 10 to 30 μm on.

According to one preferred embodiment as a finely divided mixture of calcium carbonate and carbon black lime used in the large-scale Production of cyanamide and dicyandiamide from calcium cyanamide Acid hydrolysis with carbon dioxide is obtained and preferably a residual moisture from 18 to 22% by weight.

It is to be regarded as essential to the invention that the granulation in the presence of 1 to 10 wt .-%, preferably 2 to 5 wt .-%, CaO and / or Ca (OH) ₂ is carried out as a binder, said inorganic binder before granulation is homogenized with the finely divided mixture of calcium carbonate and carbon.

The homogenization, ie the uniform incorporation of the binder in the finely divided Mi From calcium carbonate and carbon can be carried out in the usual equipment, preferably in an intensive mixer. According to a preferred embodiment, this homogenization is carried out with the aid of a star vortexer at a speed of 500 to 1000 revolutions per minute, wherein at a residence time of 1 to 5 minutes, the CaO and / or Ca (OH) _{2 is} stirred into the finely divided mixture of calcium carbonate and carbon becomes.

It especially in mixtures of calcium carbonate and carbon with a relatively low moisture content as particularly advantageous proved that during the homogenization still 0.5 to 5 wt .-%, in particular 1 to 2 wt .-% Water, based on the total weight of calcium carbonate and carbon admits.

conditioned increases by the reaction of quicklime with the added water the temperature of the mixture to about 45 ° C, wherein a microgranulate forms, which can be granulated particularly well.

The Granulation of the invention used Starting materials is relatively uncritical and can with the usual Known devices are easily performed. In the granulation have the usual Pelleting and granulating particularly proven, preferably temperatures from 10 to 80 ° C, in particular 20 to 50 ° C, be set.

To granulation, the corresponding material has a medium Particle size of 1 to 10 mm, in particular 3 to 7 mm, on.

In the subsequent stage b), the CaCO ₃ granules are dried, the oxidation of the carbon to CO _{2 is} carried out and the CaCO ₃ granules are calcined, these partial steps preferably being carried out continuously in a multiple-hearth furnace.

Of the Multi-level ovens represent a multi-level type of oven through openings on the edge or near the center connected to each other. Located in the center of the round bunker usually a rotating shaft, attached to the four arms on each floor are. The air flows through the shaft and arms and at the same time is cooled, which is why these parts can be made of steel. On the arms of the bunker are cast iron shovels, the burning calcium carbonate material in each floor of Promote edge to the center and over the center-near opening in drop the underlying floor.

The Promote arms in this floor then the material to the outside, where the opening lies to the bottom. This process principle will have several Floors practiced, in which case in the last floor of the discharge into a cooling screw and in a collection container he follows.

The drying of the CaCO ₃ granules is preferably carried out at a temperature of 20 to 400 ° C, while the oxidation of the carbon by means of atmospheric oxygen to CO ₂ at temperatures of 400 to 800 ° C is performed.

It is readily possible in the context of the present invention, during the Oxidation of the carbon in stage b) still CO-containing gas, eg. to feed the deck oven, to set the temperature targeted to 400 to 800 ° C in this way.

The actual calcination step in step b) is carried out in particular at temperatures of 900 to 1000 ° C. It has proved to be particularly advantageous that leads the exhaust gases in stage b) in countercurrent to CaCO ₃ granules. The exhaust gases are pre-cleaned by the fine dust, which is why the subsequent dust collector can be kept relatively small. The countercurrent principle also heats the freshly supplied CaCO ₃ granules and cools the exhaust gases of the firing process. This results in a very good energy utilization and an apparatus optimization, since smaller apparatus dimensions are required for the subsequent cooling of the exhaust gases.

The execution the individual sub-steps in step b) in the multi-level oven is with several Benefits connected. In the multi-level oven can be very large cross-sections be realized, resulting in very low gas velocities, which cause dust separation in the separation gas.

Of the Floor oven allows Furthermore, a flexible and accurate temperature setting and thus a very good relationship of soft and hard burned. Likewise, the firing of CO fuel gas with its low calorific value in the multi-level oven readily possible.

Finally, can the drying and calcination are carried out in an apparatus, without this the granules is destroyed, causing the apparative Effort is kept relatively low.

To the calcination step falls quicklime in the form of granules and fine material, the one CaO content of at least ≥ 90%.

The proportion of granules having an average particle size of 1 to 10 mm after calcination is about 70 to 90 wt .-%, while the fine material with an average particle size of <1 mm in an amount of 10 to 30 wt .-% is obtained.

It has proved to be particularly advantageous that the exhaust gases resulting from the drying, consisting of H ₂ O vapor, dust and small amounts of CO ₂ are purified by separation of the dust.

It is within the scope of the present invention also readily possible that one cools the exhaust gases from the carbon oxidation and calcination, consisting of CO ₂ gas with the aid of heat exchangers to 200 to 300 ° C, the dust removed by a dry gas cleaning and the cleaned CO ₂ gas after cooling to 20 to 40 ° C and optionally the compression of a recycling feeds.

Of Furthermore, it is particularly advantageous that the waste heat from the exhaust gases of the carbon oxidation and calcination for steam generation uses.

With Help of the method is it is thus possible finely divided calcium carbonate with poor performance Calcine properties to relatively pure quicklime, by the combustion of carbon and the optimal use of energy the exhaust gases is given a particularly high efficiency.

embodiment

From a production for the production of dicyandiamide, 12 t of black lime per hour in the form of a moist powder having the following composition (in wt .-%): H ₂ O 18%

After deduction of the water content, the following quantity distribution is still present: CaCO ₃ 73.3% Ca (OH) ₂ 14.0% ash 2.6% carbon 9.4% sulfur 0.6%

The fineness of the powder is
KGA X10 - value 5 μm
KGA X50 - value 22 μm
KGA X90 - 54 μm

With Conveyor belts will The powder was driven in a continuous mixer.

• – Durchmesser = 8.000 mm
• – Etagenanzahl = 12
• – Einen Stahlmantel mit 345 mm Feuerfestausmauerung.
• – Die Etagenböden sind aus Formsteinen mit Feuerfestmaterial als selbsttragende Kuppel ausgebildet.
• – Einer hohlen Zentralwelle mit je vier hohlen Kratzarmen je Etage und Kratzzähnen aus Gusseisen.
• – Zentralwelle und Kratzarme sind so ausgebildet, dass sie mit Luft durchströmt werden können und damit gekühlt werden.
• – Die Hohlwelle wird mit einem Getriebe mit Elektromotor P = 50 kW angetrieben.
• – An den Etagen 2 bis 12 sind jeweils zwei Gasbrenner mit einer Leistung von jeweils 820.000 KJ/h angebracht.
• – Die jeweiligen Luftgebläse sind installiert für die Zentralwellenkühlung m = 22.343 kg/h, für die Gasverbrennung m = 4.825 kg/h und Kohlenstoff-Oxidation m = 3.784.

Here, CaO is homogenized with an amount of 4% by weight of the black lime and 1.5% by weight of water (in each case based on the weight of the black lime) and microgranules are produced. The mixer transfers the micro-granules to a second conveyor belt. This transports the micro-granules to the granulator. Here, the grain size formation takes place up to an average diameter of 6 mm. The granulator promotes the granules formed on another conveyor belt. Here, the material flow is divided into three equal partial flows. The three product streams arrive with further conveyor belts and Doppelpendelkiappen in three deck ovens. Each of the three deck ovens is designed as follows:

• - Diameter = 8,000 mm
• - number of floors = 12
• - A steel jacket with 345 mm Feuerfestausmauerung.
• - The shelves are made of molded bricks with refractory material as a self-supporting dome.
• - A hollow central shaft with four hollow scratching arms per level and cast-iron scratching teeth.
• - Central shaft and scraper arms are designed so that they can be flowed through with air and thus cooled.
• - The hollow shaft is driven by a gearbox with electric motor P = 50 kW.
• - On floors 2 to 12, two gas burners each with a capacity of 820,000 KJ / h are installed.
• - The respective air blowers are installed for central shaft cooling m = 22.343 kg / h, for gas combustion m = 4.825 kg / h and carbon oxidation m = 3.784.

The following process steps take place in the individual floors per oven:
The 40 ° C warm granules are dried on the first floor at a gas temperature of up to 700 ° C. In this case, an amount of water of 750 kg / h with the exhaust gas T = 760 ° C discharged. In the next levels 2 to 8, the carbon m = 322 kg / h is oxidized. The granule temperature moves at 650 ° C. In levels 9 to 11 the calcination takes place at a gas temperature of 980 ° C. In the process, 2,962 kg / h of CO2 are transferred into the exhaust gas. On the 12th floor the granulate precooling and heating of the oxidation air for the carbon oxidation takes place.

The quantity balance for the residue lime granules is composed as follows per kiln: CaCO ₃ 2,504 kg / h Ca (OH) ₂ 480 kg / h ash 91 kg / h C 322 kg / h Total 3,417 kg / h H ₂ O 750 kg / h total 4,146 kg / h

The quantity balance for the calcium oxide granules is composed as follows for each furnace: CaO 1,767 kg / h ash 91 kg / h total 1,857 kg / h

The quantity balance for the exhaust gases from the respective furnaces is made up as follows: CO ₂ 2,962 kg / h H ₂ O 1,408 kg / h N ₂ 6,614 kg / h O ₂ 146 kg / h total 11,171 kg / h

The Calcium oxide granules are in cooling screws after leaving the deck ovens to a temperature of 40 ° C cooled. Then be They are loaded directly into the wagon and truck or stored in silos.

The exhaust gases leave the ovens at two openings per oven. At the first floor 1000 m ³ / h are deducted. This quantity is cooled with condensate to 200 ° C. Afterwards, the exhaust gas on bag filters is cleaned of dust and cooled to 40 ° C with a heat exchanger. The second amount of exhaust gas 7000 m ³ / h is withdrawn at the second floor and driven through a heat exchanger. The heat is removed from the exhaust gas and used for steam production. Both exhaust streams are again compressed and made available to the dicyandiamide system for reaction. The generated steam is also tendered dicyandiamide plant and used in the process.

Claims (22)

Process for the preparation of quicklime (CaO), characterized in that a) a finely divided mixture of 80 to 95 wt .-% calcium carbonate (CaCO ₃ ) and 5 to 20 wt .-% carbon after its homogenization with 1 to 10 wt. -% CaO and / or Ca (OH) ₂ and then granulated b) the CaCO obtained in step a) _3-dried granulate, the carbon contained in the granules with the aid of atmospheric oxygen to form CO ₂ oxidized and then the granulate CaCO ₃ under CO ₂ cleavage to quicklime (CaO) calcined. Method according to claim 1, characterized in that that the finely divided mixture of calcium carbonate and carbon an average particle size of 10 up to 50 μm, in particular 10 to 30 μm, having. Method according to claim 1 or 2, characterized that the finely divided mixture of calcium carbonate and carbon has a residual moisture content of 18 to 22 wt .-%. Method according to one of claims 1 to 3, characterized that as a finely divided mixture of calcium carbonate and carbon black lime used in large-scale production of cyanamide and dicyandiamide from calcium cyanamide by acid hydrolysis obtained with carbon dioxide. Method according to one of claims 1 to 4, characterized in that during the homogenization of the mixture of calcium carbonate and carbon 2 to 5 wt .-% CaO and / or Ca (OH) ₂ added. Method according to one of claims 1 to 5, characterized that the homogenization is carried out in an intensive mixer. Method according to one of claims 1 to 6, characterized that the homogenization with the help of a star vortex in a Speed of 500 to 1,000 revolutions per minute. Method according to one of claims 1 to 7, characterized that one while the homogenization still 0.5 to 5 wt .-%, in particular 1 to 2 Wt .-% water, based on the total weight of calcium carbonate and carbon, admits. Method according to one of claims 1 to 8, characterized that the granulation is carried out using a conventional pelletizing plate. Method according to one of claims 1 to 9, characterized that the granulation at temperatures of 10 to 80 ° C, in particular 20 to 50 ° C is made. Method according to one of claims 1 to 10, characterized in that in the granulation a CaCO ₃ granules having an average particle size of 1 to 10 mm, in particular 3 to 7 mm, is formed. Method according to one of claims 1 to 11, characterized in that one carries out the drying of the CaCO ₃ granules, the oxidation of the carbon and the calcination of CaCO ₃ granules in step b) continuously in a multi-deck oven. Method according to one of claims 1 to 12, characterized that one carries out the drying in step b) at a temperature of 20 to 400 ° C. Method according to one of claims 1 to 13, characterized in that the oxidation of the Carbon in stage b) is carried out at temperatures of 400 to 800 ° C. Method according to one of claims 1 to 14, characterized that one while the oxidation of the carbon in stage b) still supplies CO-containing gas to the temperature at 400 to 800 ° C adjust. Method according to one of claims 1 to 15, characterized that the calcination step in step b) at 900 to 1000 ° C is carried out. Method according to one of claims 1 to 16, characterized in that the exhaust gases are passed in step b) in countercurrent to the CaCO ₃ granules. Method according to one of claims 1 to 17, characterized the CaO content of the quicklime is ≥ 90%. Method according to one of claims 1 to 18, characterized that the quicklime after calcination to 70 to 90 wt .-% in Form of granules with an average particle size of 1 to 10 mm and 10 to 30 wt .-% as a fine material with a medium Particle size of <1 mm is obtained. Method according to one of claims 1 to 19, characterized in that the waste gases resulting from the drying, consisting of H ₂ O vapor, dust and small amounts of CO ₂ , are cleaned by removal of the dust. Method according to one of claims 1 to 20, characterized in that the exhaust gases from the carbon oxidation and calcination, consisting of CO ₂ gas, cooled by means of heat exchangers to 200 to 300 ° C, the dust removed by a dry gas cleaning and the cleaned CO ₂ gas after cooling to 20 to 40 ° C and, if necessary, the compression of a recycling can be supplied. Method according to claim 21, characterized that you have the waste heat from the exhaust gases from the carbon oxidation and calcination to Steam generation uses.