MX2008008239A - Milling process - Google Patents

Abstract

Process for milling a substance selected from sodium carbonate, sodium bicarbonate and sodium sesquicarbonate or trona, in which a cleaning agent is introduced into a mill for the purpose of obtaining a powder with a mean diameter of less than 100µm and of inhibiting the formation of incrustations in the mill.

Description

MILLING PROCESS The present invention relates to the grinding of substances selected from the group formed by sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona for the continuous production of fine powders. The invention is directed more particularly to avoid the incrustation of the facilities used to grind said substances and / or to manipulate the powders obtained from the grinding. Substances such as sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona are commonly used in the form of fine powders, for example as reagents in the decontamination of combustion gases. When the reagent is introduced into the combustion gas, it must react quickly with the impurities that the gas contains in order to obtain the desired efficiency. It is known that fine powders having an average particle size of less than 100 μm, with advantages of less than 50 μm, have a greater contact area with the combustion gas to be decontaminated and react faster. Therefore, these are preferred. In order to obtain these fine powders, an intensive grinding of the starting substances is required, preferably by means of an impact mill.

However, in certain particular impact type mills, when these are used to produce powders, for example sodium bicarbonate powder whose diameter is less than 100 μm, scale formation has been observed. It is understood that the term incrustation refers to an accumulation of ground material stuck to the wall of the mill and / or the equipment for handling the ground powder. Embedding formation has also been observed in facilities for the handling of such fine powders of sodium bicarbonate, especially in particle size classifiers located downstream of the mill, or in pneumatic installations used to transfer the ground powder to a silo or a industrial plant. The incrustation in the mills and in the powder handling facilities is obviously a disadvantage, and forces the periodic stoppage of the grinding and handling facilities for cleaning and for the removal of the formed incrustations. DE 10 357 426 discloses a grinding process in which trimethylolpropane (TMP) is introduced into the mill. However, the applicant has found that TMP can contaminate ground bicarbonate, and its presence in the bicarbonate product can cause problems in certain applications.

The object of the invention is to remedy the disadvantages mentioned above, which arise when grinding or handling sodium bicarbonate powders. More particularly, the object of the present invention is to provide a process for grinding a substance selected from the group consisting of sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona to a powder state with a smaller particle size of 100 μm, and in which scale formation is prevented in the mill and in the powder handling facilities downstream thereof, the accelerated wear of the mill is avoided, and the risks of contaminating the obtained product are reduced. Accordingly, the invention relates to a process for grinding a substance selected from the group consisting of sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona, in which a cleaning agent is added to the substance and the mixture is introduced into the mill in order to obtain a powder with a mean particle diameter of less than 100 μm and inhibit the formation of scale in the mill, the process is characterized in that the cleaning agent is selected from the group consisting of zeolites, dolomite, magnesium hydroxycarbonate, lime, hydrocarbons, talc, fatty acids, and salts of fatty acids.

In the process according to the present invention the expression "sodium carbonate" invariably refers to anhydrous sodium carbonate (Na2C03), or sodium carbonate hydrate, especially sodium carbonate monohydrate (Na2C03 -H20), sodium carbonate heptahydrate (Na2C03 • 7H20), or sodium carbonate decahydrate (Na2C03-lOH20). The average particle diameter of the powder is defined by the equation: in which neither represents the frequency (based on the weight) of the particles of diameter D. These particle size parameters are obtained by the laser scattering analysis method using a MASTERSIZER S measuring instrument manufactured by Malvern, used in wet mode with the accessory MS 17 DIF 2012. In a preferred embodiment of the present invention, the Grinding is carried out in an impact mill. Within the context of the present invention, impact mills are mills in which the material to be grinded is subjected to impact with movable mechanical parts, said impact having the effect of fragmenting the particles of the material. Mills and impact are well known in the art of fine grinding. In particular, these include (in the following non-exhaustive list): hammer mills, spindle mills, attrition mills, ball mills, and cage mills. The preferred ones are hammer mills. Hammer mills manufactured by Grinding Technologies and System SRL (model MG), by Officina 2000 SRL (model RTM), or by Hosa Alpine AG (model APP) are very appropriate. According to an essential feature of the invention, the substance to be ground is mixed with a cleaning agent. Within the context of the present invention, the term "cleaning agent" has a general definition and is related to a material capable of removing scale from sodium carbonate, sodium bicarbonate, and / or sodium sesquicarbonate or trona, present in an installation industrial, mainly in a metal installation. Such substances, for example lime, are known to clean the embedded mills. Prior to the invention, these substances were periodically introduced into the mill when unacceptable scale appeared. According to the present invention, the cleaning agent is preferably introduced continuously into the mill during its operation, and this prevents the occurrence of scale. Surprisingly, the efficiency of the mill is not affected by it. It is often improved, particularly when the substance to be ground is sodium bicarbonate, in the sense that it increases the productivity of the mill. In the process according to the present invention, the cleaning agent is added and mixed with the substance before and / or during milling. It is preferable to add it before grinding. In the process according to the present invention, the substance to be milled is normally in the form of solid grains. The particle size of said substance is not critical for the definition of the invention, but in practice it determines the most appropriate choice of the mill. It is necessary to avoid too thick a particle size in order to reduce the cost of grinding. It is also necessary to avoid too fine a particle size to avoid the progressive agglutination of the particles of the substance while it is in storage. In practice, it is recommended for the substance to select a particle size characterized by an average diameter Dm greater than twice, preferably between 5 and 10 times greater, than the average diameter of the powder obtained. Particularly recommended particle sizes have a mean diameter Dm between 150 and 250 μm. In the process according to the present invention the cleaning agent comprises at least one composition selected from the group consisting of certain abrasives, hydrocarbons, and fatty acids, or salts of fatty acids. When the cleaning agent comprises a solid abrasive, the abrasive is selected from the group consisting of silicates, oxides of metals of group 2a, hydroxides of metals of groups 2a, salts of metals, and in particular salts of metals of group 2a, ashes of combustion (the ash carried by the combustion gases, in particular in the combustion of household waste), cellulose, and starch. Zeolites, silica, dolomite, magnesium hydroxycarbonate, lime, sodium chloride, zinc chloride, sodium sulfate, and calcium fluoride are preferred. Zeolites, dolomite, magnesium hydroxycarbonate, and lime are particularly preferred. It has been observed that such substances, although they are abrasive, do not damage the grinding equipment by wear.

In one embodiment of the process according to the present invention, which is especially advantageous, the cleaning agent comprises a hydrocarbon such as fuel oil, a fatty acid or fatty acid salt selected from the group consisting of stearic acid, calcium stearate , magnesium stearate and soaps such as Marseille soap. Soaps, and in particular stearates, are preferred. The cleaning agent may comprise several of the substances mentioned. Advantageously, it consists entirely of one or more of these substances. In the process according to the present invention, the cleaning agent must be used in at least an amount sufficient to inhibit scale formation in the mill and, possibly, to ensure its removal from scale if the mill is fouled. In practice, the amount of cleaning agent to be used depends on the cleaning agent selected and the operating conditions of the mill (production, temperature, etc.). This must be determined in each particular case by routine tests. In general, it is noted that in most cases it is desirable that the cleaning agent be used in an amount greater than 0.05 (preferably at least 0.1) parts by weight per 100 parts by weight of the substance to grind. Although in principle there is no upper limit on the amount of cleaning agent used, in practice however it is useless to use too large a quantity for cost reasons. Therefore, in practice it is recommended that the amount of cleaning agent used does not exceed 20 (preferably 10) parts by weight per 100 parts by weight of substance to be ground. Amounts in the range of 0.2 to 7 gears by weight per 100 parts by weight of said substance are preferred. If the cleaning agent is selected from solid abrasives, the recommended use thereof is in the range of 0.5 to 10 (preferably 1 to 5) parts by weight per 100 parts by weight of the substance to be ground. If the cleaning agent is selected from the group consisting of fatty acids or salts of fatty acids, the recommended use thereof is in the range of 0.15 to 1.0 (preferably 0.2 to 0.5) parts in weight per 100 parts by weight of the substance to be ground. The process according to the present invention avoids the occurrence of scale on the mill and therefore increases its capacity. It has also been observed, in particular when the cleaning agent is a fatty acid or a fatty acid salt, such as stearate, that the particle size of the ground product is more constant. The process of the present invention is particularly applicable for the manufacture of sodium carbonate powders (either anhydrous or hydrated), sodium bicarbonate, and sodium sesquicarbonate or trona, which will be used in the decontamination of combustion gases contaminated with volatile acid compounds (particularly hydrogen chloride). This is because it has surprisingly been observed that the presence of the cleaning agent according to the present invention in the powder for the decontamination of the combustion gases does not impair its effectiveness. This observation is particularly beneficial in the case of salts of fatty acids. Accordingly, the invention is also related to a process for decontaminating a flue gas contaminated with a volatile acid compound, in which a reagent selected from the group consisting of sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona it undergoes a grinding operation to reduce it to powder state with an average particle diameter of less than 50 μm and said powder is injected into the combustion gas stream, the decontamination process is characterized because the grinding is carried out by means of the grinding process according to the present invention, as described above. The combustion gas decontamination process for separating volatile acid compounds by means of a powder reagent selected from the group consisting of anhydrous and hydrous sodium carbonate is well known in the art., sodium bicarbonate, and sodium sesquicarbonate or trona. This process is described in document EP 0740577B1 [SOLVAY (Société Anonyme)]. The decontamination process according to the present invention is particularly applicable to the purification of combustion gases contaminated with hydrogen chloride, particularly the exhaust gases. combustion generated by municipal or municipal waste incinerators and hazardous waste incinerators. It has been observed that the presence of a cleaning agent according to the present invention in the reagent, particularly stearates, ensures a better dispersion of the reactant in the flue gas stream. This increases the effectiveness of decontamination. When the decontamination process according to the present invention is implemented, the powder obtained from the milling operation usually undergoes a classification by particle size before being injected into the combustion gas, said injection usually being carried out by means of of a mechanical blower. The function of sorting by particle size is to divide the powder into fractions of useful particle size (to be injected into the flue gas) and into one or more fractions of particle size finer or thicker than those suitable for injecting in the combustion gas. The classification by particle size can be carried out by means of any known classifier, for example a series of meshes, or an elutriator.

The blower must be designed to transport the powder in an air stream. Substantially it is of the centrifugal type. In the process according to this invention, the blower can be located upstream or downstream of the classification by particle size. Normally, it is located under the grinding current. In the decontamination process according to the present invention, the grinding and sorting are with regulated advantages in such a way that the powder introduced into the combustion gas has a particle size characterized by an average diameter Dm of less than 50 μm and a slope of particle size less than 5, where the particle size slope is defined by the equation: s = (D90 - Dio) / Dm in which D90 (and Dio) represent, respectively, the diameter for which the 90% and 10% respectively of the powder particles (expressed by weight) have a smaller diameter of D90 and D? 0 respectively. This particle size parameter is defined by the laser scattering analysis using a measuring apparatus such as that described above. The preferred particle sizes for the powder to be injected into the flue gas correspond to a mean particle diameter between 10 and 30 μm and a particle size slope between 1 and 3. Additional information corresponding to the optimum particle size parameters can be obtained in the patent EP 0 740 577 Bl [SOLVAY (Société Anony e)] mentioned above. The classifiers by particle size and blowers used in the decontamination process according to the present invention run the risk of being progressively embedded with the reactive powder particles, which represents a disadvantage. As a consequence, the invention is also related to a process for inhibiting the scale of an installation for handling a powder with an average particle diameter of less than 50 μm, selected from the group consisting of sodium carbonate, sodium bicarbonate, and sesquicarbonate of sodium or trona, said process is characterized in that with said powder a cleaning agent is mixed according to those used and preferred in the grinding process according to the present invention. The invention is also related to a process for removing scale in a facility for handling a powder selected from the group formed by sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona, said process is characterized in that in the installation it is introduced a cleaning agent according to those used and preferred in the grinding process according to the present invention.

The process according to the present invention for respectively inhibiting the formation of scale and for descaling, is applicable in installations employing reagents used in the combustion gas decontamination process according to the present invention and which are described above. This applies especially to installations that include a particle size classifier for the treatment of said reagent and a mechanical blower to inject the reagent into the combustion gas to be decontaminated. The following examples serve to illustrate the invention. In these examples, a 0/50 TEC bicarbonate from SOLVAY (Société Anonyme), having an average particle diameter of approximately 200 μm, was used as starting material. Example 1 (not according to the invention) In this example, the aforementioned sodium bicarbonate was ground as such in an ALPINE model 100 UPZ hammer mill, equipped with rotating discs, and under the following conditions: Mill rotor: 15,000 rpm; - mass flow rate of the substance: 3 kg / h.

The grinding was regulated so as to obtain, after the same, a powder of sodium bicarbonate with a mean particle diameter of approximately 10 μm. After 6 hours of operation, the presence of a thick layer of incrustation on the rotor and on the stator of the mill was observed. The reduction of the milling speed to 1 kg / h did not modify the appearance of said incrustations. Example 2 (according to the invention) In this example with the BICAR 0/50 sodium bicarbonate, combustion ashes were mixed in an amount of 5 parts per 100 parts by weight of bicarbonate. The mixture was made in a Lódige laboratory mixer. The homogeneous mixture obtained was ground under the same conditions as in Example 1. After 6 hours of operation, no substantial incrustation could be detected on the rotor and the stator of the mill. Example 3 (according to the invention) The test of Example 2 was repeated using talc as a cleaning agent. After 6 hours of operation it was not possible to detect a substantial incrustation on the rotor and the stator of the mill. Example 4 (according to the invention) The test of Example 2 was repeated, except that 0.2% calcium stearate was used as the cleaning agent. After 6 hours of operation it was not possible to detect a substantial incrustation on the rotor and the stator of the mill, this behavior was maintained after increasing the milling speed to 6 kg / h.

Claims (9)

1. CLAIMS 1. A process for grinding a substance selected from the group consisting of sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona, CHARACTERIZED BECAUSE with the substance a cleaning agent is mixed and the mixture is introduced in a mill with In order to obtain a powder with a mean particle diameter of less than 100 μm and to inhibit scale formation in the mill, the process is characterized in that the cleaning agent comprises at least one compound selected from the group consisting of zeolites, dolomite, magnesium hydroxycarbonate, lime, sodium chloride, zinc chloride, sodium sulfate, calcium fluoride, hydrocarbons, talc, fatty acids, and salts of fatty acids.
2. 2. A process according to claim 1 characterized in that the cleaning agent comprises a fatty acid or a fatty acid salt.
3. 3. A process according to claim 2 wherein said fatty acid or fatty acid salt is selected from the group consisting of calcium stearate, magnesium stearate, and soaps.
4. 4. A process according to any of claims 1 to 3 CHARACTERIZED BECAUSE the powder has a mean particle diameter of less than 75 μm.
5. 5. A process according to any of claims 1 to 4 CHARACTERIZED BECAUSE the cleaning agent is used in an amount between 0.2 and 7 parts by weight per 100 parts by weight of the substance.
6. 6. A process according to any of claims 1 to 5 CHARACTERIZED BECAUSE the substance is used in the form of grains with a mean particle diameter 2 times greater than the particle diameter of the powder to be obtained.
7. 7. A process according to any of claims 1 to 6 CHARACTERIZED BECAUSE the mill is an impact mill.
8. 8. A process according to the preceding claim CHARACTERIZED BECAUSE the impact mill comprises a hammer mill.
9. 9. A process for decontaminating a flue gas contaminated with a volatile acid compound in which a reagent selected from the group consisting of sodium carbonate, sodium bicarbonate, and sodium sesquicarbonate or trona is subjected to a grinding operation for reducing it to a powder with an average particle diameter of less than 50 μm, and said powder is injected into the combustion gas, CHARACTERIZED BECAUSE the milling is carried out by a process according to any of claims 1 to 8.