FR2494131A1 - Processing and drying thixotropic prod. contg. solid impurities - e.g. calcium hydroxide, by kneading, homogenising, filtering or sieving and drying - Google Patents

Abstract

A process for treating and drying a thixotropic prod. contg. solid impurities in the form of insoluble grains, e.g. impure Ca(OH)2 as occurs in the residues of acetylene prodn., comprises (a) kneading the gel to render it liq. making use of its thixotropic properties, with opt. addn. of water to obtain a sufficiently low viscosity; (b) homogenising several batches of the liq. prod. in a mixing buffer tank; (c) removing the solid impurities of size above a certain value by filtration or sieving; (d) dessicating by filtering; (e) crumbling and (f) drying in a heated drier where the combustion gases do not come into contact with the prods. to be dried. The process enables chemically useful prods. to be recovered from slurries which are otherwise rejected as useless. Unlike prior art, it does not require a large amt. of water or result in the Ca(OH)2 coming into contact with CO2 in the combustion gases which might lead to undesired CaCO3 formation.

Description

Technical area.

 The present invention relates to a process for treating and drying a thixotropic product comprising solid impurities in the form of insoluble grains, such as calcium hydrate.

Problem.

 Certain manufacturing residues are rejected in the dumpster and, although chemically useful for specific uses, they cannot be used practically because of the impurities which they contain and especially of their important water content. To recover them, it would be necessary to be able to purify them and dry them economically.

 This is the case in particular of slaked lime slurry or calcium hydrate of formula Ca (OH) 2 which are the residues from the manufacture of acetylene. They are discharged to the landfill in the form of thick sludge containing at least 50% water.

 The problem is therefore essentially to dry this product and incidentally to rid it of its undesirable solid impurities.

State of the prior art and drawbacks.

 To remove water, there are two conventional techniques - on the one hand, filtration, which requires starting with a relatively fluid product; it removes a large part of the batten, - on the other hand, drying in driers which require fuel consumption which is expensive especially under current economic conditions.

 In addition, most dryers, especially rotary dryers with burners and smoke circulation in the opposite direction to the flow of products, involve contact of the combustion gases with the products themselves.

This type of dryer is inapplicable with calcium hydrate because the carbon dioxide resulting from the combustion of the burner reacts on Ca (HO) 2 to produce calcium carbonate (COCa) and we deteriorate the product we want get.

 In addition, the slurries of calcium hydrate, or slaked lime, are thixotropic, that is to say which pass from the state of gel to that of liquid by a light agitation.

 The object of the present invention is to remedy the drawbacks previously explained by using in particular the thixotropic property of slaked lime sludge.

 It is understood that the process of the invention is not limited to calcium hydrate sludge but that it can be generalized, without departing from the scope of the invention, to all sludges having this thixotropic property.

Statement of the invention.

 The invention consists essentially in subjecting the recovered products to the following successive operations a) sufficient mixing of the gel to bring it to the liquid state using its thixotropic ability with, optionally, addition of water to obtain a sufficiently low viscosity , b) homoeneisation, in a buffer tank with mixer, of several supplies of liquid product after mixing, c) filtration or screening for elimination of solid impurities of size above a certain size, d) desiccation filter, e) cleaning, f) drying in a heated drier where the combustion gases do not come into contact with the products to be dried.

 For infiltration, either a filter press or a rotary filter is used.

Solution to the problem, advantages and industrial result.

 Mixing using the thixotropic properties of calcium hydrate mud can avoid the addition of water and in any case makes it possible to use a fluid product containing a relatively low proportion of water, the fluidity of the product allowing suitable homogenization. by mixing. Drying in the filter allows a relatively dry material to be dried later, saving the energy required for this drying. Homogenization facilitates regular and metered distribution of the product in the filter or screen, then in the filter press or the rotary filter and finally in the oven. The crumbling after drying in the filter divides the product to obtain a large contact surface when drying in the oven.

Clarification on how to proceed.

 In general, calcium hydrate sludges from the manufacture of acetylene have a water content of at least 50. During the dilution this content can increase strongly but the dilution and the kneading, developing the thixotropic qualities of the mud, considerably decrease the viscosity to favor the screening and the elimination of solid impurities whose size depends on the screen mesh used. After the filter press or the rotary filter, a product is obtained having a water content of approximately 30 days.

 The crumbling of boats leaving filter presses or rotary filters is carried out in trommels.

 Drying must be carried out with a gas stream hungry for water and free of carbon dioxide. This eliminates all dryers whose drying gas is heated directly by a burner. It is compulsory to have a separate combustion chamber and to circulate, for example, hot air. The yield can be improved by any useful means; for example, using gases other than air, provided that it is a gas which does not react on calcium hydrate; we can also use the vacuum.

Claims (3)

 1. Process for treating and drying a thixotropic product containing solid impurities in the form of insoluble grains, such as impure calcium hydrate as it exists in acetylene manufacturing residues, for example, character risd by the fact that the product is subjected to the combination of the following successive operations a) sufficient mixing of the gel to bring it to the liquid state using its thixotropic ability with, optionally, addition of water to obtain a sufficiently viscosity weak, b) homogenization, in a buffer tank with mixer, of several supplies of liquid product after mixing, c) filtration or screening for elimination of solid impurities of size above a certain size, d) desiccation with the filter, e) crumbling, f) drying in a heated drier where the combustion gases do not come into contact with the products to be dried.  2. A method of treatment and drying as defined in claim 1, c a r a c t é r i sé by the fact that a filter press is used.  3. A method of treatment and drying, as defined in claim l, c a r a c t é r i sé by the fact that a rotary filter is used.