Technical Field

The invention relates to an apparatus for the granulation of molten alkali hydroxide, in particular for the production of sodium and potassium hydroxide prills.

Background

Alkali hydroxides are basic chemicals used in industry. They are generated by electrolysis of the respective alkali salts, namely chlorides. The caustic solution resulting therefrom is concentrated in several steps. In the final step, the molten alkali hydroxide is cooled down and solidified to form flakes or prills. The world wide production of sodium hydroxide alone amounts to approximately 60 million tons per year.

Forming prills from alkali hydroxides starts with the generation of droplets of molten hydroxide, which are solidified in a cooling medium, conventionally dry air. The droplets are formed at the top of a cooling tower with an airflow inside and solidify on their way down through the tower. As air is a bad heat conductor and its specific heat capacity is low, huge quantities of air are needed for cooling the hydroxide droplets down, this requiring cooling towers of considerable height and large volumes of air. Space requirements of the cooling towers require high investment costs.

Use of a denser cooling medium would lower the space requirements of the cooling facility and the investment costs for a cooling tower. It is therefore an object of the invention to provide a cooling medium with a substantially higher heat capacity than air for cooling down droplets of molten alkali hydroxide to form granules or prills. Metal powders are regularly prepared by spraying liquid metal into cold water. The metal spray solidifies immediately upon contact with water. However, due to the great solubility of alkali hydroxides in water, water cannot be used as a cooling medium.

Organic solvents are used as granulation liquid for solidifying liquid plastics. Alkali hydroxides, however, are very reactive and undergo rapid reactions with a great number of organic solvents. What is needed is a cooling medium that does not react with or dissolve alkali hydroxides, have a reasonably heat capacity and are easy to handle.

Summary

Accordingly, the invention relates to an apparatus for granulation of molten alkali hydroxide comprising

- a feeding line for the molten alkali hydroxide,

- a droplet generator for generating droplets of the molten alkali hydroxide,

- a granulation vessel comprising a liquid granulation medium, which is a non-solvent for the alkali hydroxide, the liquid granulation medium having a boiling point of at least 100°C,

- at least one conveyor for recovering alkali hydroxide granules from the liquid granulation medium,

- a drying unit for the recovered alkali hydroxide granules, and

- a cooling unit for cooling the liquid granulation medium.

The cooling unit can be combined with or represented by a heat recovery unit as the temperature level of the liquid granulation medium is considerably higher than that of the heated cooling air of a prill tower or that of the cooling water of a flaking machine.

Non-solvents for alkali hydroxides are in particular hydrocarbons. Accordingly, hydrocarbons and hydrocarbon mixtures having a boiling point of at least 100°C are preferred as liquid granulation media. In particular, the granulation medium should have a boiling point in between 120°C and 220°C. The hydrocarbons or hydrocarbon mixtures are preferably dearomatized refined petroleum distillates.

The boiling temperature of the liquid granulation medium is of some importance for the process. If too low, losses due to evaporation occur, which require either substitute solvent or costly recovery facilities. If too high, removal of residues of the liquid granulation medium from the alkali hydroxide granules is tedious. The range of 120°C to 220°C has proven to give best results. Moreover, hydrocarbons having a boiling point above 100°C are much less inflammable than low boiling hydrocarbons.

The apparatus of this invention is particularly suited for the preparation of sodium and potassium hydroxide prills. Upon tests, it was found that prilling in hydrocarbons as granulation media gives good results and that residual hydrocarbons on the surface of the resulting prills were reduced successfully below the detection limit by drying with hot air on a grid only.

The apparatus of the present invention requires a feeding line for the molten alkali hydroxide ending in a droplet generator, the latter being arranged on top of a granulation vessel. The droplet generator generally has a number of nozzles for providing droplets of the required size. The droplets fall down into the vessel, which is filled with the liquid granulation medium. There the droplets are cooled down below solidification temperature, solid granules sinking to the bottom. The heat contained in the droplets is transferred to and absorbed by the liquid granulation medium.

A conveyor recovers the granules from the bottom of the granulation vessel and conveys them to a drying unit. The conveyor may be a conventional trough chain conveyor, tube conveyor, bucket conveyor or screw conveyor, a conveyor being preferred, where the granules can lose at least part of their hydrocarbon freight.

According to a preferred embodiment, the apparatus comprises a first conveyor taking up the granules from the bottom of the vessel and delivering them to a second conveyor for transport of the granules to the dryer. The second conveyor preferably is a drip sieve vibration conveyor or conveying the granules into and through the drying unit. Preferably, the first and second conveyors allow adhering liquid granulation medium to drip off and flow back into the granulation vessel.

The drip sieve conveyor forwards the granules to the drying unit, where hot dry air is passed through the conveyor and over the granules. Adhering liquid is evaporated. The drying unit comprises a housing, a hot air generator and a ventilator for removing air loaded with liquid granulation medium. The air containing evaporated granulation liquid is preferably used in a combustion unit for heating the alkali hydroxide to be granulated. However, it is also possible to condensate evaporated granulation liquid in a cooling unit and to feed it back into the granulation vessel.

After drying, the dried granules pass a classifying unit, which preferably consists of a two deck sieve, fractionizing solids into an oversize fraction, a main fraction with the desired prill size and an undersize fraction. The oversize fraction and the undersize fraction are returned into the granulation process.

The granulation liquid dripping off from the granules on the conveyors is collected and transferred back into the granulation vessel via a collecting shaft or channel.

Since the granulation liquid in the granulation vessel heats up in the cause of the granulation process, a cooling circle is required. Preferably, liquid granulation medium from the bottom of the granulation vessel is passed through a cooling unit and recirculated, after cooling, to the top of the granulation vessel. Since the hot granulation medium may contain alkali hydroxide particles, the circulation line contains a filter. The cooling unit is preferably cooled with water. Of course, it is possible, to circulate the liquid granulation medium from the top to the bottom, which may avoid the filter. Moreover, it is possible, to integrate the cooling unit into the granulation vessel.

According to a preferred embodiment, hot granulation liquid, before being passed through the cooling unit, is used to heat the air used for drying the granules passing through the drying unit. This will lower the energy costs for the drying air and cooling costs in the cooling unit.

Also, other forms of heat recovery from the granulation liquid may be integrated in the liquid cicuit like heating steam generation or disctrict heating. The present invention also refers to a process for the granulation of molten alkali hydroxide, which comprises feeding the molten alkali hydroxide to a droplet generator, generating droplets of the molten alkali hydroxide on top of granulation vessel, the droplets falling into the granulation vessel and passing through a liquid granulation medium contained in the granulation vessel, the liquid granulation medium being a non-solvent for the alkali hydroxide, conveying the alkali hydroxide granules from the bottom of the liquid granulation vessel to a drying unit and drying the granules with hot dry air, the liquid granulation medium being cooled by a cooling unit.

Preferably, the hot liquid granulation medium is circulated through a cooling unit. Before being recirculated to the granulation vessel, it is used to heat the drying air in the drying unit. The cooling unit makes use of cooling water.

Collecting the alkali granules from the bottom of the granulation vessel takes place by means of a conventional conveying unit, as described above. Preferably, the conveyor consist of a first conveyor and the second conveyor, both allowing adhering liquid to drip off from the granules. As to the other preferred features, it is referred to the description of the apparatus and the attached drawing. The attached drawing shows a preferred embodiment of a granulation apparatus of the invention.

Brief description of drawings

FIG.1 is a schematic diagram of an apparatus of granulation of molten alkali hydroxide according to the present application.

Detailed description

Figure 1 shows a granulation vessel 2, which is filled with a liquid hydrocarbon having a boiling point above 120°C. The alkali hydroxide melt is fed in at the top through a droplet generator 1 , generally comprising a multitude of nozzles separating the alkali hydroxide melt into individual droplets. The droplets fall down through the liquid hydrocarbon and collect at the bottom of vessel 2, where a first conveyor 3 conveys the solids within a tube 4 to a level above the liquid level within granulation vessel 2. From the top of the first conveyor 3 the granules fall on a second conveyor 5, a dripping sieve conveyor. Liquid hydrocarbon adhering to the granules drips off within conveyor 3 and dripping sieve conveyor 5. Dripping sieve conveyor 5 passes the granules on to the drying unit 6 by means of vibration.

Drying unit 6 comprises a hot air generator 7, a ventilator and a housing 9 allowing to collect air loaded with hydrocarbone vapour.

In drying unit 6 hot dry air is blown in, which passes through the dripping sieve 5 and the granules and takes up remaining liquid hydrocarbon. The exhaust air from drying unit 6 is passed to a combustion unit in a manner well known in the art, for example by an exhaust fan 8 to melt alkali hydroxide. The dried granules are passed on to a classifying unit 10, where the alkali hydroxide is fractioned into an oversize friction, a desired prill size fraction and an undersize fraction. The oversize and the undersize fraction are returned into the alkali hydroxide melt stream.

In order to cool the liquid hydrocarbon heated up by the granulation process, hot hydrocarbon is withdrawn from the bottom of vessel 2 and pumped by pump 11 through a recirculating filter 12 and air/liquid heat exchanger 13 at the top of the drying unit. Air/liquid heat exchanger 13 heats the drying air for the drying unit 6. The partly cooled down liquid hydrocarbon then is passed on to heat exchanger 14, where the liquid hydrocarbon is further cooled down by means of cooling water, before being passed on to the top of granulation vessel 2.

The letter M designates a driving motor.

It is to be understood that the embodiment of figure 1 is a preferred embodiment of the invention, other embodiments with features, that serve the same purpose, being possible and embraced, especially that of possible heat recovery.