Method for producing salts with a reduced water of crystallisation content

Abstract

The invention relates to a method for drying salts with a water of crystallisation content in convective apparatuses that can additionally be indirectly heated. The invention is primarily characterised in that the drying process takes place with a moisture content of the drying gas above a determined level. The gas surrounding the salt particles during the drying process therefore has a specific humidity. In this way, the rate of drying is positively influenced.

Claims

1. A method for drying moist salts containing water of crystallisation, comprising drying the moist salts with a hot drying gas having a moisture content and temperature in a convective dryer whereby water is evaporated from the moist salts as the moist salts are dried and said evaporated water increases the moisture content of the drying gas in the dryer; discharging the moist drying gas from the dryer into a loop that fully returns the discharged drying gas into the dryer; in said loop, removing moisture from the discharged moist drying gas by condensation at a controllable condenser temperature; and maintaining an absolute moisture content of the drying gas in the dryer above a predefined moisture content value by controlling the moisture content of the drying gas that returns to the dryer through said loop, in dependence on controlling the temperature of a condenser.

2. The method according to claim 1, wherein the moisture content in the drying gas in the dryer is maintained above said predefined moisture content value, in dependence on sensing a rate of evaporation of water from said moist salts and the temperature of the drying gas.

3. The method according to claim 2, wherein said absolute moisture content is at least 10%.

4. The method according to claim 2, wherein the moist salts containing water of crystallisation is iron sulphate heptahydrate; said moist salts are dried to a monohydrate product having a bulk density of more than 1100 g/I; the moisture of the drying gas increases as the drying gas passes through the dryer until it is discharged; and the absolute moisture content of the drying gas is greater than 15% when it is discharged from the dryer.

5. The method according to claim 1, wherein the moisture content in the drying gas in the dryer is maintained above said predefined moisture content value, in dependence on additional heat supplied to the dryer through heat exchanger surfaces.

6. The method according to claim 5, wherein said absolute moisture content is at least 10%.

7. The method according to claim 5, wherein the moist salts containing water of crystallisation is iron sulphate heptahydrate; said moist salts are dried to a monohydrate product having a bulk density of more than 1100 g/I; the moisture of the drying gas increases as the drying gas passes through the dryer until it is discharged; and the absolute moisture content of the drying gas is greater than 15% when it is discharged from the dryer.

8. The method according to claim 1, wherein said absolute moisture content is at least 10%.

9. The method according to claim 8, wherein the moist salts containing water of crystallisation is iron sulphate heptahydrate; said moist salts are dried to a monohydrate product having a bulk density of more than 1100 g/I; the moisture of the drying gas increases as the drying gas passes through the dryer until it is discharged; and the absolute moisture content of the drying gas is greater than 15% when it is discharged from the dryer.

10. The method according to claim 1, wherein the moist salts containing water of crystallisation is iron sulphate heptahydrate; said moist salts are dried to a monohydrate product having a bulk density of more than 1100 g/I; the moisture of the drying gas increases as the drying gas passes through the dryer until it is discharged; and the absolute moisture content of the drying gas is greater than 15% when it is discharged from the dryer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following figures, the variants of the method are described using a fluidized bed dryer as an example. These figures also apply by analogy to the other types of dryer mentioned above.

(2) FIG. 1 shows a system according to the invention with a flow of recirculated material,

(3) FIG. 2 shows a system according to the invention with partial gas recirculation, and

(4) FIG. 3 shows a system according to the invention with full gas recirculation.

DETAILED DESCRIPTION

(5) The supply air fan 10 (see FIG. 1) pushes ambient air through a heater battery 11 into the drying device (in this case a fluidized bed dryer) 12, in which the salt can be heated optionally by the heat exchanger 13, and releases the water of crystallisation. The moist drying gas leaves the dryer, the dust is removed from the gas in a filter or cyclone 14, then the drying gas passes through the exhaust air fan 18 into the fresh air 19 after undergoing optional, additional gas cleaning. The salt 20 with a moist surface and containing water of crystallisation is coated with the dust separated in the cyclone or filter 14 and optionally with recirculated product in the mixer 21 and fed to the dryer 12. After the drying process, the salt leaves the dryer 12 and the oversized grains are removed on an optional screen 22, ground 23 and fed to the mixer 21. The accept fraction enters an overflow silo 24, from which recirculated product for the mixer 21 is taken optionally by a dosing screw 25 and to which the final product 26 is fed for further processing.

(6) FIG. 2 shows the variant of the drying process with partial gas recirculation in order to set a defined gas moisture content, i.e. a defined moisture content in the dryer. The product is treated in the same way as described in FIG. 1, but is not shown here for the sake of maintaining clarity. The fan 10 presses the gas in the circulating system through the heater battery 11, dryer 12 and dust remover 14. A partial flow of fresh air, which is cleaned with the optional filter 15, is added in metered doses with the aid of the throttling device 16. In this way, the moisture content of the gas is set in the dryer 12. The corresponding amount of moist waste gas is discharged through the pressure-controlled throttling device 17 on the discharge side of the fan. This flow of waste gas 19 also contains the evaporated water from the product. The smaller the partial flow of fresh air, the higher the moisture content in the gas.

(7) If the circulating system is completely closed (see FIG. 3), the evaporated water must be removed selectively from the circulating gas. This can usefully be achieved by condensation in a surface or a wash condenser 30. The wash water is pumped 31 through the circulating system and the condensation heat is removed via the cooler 32. The volume of condensed water, which also corresponds to the volume of water that has evaporated from the product, is removed from the process with level control 33. With the aid of the cooler 32, the temperature of the wash water can be set such that the initial moisture content needed in the circulating gas results from the partial steam pressure of the water. The initial moisture content is calculated from the desired moisture content in the waste gas less the proportion generated by water evaporation from the product. The circulating gas is conveyed in a loop by the fan 10 through the heater battery 11, the dryer 12, the dust separator 14, and the washing condenser 30. The product route is the same as is shown in FIG. 1, but is not repeated here for the sake of maintaining clarity.

(8) The systems shown are only intended as examples and are illustrated using a fluidized bed dryer (with or without built-in heat exchanger) as an example. However, rotary dryers (drum dryers), flash tube dryers or fluidized bed spray granulators could also be used.