INTRODUCTION

Since the early days, man has been using common salt as food additive in the food to improve its taste and to meet the physiological requirements of the human body. During the coarse of development of the society as man needed a wide range of material to meet its need it became necessary to develop range chemicals that would meet these requirements. Salt became an important ingredient and raw material for manufacture of a range number of these chemicals. Salt is reported to have over 14000 uses. Some of the major uses are here under :
(1 .)Chloralkali Industry, ( 2.) Textile Dying, (3.) Dyes & Colors, (4.) Food & Fish preservation & processing, (5.) Aluminium Industry, (6.) Soda Ash Industry, (7.) Road Delcing (8.) Animal Consumption, (9.) water softening, (10.) Ice & Cold Storage, (11.) Curing & Tanning of hides & Skins, (12.) Human Consumption.

PROCESS OF SALT REFINING

(a) Raw Salt Feeding, (b) Salt Washing & Milling, (c) Dosing of Additive & Coating, (d) Centrifuging, (e)Drying, Sizing & Conditioning, (g) Packing, (h) Brine Recycle.

a) Raw salt is fed to the raw salt Hopper from the stockyard or directly unloaded from the dumper. Below these hopper there is a feeder arrangement. This feeder feeds material to a conveyor. This conveyor has Magnetic Separators at the feed and discharge point to remove iron pieces like nails, nuts and bolts and other magnetic debris. The conveyor discharges raw salt to the feed hopper of the Wet Mill.

b) Wet Mill grids and control the size of the product and also removes soluble entrapped in grained impurities present in the raw salt crystals. In this process raw salt is milled with brine to achieve the desired result. Salt slurry from the Wet Mills falls by gravity in to a Slurry Tank below. In the slurry tank more brines is added to carry out primary elutriation. Overflow of the Slurry Tank is carried to the setting ponds. Salt Slurry is pumped to the wash tank for main elutriation. The overflow brine from wash tank is taken into a Clarifier for purification and to recycle it back into the system.

c) Salt Slurry from the Wash Tank is pumped to a thickener where the slurry is conditioned for centrifuge. This is a dual purpose Tank that also carries our secondary elutriation as well as thickening of the slat slurry. This slurry is then fed to the centrifuge. Salt cake from the centrifuge is taken up by a Screw Feeder and the mother liquor is discharge to slurry tank below.

d) The salt cake discharged into the Screw Feeder is dosed with Iodine and other additive anti caking agent before it is carried to the Fluid Bed Dryer.

e) The Fluid Bed Dryer is an internationally accepted option for drying salt cake. It is an integrated system involving two section for heating and subsequent cooling the product. Hot air is blown with the help of fan into the salt bed to achieve the required dryness of final product. Ambient air is blown into the cooling section to cool the hot may be packed in sacs and sealed. Salt from the outlet of the dryer is carried up through a Bucket Elevator for sizing and conditioning.

f) The dry salt from the Bucket Elevator is fed to a Gyro Screen. Oversize from the Screen is sent to a Dry Mill (Pin Mill) for regrinding. The sized product is fed to the Blender for conditioning to make it free flowing or imparting properties and composition that may be required. Output from the dry mill is sent back to the screen to avoid any mixing of oversize with the final product.

g) Final product from the Blender is stored in a silo. The silos feed the packing machine to pack the final product to pack the production 1 Kg. to 50 Kg. bulk packing as may be required.

h) Clear brine that overflows from the clarifier is taken to a Brine Tank. Sludge that settles in the Clarifier is discharged into the setting Tank. Brine from the Brine Tank is pumped into a Brine Header. This Brine header distributes brine at predetermined rates to various tanks. Run-off brine from the setting pond is recovered and pumped by a transfer pump back into the process. Sludge that settles at the bottom of the pond is scrapped and disposed of the salt pans or any other approved site.

The close circuit flow of brine is briefed as under.

Brine Header (B-11) to Wash Tank (B-6) to Reaction Tank (E-1) to Clarifier (B-12) and Brine Tank (B-9) and  the Brine Pump (B-10 to B-11). The impurities are withdrawn at the clarifier and discarded.

The material from wash tank (B-6) id received into the Thickener (B-8). Thickener is an equipment where the slurry containing crystalline salt as well as brine is permitted to thickened by controlled setting of salt and uniform withdrawal of brine by overflow for feeding tank into wash tank (B-6). This flow between B-11, B4, B-5 & B6 is also in continuous close circuit.

The attended flow of brine from the brine header  (B-11) on the topside of the thickener  helps to carry upward portion the unwanted material. A Rotary Feed valve is provided at the discharge end of the thickener.  The purpose of this is for uniform discharge of thickened slurry to pushes to the centrifuge.

The Pusher Type Centrifuge (C2) is Equipment, which eliminates moisture from the thickened slurry (received from thickener ). This is continuously operating equipment. The specialty of the equipment is that a high speed equipment is able to receive salt crystals and brine and separate brine and salt. The discharge by the “pushing’ action of the centrifuge for retain the crystal structure of the salt and ensuring minimum breakages of the crystals.
The material from centrifuge is received into the screw feeder (C3). It is a paddle type of conveyor, which achieves certain mixing operation. Iodine solution from Iodine Tank (E4) is comes into the regulated manner by dosing pump (E-5). The product discharged  from screw feeder is almost dried crystalline salt with uniformly distributed iodine compound.

The material from the screw feeder is discharged through a division valve to two section, one of iodine salt section and another to washed salt section (2.5 TPH & 8 TPH respectively).

The two conveyors carrying this salt are not shown in the drawing. The iodized material is then fed into Fluid Bed Dryer of Fluidization Chamber (D5). In this equipment there is a constant flow of hot air through a perforated screen. The salt crystals “float” on the air stream and subjected to drying. Very fine salt is carried always at the discharge of the air and collected in the cyclone (D7). Air sucked by ID Fan (D3) and released into atmospheric exhaust chimney ( D-10).

The hot air for the dryer is supplied by headers, which receives the hot air from heat exchanger (D6). The heat exchanger is supplied by heat with thermic fluid by heater (D-9). Fan (D-1) takes the air through filter (D-8) so that clean air is sending for heating and drying.

C. Dosing Section

The dosing section are essentially for uniform feeding of reaction agent for precipitation of unwanted salt. The Dosing section after the centrifuge is for introducing iodine. The Dosing section consists of reaction tank. (E1), Dosing Tank (E2), Dosing Pump (E3) and this section is for elimination of unwanted salts like Calcium & Magnesium. Iodine Tank (E4) and Dosing Pump (E5) together from iodizing equipment.

D. SIZING CONDITIONING SECTION

In this section the material from dryer is graded and suitable FFA (to introduce free flow characteristics) are added. The material from the dryer is lifted by Bucket Elevator (F1) and fed into Gyro Screen (F2). The oversized particles are taken for regrinding into the pin mill (F5) and fine particles from this is introduced into the main stream into the Bucket Elevator. The Pin Mill contains Aspiration Equipment by Cyclone.

Free flowing additive is added in a regulated manner to feed the screw blender (F3) for processing free flowing salt. The material is stored in the silo for packing in packing machines to large capacity or strip packing Equipment.