DESCRIPTION OF FUNCTIONING OF A CONCENTRATION PLANT WITHOUT VAPOR HEAT RECUPERATION
The concentration plant consists mainly of a final concentrator (1) which is heated by molten salt used as heat transfer medium. The concentrator operates under normal atmospheric pressure. The feed liquor will be supplied by means of a centrifugal pump (2) via a rota-transmitter (3) (rota-meter with magnetic transmission of the float movement and pneum. transmitter of the flow measurement), a pneum. controller/recorder (4) (on which the production capacity will be adjusted) and a pneum. control valve (5). The feed liquor will be distributed by means of one or two collecting pieces in a uniform quantity to each concentrator element (6) of the final concentrator (1). The feed liquor flows along the inner walls of the concentrator element (6) from top to bottom. Counter current to it flows the heat transfer medium from bottom to top in the heating jacket surrounding the concentrator element. The vapor (evaporated water) produced during the concentration process, reaches together with the concentrated caustic soda a horizontal collecting channel (7). (To separate the caustic soda and the vapor, conic baffles are installed in the collecting channel). Guided by the horizontal collecting channel (7), the vapor reaches the attached centrifugal separator (8). There, the caustic drops contained in the vapor will be separated. The cleaned vapors leave the concentrator via the top part of the separator and will be guided by a stack (9) over the roof into the open air. The concentrated caustic soda leaves the separator (8) in the lower part and will be directed by means of the distribution device (10) to the respective consumers (flaker (11) and/or drum filling device (12)).
The heating device using a mixture of molten salt as a heat transfer medium, consists mainly of a storage tank (13) with installed circulation pump (14) a heater (15) with installed burner (16) (oil or gas) a flue duct (17) and an air preheater (18) with mounted flue gas stack (19).
The combustion air will be supplied via the air preheater (18) to the burner (16). The flame burns from top to bottom in the combustion chamber formed by a multi-pitch cylindrical top coil (radiation zone). The flue gases leave the heater in the lower part which is formed by several parallel wound heater coils (convection zone) and let the flue gas duct (17) to the air preheater (18). There the combustion air will be preheated. The cooled down flue gases leave the air preheater (18) by the mounted flue gas stack (19) and are led over the roof into the open air.
The heat produced by the burner (16) will be transferred to the heat transfer medium circulating in the top coil in the upper part of the heater (radiation zone) by the radiation of the flame in the lower part (convection zone) by convection (direct contact of the hot flue gases with the heater coil wall). The heat transfer salt will be supplied by means of a pump (14) from the storage tank (13) into the tube coil of the heater (15). There, the heat transfer salt circulated from top to bottom. At the heater outlet, the heat transfer salt is guided to the lower collectors of the final concentrator (1) where it will be distributed to the respective concentrator elements (6). The heat transfer salt circulates there from bottom to top and leaves the concentrator elements (6) via collectors and is guided back into the storage tank (13).
By means of a by-pass valve (20), the heat transfer quantity circulating in the final concentrator (1) can be controlled. The control takes place by deviation of a part of the heat transfer salt quantity directly back into the storage tank (13). The valve (21) installed in the supply line to the final concentrator (1) will be shut-off the later to circulate the heat transfer medium over the heater coil to cool down the heat transfer salt after a shut-off of the plant provoked by a current failure. By this manner, it is possible to cool down the heat transfer medium by means of the fan (22) for the combustion air, for this operation. By this operation, a by-pass valve (20) will by completely opened.
For preheating of the heat transfer system, the whole piping system and each single concentrator element is equipped with a steam heating jacket, The condensed steam leaves the heating jacket via steam traps (23). In the supply line to the steam traps a vent valve (24) is installed. This vent valve (24) will be opened completely before starting the circulation of the heat transfer salt. By this precaution it will be avoided that during the heating up of the heat transfer salt, a pressure rise, created by the over-heated steam contained in the heating jackets, occur. As a supplementary safety device, a safety valve (25) will be installed in each steam supply line.
For the preheating of the nickel parts (7) and (8) of the final concentrator (1) which are not equipped with a salt heating, the concentrator element installed furthermost away from the centrifugal separator (8) is connected to the heating steam system. By means of this supply line, steam will be led into the concentrator. The steam will be heated by contact with the inner wall of the concentrator element. The over-heated steam flows along the lower nickel parts (7) and (8), and heats them up. The thereby created condensate leaves the plant via the drum filling station (12). By this preheating, the temperature shock created at the start-up with cold feed liquor will be reduced to a minimum.
The heat transfer salt will be filled into the storage tank (13) in a granular shape and melted by means of an installed steam heating coil.
To preheat the salt coil of the heater (15), the burner (16) will be operated with a reduced capacity.
To drain the heat transfer medium out of the piping system, the later can be vented with nitrogen by means of the valves (27) installed at the highest points of the piping system. By this venting, the whole heat transfer salt content drain automatically into the storage tank (13). To avoid a decomposition of the heat transfer salt during operation of the plant, (decomposition by pick-up of oxygen contained in the ambient atmosphere) the ambient air is removed by nitrogen superimposed in the storage tank (13).
The circulation pump (14) of the heat transfer salt is blocked by means of a temperature safety switch which allows the start of the pump only if the heat transfer salt in the storage tank (13) has exceeded a lower temperature limit, i.e., the heat transfer salt is melted.
As a protection of the nickel parts against corrosion by the caustic soda, a small quantity of sugar will be added to the feed liquor. The sugar will be dissolved in the tank (28) by means of clean water and supplied by the dosing pump (30) into the supply pipe for the feed liquor. To dissolve the sugar two identical tanks (28) are at disposal. The tanks are connected in such a way that it is possible to clean the tank and prepare a new solution while from the other one the dosing pump (30) is supplied without interruption.
The piping system for the molten caustic soda will be preheated by means of a gas heating system (31). During the continuous operation of the plant with nominal load the gas heating may be shut-off.
The flaker (11) operates according to the principle of rotating cooling cylinder. The later dips into a bath of molten caustic which will be maintained in a dipping vat. By this, a thin film of crystallized caustic soda will be created on the surface of the cooling cylinder. During the further rotating movement of the cylinder, this film will be cooled down to approx. 60°C and taken off the cylinder surface by means of a scraping knife/blade. After that, the flakes fall into a collecting channel (32) which leads them to the falling devices (33) (hand or automatic operated weighing device, bag filling device or similar). The cooling of the cylinder takes place by spraying cooling water to its inner surface. The cooling water will be discharged off the cooling cylinder by means of a dipping tube and led into the sewer system. A slight air overpressure maintained inside the cooling cylinder forces the cooling water out of the cylinder, so that only a very low level remains. Below the flaker a safety vat is installed which protects the operators by directing eventually over-flowing caustic soda into a safety over-flow drum (37).
The drum filling takes place by means of a supply channel (12) with two outlets. During the filling process, the molten caustic reaches alternatively one of the drums (38) through the left or the right outlet. This device has no collecting volume at all. For this reason, it is necessary to exchange the filled drums immediately by empty ones to avoid any interruption in the concentration process.
