The EARS (Enhanced Acid Regeneration System) process is a highly efficient means to regenerate strong hydrochloric acid from spent iron chloride leach liquor produced during the manufacture of synrutile by the ERMS SR process.
Benefits of the EARS Process:
- The process is not fuel specific. In addition to liquid or gaseous fuels, solid fuels such as coal or char can be used or a combination of these fuels.
- Regeneration of hydrochloric acid by the EARS process is cost effective, which is very important to the overall economics of synthetic rutile production. Significant savings are achieved through lower capital cost (smaller equipment is required) and reduced operating cost (by using high strength acid and low cost fuels) as compared to the alternative processes available.
- Pyrohydrolysis is carried out in a fluid bed. In these units, the solid oxide discharge is in the form of small pellets as opposed to the fine powder of spray systems. This is a decided advantage when it comes to subsequent handling.
- There are substantial environmental benefits. The iron fraction from the ilmenite is converted into Austpac’s Iron product, a suitable feedstock for use in foundries or for steel production. Other acid regeneration processes produce iron oxide, either as low value iron oxide pellets or fine red mud waste mud, and so do not recover the value of the iron metal. This allows ERMS SR plants to be located near cheap energy and good port facilities, rather than near a mine site, as is normally the case where iron oxide waste disposal is necessary.
- Importantly for the environment, the EARS process produces below limit of detection levels of dioxins or furans that are commonly emitted by other acid regeneration processes.
- Super-azeotropic HCl acid concentrations (i.e. above 20.4% concentration) can be produced compared with 18% acid made by the alternative systems. Super-azeotropic acid increases the leaching rate of ilmenite in the ERMS SR process, and significantly reduces the quantity of water recirculating with the acid, which lowers the fuel requirement for regeneration.
- The recovery of chloride (and hence HCl) from spent leach liquors is very high (>98%).
The EARS hydrochloric acid regeneration process was developed by Austpac to enhance the economics of the ERMS synthetic rutile production. Spent leach liquor produced from leaching ilmenite in hydrochloric acid must be converted to acid for synthetic rutile to be produced economically.
There are several possible process routes for the regeneration of hydrochloric acid, but commercially, the most popular is by pyrohydrolysis, and the EARS system is based on this process. The predominant pyrohydrolysis reaction is:
3FeCl2 + ½O2 + 3H2O ⇔ Fe3O4 + 6HCl
Other metal chlorides behave similarly and high recoveries of chloride are achieved.
The overall economics of an ERMS SR plant can be significantly improved by metallising the iron oxide from pyrohydrolysis to form a direct reduced iron pellet. This is achieved by gasifying the fuel required for evaporation and passing the reducing gases through a separate fluid bed. Therefore, very little additional fuel is required. The off-gas from metallisation is after-burnt and the hot gas is fed to the evaporator.
1. Fluid Bed Evaporation
Spent leach liquor is fed to a fluidised bed of metal chloride (MeCl) pellets of between 1 to 2 mm in size. A fraction of the total fuel requirement for pyrohydrolysis is mixed with the spent leach liquor as fine coal. This has many advantages for the operation of both fluid bed evaporation and pyrohydrolysis. Hot gas derived from gasification and metallisation is fed to the fluid bed evaporator, which is maintained at ~140°C bed temperature. This temperature affects the water of hydration of the MeCl pellets, which is adjusted to suit the subsequent pyrohydrolysis reaction. Off-gas treatment consists of venturi cleaning combining evaporative quenching with feed spent leach liquor and adiabatic absorption in packed towers. Absorption liquor comes from water washing of the leach solids and the tail gas scrubber. The final weak acid liquor from fluid bed evaporation absorption is fed to pyrohydrolysis absorption to increase the HCl concentration to above azeotropic strength.
Pelletised MeCl is fed to an air-fluidised bed of metal oxide pellets, also in the size range of 1 to 2mm. The bed temperature is maintained at ~940°C. Additional gas is used to control the temperature. The off-gas generated is high in HCl and passes through venturi cleaning and evaporative quenching with spent leach liquor, absorption and tail gas scrubbing. Final absorption liquor acid concentration is nominally 25% HCl. At this strength acid, a negative water balance can be maintained.
Hot metal oxide pellets from pyrohydrolysis are passed to a separate fluid bed in which strongly reducing conditions are created by sub-stoichiometric combustion. The oxide pellets are therein reduced to iron metal pellets (AustpacIron) using Austpac’s proprietary iron reduction process, and are then cooled in an anaerobic cooler before discharging.