ACID REGENERATION

Abstract: Regeneration of hydrochloric acid by hydrolysis of iron chloride in aqueous solution employs a Pelletiser (5) after preconcentration of acid solution in a "Pre-Evaporator" (1). Pelletiser (5) controls pellet mixture to between 12 % and 14 % free water by recycling dried solids from Roaster (3), from Screen (7), or from dust removal Cyclone (9), and waste liquid from Preconcentrator (1). Preconcentrator (1) is contacted with superheated gas from Fluid Bed Roaster (3). Rotary Drier (6) dries pellets with a con-current hot gas stream and the pellets are screened at (7) with oversize being sent to a Mill (8), undersize recycled to Pelletiser (5), and sized feed delivered to Roaster (3). HC1 is produced as gas offtake from Preconcentrator (1) and passes via a demisting Cyclone into adiabatic Absorption Process section (11), where weak acid liquor and water (10) are added as required.

Description: ACID REGENERATION TECHNICAL FIELD The present invention relates particularly to regeneration of hydrochloric acid from the iron chloride liquor that results from the leaching of iron from ilmenite, to form synthetic rutile. It applies also to regeneration of hydrochloric acid from pickle liquor produced from treating the surface of steel with hydrochloric acid. The relevant reactions for ilmenite are: A( i ) FeO Ti0 2 + 2HC1 -> Ti0 2 + FeCl 2 + H 2 0 A( ii ) 2 Fe Cl 2 + 2 H 2 0 + 0 . 5 0 2 -> Fe 2 0 3 + 4HC1 ( gas ) BACKGROUND ART Because of the cost of new acid, and/or the cost of disposal of iron chloride liquor, the commercial use of hydrochloric acid as a leachant for ilmenite depends on the availability of a technically viable and cost-effective method of regenerating and recovering the hydrochloric acid. Regeneration of hydrochloric acid is in common use in the steel industry world-wide. For example the acid is used for cleaning steel and wire before galvanising. This process is known as "pickling" and the resultant liquor, consisting largely of iron chlorides and some free acid, i known as "pickle liquor". The regeneration technology uses either a spray roasting, (Ruthner), process, or a fluidised bed, (Lurgi), process, both of which produce by-product iron. The reactions: B(i) FeO + 2HC1 -> FeCl 2 + H 2 0 B(ii) 2FeCl 2 + 2 H 2 0 + 0.5 0 2 -> Fe 2 0 3 + 4HC1 (gas) are essentially similar to those involving ilmenite. In both cases the acid can be regenerated by hydrolysis of the iron chlorides. Excess water is removed by heat, and the iron oxide as dust, or as a pellet by-product. Although the hydrolysis reaction is exothermic ( H800°C = - 112.6kJ [- 26.9kcal]) the quantity of water that has to be boiled off requires an external heat source. The principal difference between regeneration of pickle liquor and ilmenite leach liquor is that a significantly greater volume of liquor has to be processed when ilmenite is leached in commercial quantities, than is commonly necessary in the steel industry. Simplistically, this requires either a very much larger reactor or multiple reactors for an ilmenite leach operation, the size or number required depending on the volume constraints imposed by reactor design. The volume of concentrated iron chloride liquor which can be injected into a Lurgi - type fluidised bed reactor is limited by the need to achieve uniform distribution of liquor over the whole fluid bed. A large increase in feed volume cannot be accommodated by simply increasing reactor size (diameter) . As an example, the leach liquor from a 50,000 tonnes per year synthetic rutile production facility is estimated at approximately 54 tonnes per hour. Using the Lurgi pickle liquor regeneration technology, 4 reactors would be required. The system described herein would require only a single reactor. SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a process for the treatment of spent acid leach liquor containing iron chloride in aqueous solution including the steps of concentrating the aqueous solution to form a liquid concentrate, and forming the liquid concentrate into pellets. Additionally, the pellets may be dried and sized. The pellets are formed by adding dried solids or liquid, as the case may be. According to a second aspect of the invention there is provided a process for the regeneration of hydrochloric acid from spent acid leach liquor containing iron chloride in aqueous solution including the steps of in sequence, concentrating the aqueous solution to form a liquid concentrate, pelletising the liquid concentrate to form pellets, and hydrolysis of the pellets in a reactor. Additionally, the pellets may be dried and sized before hydrolysis in a reactor. The reactor is preferably a fluidised bed reactor but may also be a rotary kiln or other known type of reactor. The hydrochloric acid given off by the hydrolysis reaction is then recovered in a hydrochloric acid absorption circuit well known in the art. Preferably, the iron chloride in aqueous solution, the leach liquor, is concentrated to a nominal 250g/L (grams per litre) of iron (equivalent to 500 g/L of 40%FeCl 2 ). BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described with respect to Fig. 1, which shows a flow sheet of a process according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The flow sheet (Figure 1) is essentially a combination of four integers, each derived from known technologies. The first integer involves concentration of the leach liquor to a nominal 250g/L of iron. The second is a pelletising and drying process similar to common practice in the fertiliser industry. The third involves hydrolysis by heating the pelletised feed in a fluid bed reactor, and the fourth integer is a normal hydrochloric acid absorption circuit. The novelty of the inventive process lies in the arrangement of these integers so that the feed to the fluid bed reactor is presented in a pellet form from which most of the uncombined water has been removed, thereby reducing the volume of spent acid to readily manageable proportions. It also allows a closely-sized, reactive feed to be presented to the roaster, giving the added advantage of bed stability in the roaster reaction. The flow sheet includes the following steps: a) Preconcentration of 25% FeCl 2 spent acid solution by introducing the spent acid into a Lurgi-type "Pre-Evaporator", Preconcentrator (1) by a venturi or spray system (2) and contacting with superheated gas from a Fluid Bed Roaster (3) (step (e)). b) The concentrated (40%FeCl 2 - 500 g/L) solution is piped to a surge/storage tank (4), and thence to a Pelletiser (5) - a blunger or, alternatively, a rotating disc or drum-type pelletiser. Here the mixture is controlled to between 12% and 14% free water by recycling dried solids from the Roaster (3), from the Screen (7), or from the dust removal Cyclone (9), and waste liquid from the Preconcentrator (1) . c) The mixture from the Pelletiser (5) is fed to a Rotary Drier (6) where the pellets are dried with a con-current hot gas stream. Typically, the operating conditions within the drier (6) would be: Inlet gas temperature 600°C - 800°C Inlet moist solid feed 50°C - 80°C Outlet gas temperature 160°C - 180°C Outlet dried pellets 130°C - 150°C d) The pellets are screened at (7) with oversize being sent to a Mill (8), and undersize recycled to the Pelletiser (5). The sized feed is delivered to the Roaster (3) where hydrolysis occurs. e) The superheated gas stream from the Roaster (3) (including HC1, excess 0 2 , diluent N 2 , and water vapour) is fed back to the Preconcentrator (1) after dust removal for example, by Cyclone (9). In this system, a portion or all of the separated dust may advantageously be returned to the Pelletiser (5). The heat from the stream is used to concentrate the spent acid from the leach plant (step (a)). f) HC1 gas offtake from the Preconcentrator (1) is then delivered via a demisting Cyclone into a standard hydrochloric acid Absorption Process section, where weak acid liquor and water (10) are added as required, and absorbed with the HC1 stream in an adiabatic Absorber (11). The output of the Absorber (11) is then preferably cooled before becoming the product, 20% HC1 azeotrope (12). g) The 20% HC1 azeotrope (12) is then stored. In processing ilmenite ores the azeotrope (12) would be recycled to the leach plant to produce synthetic rutile (reaction A(i)). In a steel plant it would be used for pickling (reaction B(i)). Though the present invention has been described above with respect to a particular embodiment thereof it is be understood that the invention is not limited thereto but is capable of variation within the knowledge of a person skilled in the art.

Claims: CLAIMS 1. A process for the regeneration of hydrochloric acid from spent acid leach liquor containing iron chloride in aqueous solution including the steps of in sequence, concentrating the aqueous solution to form a liquid concentrate, forming the liquid concentrate into pellets, and hydrolysis of the pellets in a reactor. 2. A process for the regeneration of hydrochloric acid from iron chloride in aqueous solution as claimed in claim 1 wherein the free water content of the pellets is maintained in a range of 12-14% free water, and said pelletising step further includes the steps of drying and sizing the pellets before hydrolysis. 3. A process for the regeneration of hydrochloric acid from iron chloride in aqueous solution as claimed in claim 2 wherein the free water content of the pellets is maintained in the range of between 12% and 14% free water by recycling dried solids or waste liquid, as the case may be, respectively from said pelletising or said concentrating steps, and wherein said screening of the pellets includes undersized pellets being recycled to the pelletising step, and sized pellets being delivered to the roaster; and further including feeding gaseous products of said hydrolysis step back to said concentrating step and recovering therefrom hydrochloric acid in an adiabatic absorber. 4. A process for the regeneration of hydrochloric acid from iron chloride in aqueous solution as claimed in claim 1 wherein a range of between 12% and 14% free water is maintained in said pellets by adding dried solids or waste liquid, as the case may be, and further including the step of drying the pellets before hydrolysis. 5. A process for the regeneration of hydrochloric acid from iron chloride in aqueous solution as claimed in claim 4 wherein said roaster is a fluid bed roaster. 6. A process for the regeneration of hydrochloric acid from iron chloride in aqueous solution as claimed in claim 5 wherein said steps further include feeding gaseous products of said hydrolysis step back to said concentrating step and recovering therefrom hydrochloric acid in an adiabatic absorber. 7. A process for the regeneration of hydrochloric acid from iron chloride in aqueous solution as claimed in claim 6 wherein said waste liquid is taken from said concentrating step and further including the step of screening the pellets after drying, with oversize being sent to a mill, undersize recycled to the pelletising step as said dried solids, and sized feed delivered to the roaster. 8. A process for the regeneration of hydrochloric acid from spent acid leach liquor containing iron chloride in aqueous solution including the steps of: a) Concentration of the spent acid leach liquor by introducing the spent acid into a Preconcentrator (1) a venturi or spray system (2) and contacting with superheated gas from a Fluid Bed Roaster (3) (step (e)) b) Transfer of the concentrated solution to a Pelletiser (5) of a blunger or a rotating disc or drum-type wherei the pellet mixture is controlled to between 12% and 14% free water by recycling dried solids from the Roaster (3), from the Screen (7), or from the dust removal Cyclone (9), and waste liquid from the Preconcentrator (i); c) Feeding the mixture from the Pelletiser (5) to a *» Rotary Drier (6) where the pellets are dried with a con-current hot gas stream with operating conditions <* within the drier (6) being within the ranges: 5 Inlet gas temperature 600°C - 800°C Inlet moist solid feed 50°C - 80°C Outlet gas temperature 160°C - 180°C Outlet dried pellets 130°C - 150°C 10 d) Screening the pellets at (7) with oversize being sent to a Mill (8), undersize recycled to the Pelletiser (5), and sized feed delivered to the Roaster (3); 15 e) Feeding the superheated gas stream from the Roaster (3) (the stream including HC1, excess 0 2 , diluent N 2 , and water vapour) back to the Preconcentrator (1) after dust removal by Cyclone (9) with a portion or 20 all of the separated dust being returned to the Pelletiser (5) and the heat from the stream used to concentrate the spent acid (step (a)); f) Delivering the HC1 gas offtake from the 25 Preconcentrator (1) via a demisting Cyclone into a hydrochloric acid Absorption Process section, where weak acid liquor and water (10) are added as required, and absorbed with the HC1 stream in an adiabatic Absorber (11), and cooling the output of the absorber 30 (11) to form the product, HC1 azeotrope (12). 9. A process for the treatment of spent acid leach liquor containing iron chloride in aqueous solution including the 35 steps of in sequence, concentrating the aqueous solution of iron chloride to form a liquid concentrate, and forming pellets from the liquid concentrate by adding dried solids or liquid as the case may be. 10. An apparatus for the regeneration of hydrochloric acid from spent acid leach liquor containing iron chloride in aqueous solution including: a) a Preconcentrator (1) concentrating the spent acid leac liquor containing iron chloride in aqueous solution by introducing the spent acid thereinto by a venturi or spray system (2) and contacting with superheated gas f a Fluid Bed Roaster (3); b) a Pelletiser (5) of a blunger or a rotating disc or drum-type to which the concentrated spent acid solution from the Preconcentrator is feed and wherein the pellet mixture is controlled to between 12% and 14% free water by recycling dried solids from the Roaster (3), from a Screen (7), or from a dust removal Cyclone (9), and was liquid from the Preconcentrator (1); c) a Rotary Drier (6) to where the pellets are feed from t Pelletiser (5) and are dried with a con-current hot gas stream with operating conditions within the drier (6) being within the ranges: Inlet gas temperature 600°C - 800°C Inlet moist solid feed 50°C - 80°C Outlet gas temperature 160°C - 180°C Outlet dried pellets 130°C - 150°C d) a Screening device (7) for the dried pellets with oversize being sent to a Mill (8), undersize recycled to the Pelletiser (5), and sized feed delivered to the Roaster (3) ; e) wherein the superheated gas stream from the Roaster (3) (the stream including HC1, excess 0 2 , diluent N 2 , and water vapour) is fed back to the Preconcentrator (I) after dust removal by Cyclone (9) with a portion or all of the separated dust being returned to the Pelletiser (5) and the heat from the stream used to concentrate the spent acid; and the HCl gas offtake from the Preconcentrator (1) is delivered via a demisting Cyclone into a hydrochloric acid Absorption Process apparatus where weak acid liquor and water (10) are added as required, and absorbed with the HCl stream in an adiabatic Absorber (II) with the output of the absorber, (11) forming the product, HCl azeotrope (12).

Inventors: WALPOLE ERNEST ALAN, WALPOLE, ERNEST, ALAN
Applicants: AUSTPAC GOLD NL, AUSTPAC GOLD N.L.
Application year: 1993
Main classification: C01G23
IPC codes: C01B7, C01G23, C01G49, C22B1, C23G1