FIELD OF THE INVENTION

The invention relates to a combined heater-decanter in a nickel laterite process, and more particularly to a combined heater-decanter as defined in the preamble of claim 1 .

The invention also relates to a method for increasing density of feed slurry in a combined heater-decanter, and more particularly to a method as defined in the preamble of claim 10. BACKGROUND OF THE INVENTION

In extractive metallurgy of nickel laterites high-pressure autoclaves are extensively utilized to allow increased operating temperature for extraction and precipitation reactions. Typical temperatures of High Pressure Acid Leaching (HPAL) autoclaves are 240 to 270°C. While the rate of nickel leaching from a laterite is enhanced using higher temperature the primary reason for operating at elevated temperature is to precipitate iron as hematite thereby regenerating sulphuric acid in-situ substantially reducing the cost of acid for the process. In HPAL conditions iron, aluminum and other elements have limited solubility and will not form stable sulphates thereby minimizing acid usage. In particular trivalent metals like ferric and aluminum have much lower solubility at high temperature than at temperatures less than 100°C. In HPAL of nickel laterite the precipitation of iron and aluminum is the primary driver behind the usage of high temperature and pressure autoclaves.

It is possible to leach nickel from laterites at atmospheric temperatures however large amounts of iron are also solubilized with much increased acid consumption. Attempting to separate nickel and cobalt from excessive iron is very complex and expensive and finally the acid added to the atmospheric leach must be eventually neutralized with limestone or lime at significantly increased cost. By using an autoclave at temperatures greater than 240°C most of the iron precipitates as hematite leaving minimal iron in solution and thereby greatly reducing the cost of nickel recovery.

While HPAL process chemistry is elegant and cost effective, the cost of constructing HPAL plants for treating nickel laterites has proven to be very expensive. Large pressure vessels operating at high temperatures and containing acidic solutions require use of large amounts of titanium and other expensive materials at large capital cost. Particularly, the HPAL requires a large amount of energy for heating the ore material. The acid and the hot acidic environment causes wear and tear upon plant and equipment. Due to resulting high energy costs lower grade ores have not been utilized in such processes.

Since laterite leaching does not generate energy, as occurs in the leaching of sulfide ores and concentrates, steam must be added to the autoclave. To minimize the cost of the steam addition energy recovery is also practiced. Flashed steam is recycled back into the laterite feed stream for preheating the slurry before adding it to the autoclave. However since heating occurs by direct contact, flash steam dilutes the incoming slurry. The dilution significantly increases the volumetric flow rate of slurry entering the autoclave and for a fixed leach residence time the size of the autoclave increases significantly.

Indirect heating of the slurry before its entry into the autoclave would solve the problem of the dilution of the slurry. However, the tubes of an indirect heater scale very quickly if flash steam obtained from the pressure let-down of the autoclave discharge slurry is utilized in the indirect heater. Further, nickel laterite slurries are notoriously difficult slurries to transport when thickened and due the rheology, the heat transfer co-efficient of these slurries in indirect heater is inherently very low.

Publication US 5,407,561 discloses an apparatus for separating solids from liquid under pressure and a pressurised decanter. This pressurized decanter is for alumina industry. The apparatus comprises a vessel having an elongated cylindrical vessel wall, a closed upper end and a bottom section for accumulation of solids. The vessel includes a stirrer mounted for rotation along an inside surface of the bottom section and a discharge spool mounted beneath the bottom section including at least one opening. An underflow pump is provided for discharging separated solids without loss of pressure in the apparatus. The apparatus further includes means for detecting solids level in the apparatus, which preferably operates without disturbing settling of the solids therein. Also, the apparatus includes a feedwell for receiving an incoming pressurized slurry stream, the feedwell having an opening a predetermined distance above a level in the apparatus defined by settled solids. The above mentioned prior art discloses a pressurized decanter for alumina industry. As explained earlier above extractive metallurgy of nickel laterites require large amount of energy and the problem with the prior art publication is that it does not save in operating costs relating to the energy consumption.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is thus to provide a combined heater-decanter and a method for increasing density of feed slurry in said combined heater-decanter so as to alleviate the above disadvantages. The objects of the invention are achieved by a combined heater-decanter and a method which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on the realization that by combining the pre- heating of slurry and the removal of excess water which allows higher density slurry to be produced, the capital and operating cost of nickel laterite processing can be substantially reduced.

The invention relates to a combined heater-decanter unit in a nickel laterite process and in particularly to a combined heater-decanter unit comprising a vertically elongated vessel comprising a heating section arranged in the upper part of the vessel, a pressure decantation section arranged below the heating section and a passage extending from the heating section to the pressure decantation section. The heating section comprises a first inlet for slurry to enter the vessel and a second inlet for heating fluid to enter the heating section. The pressure decantation section comprises a tapered part for slurry to settle and a first outlet for discharging the slurry.

The heating fluid is preferably saturated steam that is fed to the heating section such that it enters the heating section below the point in which the slurry enters the heating section so that the steam flows upward in the heating section and slurry falls downward by gravity in the heating section and at the same time the slurry becomes heated by the saturated steam. In other words the inlet for heating fluid to enter the heating section is in the heating section such that it is below the inlet for slurry to enter the heating section, or in other words it is below the inlet for slurry to enter the vessel. The inlet for slurry is referred as a first inlet and the inlet for heating fluid is referred as a second inlet in this application. So the first inlet is arranged above the second inlet in the heating section. The heating section preferably comprises a further outlet for excess heating fluid, preferably excess steam, to be released away from the vessel. The excess steam is preferably released under pressure control through a stream in the top part of the heating section. The excess heating fluid is the heating fluid that has not condensed on the cold slurry and therefore becomes useless in this particular process. So the heating section comprises an outlet for an excess heating fluid to exit the vessel under pressure.

The heating section comprises at least one gas-liquid contact surface to be heated with the heating fluid for heating the slurry. The gas-liquid contact surface can be made as an integral part of the vessel or as a separately attached part. The gas-liquid surface is preferably a tray or a plate which is arranged in the heating section such that the cold slurry that has entered the heating section flows by gravity over the gas-liquid surface or a series of gas-liquid surfaces and during flow of said cold slurry that comprises liquid a contact is formed between the slurry and said steam to effect heat exchange there between. The steam also heats the gas-liquid surfaces that promote heating of said slurry.

The pre-heated slurry that is heated in the heating section flows through a passage to the pressure decantation section. The pressure decantation section forms the lower part of the vertically elongated vessel and comprises a part that is tapered or that has a section comprising inclined sides that make an angle between 30-60° from the horizontal. In the pressure decantation section the heated slurry settles and thickens, especially when the tapered space forms sufficient static head to promote dewatering under the weight of the settled solids and a relatively clean overflow is generated. The thicker slurry in the bottom part of the pressure decantation section is eventually discharged through a first outlet for discharging said slurry in the bottom of the vessel. The overflow liquid that has generated on the settled slurry can be discharged through a second outlet in the upper part of the pressure decantation section for discharging overflow liquid.

The passage between the heating section and the pressure decantation section is preferably a down comer pipe that is arranged to extend from the heating section to the pressure decantation section. The down comer pipe preferably extends from the lower part of the heating section to the pressure decantation section and preferably to the middle of the pressure decantation section in height direction such that the incoming slurry is directed into the settled solids bed via said pipe such a way that the settled solids act as a clarifier for the overflow solution.

The vessel may also comprise a rake in the pressure decantation section to promote solids dewatering.

The invention also relates to a method for increasing density of feed slurry by preheating and removing water from the slurry in a combined heater- decanter unit. The method comprises the steps of feeding slurry through the first inlet to the heating section, feeding heating fluid through the second inlet to the heating section, arranging the slurry to flow by gravity through the heating section and through the passage from the heating section to the pressure decantation section and settling the slurry in the pressure decantation section.

In the method the step of feeding heating fluid preferably comprises a step of arranging the heating fluid to flow to an opposing direction as the slurry through the heating section for heating said slurry. The heating fluid is preferably saturated steam.

An advantage of the combined heater-decanter of the invention is that it reduces the volumetric flow rate of slurry entering HPAL autoclaves thereby allowing lower cost equipment for HPAL processing at significantly reduced operating cost in terms of lower boiler steam, acid and other consumables.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawing, in which

Figure 1 shows a heater-decanter unit according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a heater-decanter unit according to the invention in which the heater-decanter unit comprises a vessel 1 that is vertically elongated and comprises a tapered portion in the lower part of the vessel 1 .

The cold slurry is fed through a first inlet A which is in the top part of the vessel 1 and especially in the heating section 2. The heating fluid which is preferably saturated steam is fed through a second inlet B which is below the first inlet A and preferably in the lower part of the heating section 2. The first and second inlet A, B are arranged such that the slurry falling down by gravity will be exposed to the heating fluid flowing upward in the heating section such that the cold slurry is heated by the heating fluid during the flow through the heating section 2. In the upper part of the heating section is preferably an outlet D for discharging excess heating fluid. Said outlet D comprises a pressure control so that excess steam can be released under pressure control through stream. The heating section 2 may comprise a gas-liquid contact surface 5 which is preferably a plate or a tray but even the inner surface of the heating section that comprises surfaces with which the cold slurry may come in contact with the heating fluid may suffice. In this figure a series of plates are arranged in an inclined position protruding from the inner surface of the vessel 1 in the heating section 2 such that the slurry fed through the first inlet A falls down via said plates.

The heated slurry falls from the heating section 2 to the pressure decantation section 3 through a passage 4 which in this embodiment is a down comer pipe extending from the lower part of the heating section 2 to a midway of the pressure decantation section 3. The passage extending between said heating section 2 and said pressure decantation section 3 is preferably such that is extends below the level in which the slurry has settled in the pressure decantation section 3.

The pressure decantation section 3 comprises a tapered portion in the bottom part of the pressure decantation section 3 and preferably in the very bottom is an outlet C for discharging the thickened slurry. The pressure decantation section 3 further comprises a second outlet E for discharging the overflow liquid that has generated on the settled slurry.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.