DESC:TECHNICAL FIELD OF THE INVENTION
The present invention relates to decanting systems and more particularly to a decanter apparatus for solid and liquid separation.
BACKGROUND OF THE INVENTION
Bayer Process is a process in which pure aluminium oxide (Al2O3) is extracted from bauxite. Aluminium oxide normally exists in bauxite in two forms namely, monohydrate (A12O3.H2O) or Boehmite and trihydrate (Al2O3.3H2O) or Gibbsite. Impurities in bauxite are mainly present in the form of oxides of iron, silicon and titanium with traces of other minerals. In the Bayer process, bauxite is dissolved in a hot caustic solution, commonly referred to as ‘Mixed liquor’ to produce a liquor supersaturated with alumina referred as ‘Pregnant liquor’. Bauxite residue is separated from the pregnant liquor before precipitation process. Alumina hydrate is precipitated by cooling of pregnant liquor and seeding with previously precipitated hydrate. This hydrate is finally calcined in order to remove the chemically bound water to yield the final product alumina (Al2O3), which is, in turn, processed into aluminium metal through Hall-H´eroult smelting process. The separated bauxite residue is often present in the form of a slurry that contains a significant volume of caustic liquor which needs to be recovered back to the circuit to maintain the caustic balance of the refinery.
Conventionally, a decanter or settler is used to flocculate the bauxite residue slurry, thereby leading to separation of bauxite residue from pregnant liquor. The decanter operation is a complex phenomenon involving simultaneous flocculation, settling and separation in highly turbulent velocity zones. However, the conventional decanters or settlers are opaque, hence, the aforementioned phenomena occurring therein cannot be observed. It is important to study the various steps involved in the steady operation of decanter viz., flocculent addition ratios in feed line and feed well, settling zones, effect of bauxite residue level in compaction zone on solid consistency, effect of slurry flow-rate on turbulent velocity zones and overflow clarity. The study of the aforementioned parameters helps in validating the hydrodynamic model, which, in turn, makes it possible to choose the best variant for upgrading the existing decanter equipment without the need for large capital investment.
At laboratory scale, the settling rate, flocculant dosage and overflow clarity are determined through a simple jar test. Based on the jar test, even if these parameters are further scaled up to the plant level, the impact of turbulent velocity profiles on the flocculant mixing with bauxite residue slurry cannot be predicted.
Therefore, there is felt a need of a decanter apparatus which facilitates clear visualisation of the steps involved in the operation of decanters.
SUMMARY OF THE INVENTION
The present invention envisages a decanter apparatus. The apparatus comprises a slurry holding vessel, at least one flocculant holding vessel, and a decanter vessel. The slurry holding vessel is configured to hold residue slurry therein. The holding vessel has a stirrer positioned therein. The flocculant holding vessel is configured to store a flocculant therein. The decanter vessel is in fluid communication with the slurry holding vessel to receive slurry and in fluid communication with the flocculant holding vessel to receive flocculant. The decanter vessel is configured to facilitate mixing of the slurry with the flocculant therein. The decanter vessel has a rake assembly positioned therein. The rake assembly is configured to displace residue particles towards center of the decanter vessel. A lid having a plurality of holes is configured thereon.
The decanter vessel is made of a transparent material to facilitate observation of all the process steps and zone formations occurring in the decanter vessel.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 illustrates a front view of an apparatus, in accordance with an embodiment of the present invention.
Figure 2 illustrates a side view of the apparatus of figure 1.
Figure 3 illustrates a schematic view of a bauxite residue slurry holding vessel of the apparatus of Figure 1.
Figure 4 illustrates a top view of a lid of the bauxite residue slurry holding vessel of Figure 3.
Figure 5 illustrates an exploded view of a stirrer arranged in the Bauxite Residue Slurry holding vessel of Figure 3.
Figure 6 illustrates a schematic view of a flocculant holding vessel of the apparatus of Figure 1.
DETAILED DESCRIPTION OF THE INVENTION
Although specific terms are used in the following description for sake of clarity, these terms are intended to refer only to particular structure of the invention selected for illustration in the drawings, and are not intended to define or limit the scope of the invention.
References in the specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
A decanter apparatus is now described with reference to accompanying figures 1 to 6. The apparatus comprises a decanter vessel which is made of transparent material. In an embodiment, the decanter vessel is made of glass. However, the decanter vessel can be made of other transparent materials in other alternative embodiments of the decanter vessel.
Referring to figures 1 to 6, a decanter apparatus 100 is shown. The decanter apparatus 100 facilitates solid and liquid separation under an atmospheric pressure wherein the bauxite residue slurry is flocculated to separate bauxite residue from pregnant liquor. The apparatus 100 comprises a Bauxite Residue Slurry holding vessel 110, at least one flocculant holding vessel 120, and a decanter vessel 130. The vessels 110, 120, 130 can be supported on a stand 105.
The bauxite residue slurry holding vessel 110 is configured to hold residue slurry therein. More specifically, the bauxite residue slurry holding vessel 110 serves for mixing the concentrated blow off slurry solids. Typically, the composition of the slurry solids is concentrated up to 60 wt%. The bauxite residue slurry holding vessel 110 is provided with a lid 112. The lid 112 has two holes 114 configured thereon. The holes 114 facilitate insertion of thermocouples therethrough. The lid 112 has another set of two holes 116 acting as feed points. The slurry vessel 110 has a stirrer 118 positioned therein. The lid 112 has a central hole 117 through which a stirrer 118 is positioned in the bauxite residue slurry holding vessel 110. A heater is provided in the bauxite residue slurry holding vessel 110.
The apparatus 100 includes at least one flocculant holding vessel 120 configured to store a flocculant therein. In one embodiment, the apparatus 100 includes two flocculant holding vessels 120. However, in other embodiments, the number of the flocculant holding vessels 120 may vary depending upon the application requirement of apparatus 100. In this embodiment, the apparatus 100 includes two flocculant holding vessels 120 for storing hydroximated polyacrylamide (Hx-PAM) and polyacrylate (PA). The hydroximated polyacrylamide (Hx-PAM) and polyacrylate (PA) are pumped to the decanter vessel 130 using peristaltic pumps. The flocculant vessels 120 are provided with a lid 122.
The apparatus comprises the decanter vessel 130 in fluid communication with the slurry holding vessel 110 to receive slurry and in fluid communication with the flocculant holding vessel 120 to receive flocculant. The decanter vessel 130 is configured to facilitate mixing of the slurry with the flocculant therein to provide separation thereof.
The decanter vessel 130 is surrounded by a feed well 132. The decanter vessel 130 is in fluid communication with the feed well 132 via a feed line. The polyacrylate (PA) flocculant is added in the feed line. The feed line is connected to the feed well 132. The feed well 132 is a cylindrical vessel in which mixing of the bauxite residue slurry and flocculant takes place.
The feed well 132 supplies mixture of slurry received from the slurry holding vessel 110 and flocculant received from the flocculant holding vessel 120 to the decanter vessel 130.
The apparatus 100 comprises a rake assembly 134 disposed within the decanter vessel 130. In an embodiment, the rake assembly 134 is centrally connected in the decanter vessel 130. The rake assembly 134 helps in moving the settled bauxite residue particles towards the centre for removal through the outlet valve.
The decanter vessel 130 is made of transparent material. Due to the transparent material, all the processes occurring within the decanter vessel 130 can be observed. In an embodiment, the decanter vessel 130 is made of glass. However, the decanter vessel 130 can be made of any other suitable transparent material in alternative embodiments of the present invention.
In another embodiment, the slurry holding vessel 110 and the flocculant holding vessel 120 are made of transparent material. In one embodiment, the slurry holding vessel 110 and the flocculant holding vessel 120 are made of glass. However, the slurry holding vessel 110 and the flocculant holding vessel 120 can be made of any other suitable transparent material in alternative embodiments of the present invention.
The decanter vessel 130 has large diameter with shallow depth tanks and has slowly revolving rakes for removing and consolidating the sludge. The slurry is preferably fed at the centre of the feed well 132 and clear liquor overflows around the top edge of the feed well 132. The rake 134 scrapes the sludge settled towards the centre and at the bottom for discharge. The motion of the rake 134 gently stirs only the sludge layer, which aids in liquor removal from the sludge, thus consolidating the bauxite residue.
In context of the present invention, several zones of different concentrations are established during the settling process in the decanter vessel 130, namely a compression zone, a transition zone and a clarification zone. The compression zone is a bottom most zone which predominantly consists of heavier and faster settling particles. In this zone, sludge undergoes ‘compression’. The transition zone is defined above the compression zone which is a region of variable size distribution and non-uniform concentration. The transition zone is also called as a ‘critical zone’. The clarification zone is located above the transition zone which is a uniform concentration zone of approximate feed concentration, wherein entry of feed slurry is facilitated. The clarification zone is the zone from where clear liquid overflows.
As the decanter vessel 130 is made of transparent material, all the three zones, i.e., compression zone, transition zone and clarification zone, can be clearly seen and observed from outside the decanter vessel 130.
The decanter vessel 130 has an inlet for receiving slurry. The slurry includes liquid with suspended solid particles. The slurry components are separated in decanter vessel 130 into an overflow phase (i.e., a primarily clarified liquid) and underflow phase (i.e., a primarily high solid concentration). The decanter vessel 130 includes a tapering conical bottom, having an apex connected to an underflow discharge port for the discharge of the thickened, underflow phase of the slurry that has been separated in the decanter vessel 130.
The Bayer process waste comprises slurry containing bauxite residue and dissolved alumina. The process for treating Bayer process waste includes the steps of supplying the waste to a settling area to cause the bauxite residue to settle out of the slurry, thereby producing supernatant liquor.
A variable speed pump and motor communicate via a valve with discharge port for the controlled feed rate of the slurry to the decanter vessel 130. A pump is used for the recirculation of the slurry. A manual valve is provided as a discharge port to control the removal of the underflow.
The apparatus 100 includes an overflow weir which is defined around the periphery of the decanter vessel 130 for removal of the overflow phase, separated from the slurry in the decanter vessel 130. A manual valve is provided as a port to collect the overflow of the decanter vessel 130.
A cylindrical wall in decanter vessel 130 forms an annular chamber and defines a fluid barrier between the annular chamber and the remainder of the vessel interior. The slurry enters the annual chamber through a tangential pipe to form the vortex in annular chamber which help in the floc formation and separation of BR from the liquor.
In the context of the present invention, a settling agent, such as a Polyacrylate (PA) and Hydroximated Polyacrylamide (Hx-PAM) are delivered as an additive to the slurry to facilitate flocculation in decanter vessel 130, preferably to facilitate free settling of BR in the decanter vessel 130. The settling agent is stored in flocculant holding vessels 120 from where fluid flow is transferred to the decanter vessel 130. A fluid flow control device, such as a peristaltic pump, regulates the dosage of settling aid delivered to the slurry and thus controls the extent of floc formation within the decanter vessel 130.
The slurry received and held in the decanter vessel 130 gets separated by gravity into a hindered settling zone which is located above a compaction zone and below an interior free Settling clarification zone and an overflow clarification zone in the annular chamber. Within hindered settling zone, the liquid and solid particles tend to get separated with the solid particles tending to flow downwardly to the compaction zone and with the clarified liquid flowing up to clarification zone. Within compaction zone, the solids concentration increases to the highest level of concentration at discharge outlet at the bottom of the decanter vessel 130. A process for treating a suspension of solid particles in a carrier liquid comprises subjecting the particles in the suspension to gravity settling in a treatment zone.
The rake assembly 134 is mounted for rotation as indicated at the bottom of the decanter vessel 130. The rake assembly 134 has a plurality of generally vertical oriented pickets. The pickets typically extend parallel to each other. The pickets extend into the compaction zone. The pickets form channels in the slurry held in the lower portion of the decanter vessel 130 for releasing liquid from the lower settling zones. As the rake assembly 134 rotates, the pickets travel through the slurry and form temporary channels in the slurry held in the lower portion of the decanter vessel 130. These channels release free or interstitial liquid in pockets from the lower settling zones. From there, the liquid is free to flow on to the free settling zone. Rotation of the rake assembly 134 at its upper end is done through connection to a motor via a drive head. The motor runs at fixed or variable speed generally at 1 rpm. The rake 134 is designed with vanes at a calculated angle to transfer torque generated from motor for homogenization and dewatering of bauxite residue sludge for final separation through drain. The rake assembly 134 includes a rotary drive for rotation of the rake assembly 134 about a vertical axis.
As the decanter vessel 130 is made of transparent material, all the process steps and zone formations occurring in the decanter vessel 130 can be easily observed and studied. Also, the impact of the various process parameters such as slurry flow rate, flocculant concentration, flocculant dosing ratio in Feed line and Feed well can be studied. Additionally, using the apparatus 100, the higher inventory of settled bauxite residue sludge at the bottom of the decanter vessel 130 on the velocity profiles in the decanter vessel 130 and its subsequent impact on settling performance can be observed and studied.
The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others, skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.
LIST OF REFERENCE NUMERALS
100 – Decanter apparatus
105 – Stand
110 – Slurry holding vessel
112 – Lid of slurry holding vessel
114, 116 – Holes on the lid of slurry holding vessel
117 – Central hole on the lid of slurry holding vessel
118 – Stirrer
120 – Flocculant holding vessel
122 – Lid on flocculant holding vessel
130 – Decanter vessel
132 – Feed well
134 – Rake assembly
,CLAIMS:WE CLAIM
1. A decanter apparatus (100) comprising:
a slurry holding vessel (110) configured to hold residue slurry therein, said slurry holding vessel (110) having a stirrer (118) positioned therein;
at least one flocculant holding vessel (120) configured to store a flocculant therein;
a decanter vessel (130) being in fluid communication with said slurry holding vessel (110) to receive slurry therein, said decanter vessel (130) being in fluid communication with said flocculant holding vessel (120) to receive flocculant therein, said decanter vessel (130) configured to facilitate mixing of said slurry with said flocculant therein, said decanter vessel having a rake assembly (134) positioned therein, the rake assembly (134) configured to displace residue particles towards center of said decanter vessel (130), wherein said decanter vessel (130) is made of a transparent material; and
a lid (112) having a plurality of holes (114, 116, 117) configured thereon.
2. The apparatus (100) as claimed in claim 1, wherein said slurry holding vessel (110) and said flocculant holding vessel (120) are made of transparent material.
3. The apparatus (100) as claimed in claim 2, wherein said slurry holding vessel (110), said flocculant holding vessel (120), and said decanter vessel (130) are made of glass.
4. The apparatus as claimed in claim 1, wherein said rack assembly (134) is disposed at bottom of said decanter vessel (130).
5. The apparatus as claimed in claim 1, wherein said rake assembly (134) has a plurality of vertically oriented pickets extending parallel to each other.
6. The apparatus (100) as claimed in claim 1, wherein said apparatus includes two flocculant holding vessels (120) configured to store hydroximated polyacrylamide (Hx-PAM) and polyacrylate (PA).
7. The apparatus (100) as claimed in claim 1, wherein the holes (114, 116, 117) on the lid (112) facilitate insertion of said stirrer, a heater, and thermocouples into said slurry holding vessel (110).
Dated this 1 June 2020
M. Kisoth
IN/PA-2259
Agent for the Applicant