DESC:FIELD OF THE INVENTION
[001] The present invention relates to a process for beneficiation of bauxite. More particularly, the present invention provides a single step process for beneficiation of bauxite and a flexible apparatus that consumes less energy while carrying out the said process.
BACKGROUND OF THE INVENTION
[002] Bauxite is the main raw material used in the production of alumina by Bayer process. Apart from Al-minerals, bauxite contains Si- and Fe-minerals as major impurities. Reactive Si-minerals such as clay has a strong affinity to soda, which react with soda in the Bayer alumina process to form complex sodium alumino-silicates and leads to loss of both alumina and soda in the red mud. The Si- & Fe-minerals gangue in Bauxite to convert into bauxite residue / red mud in Bayer process. This bauxite residue is highly alkaline, which makes its utilization & disposal very difficult. Presently, the only commercial method used for disposal of bauxite residue is either wet or dry disposal.
[003] Quality, availability & cost of high grade bauxite and stringent environmental regulations are major challenges for sustainability of alumina refineries. Therefore, it is necessary to upgrade the bauxite ores to reduce the generation of bauxite residue. The upgradation not only helps to reduce the bauxite residue generation but also helps to utilize the low grade bauxite, thereby increasing the availability of bauxite for alumina production.
[004] A major challenge in beneficiation of bauxite is the complex association & texture of these minerals in bauxite ore body. Various beneficiation & upgradation technologies have been developed over the years considering both wet and dry methods. Few bauxite mining companies follow simple screening or desliming technique to remove the fines from bauxite such that the total silica content reduces in the bauxite. Similarly, crushing & grinding followed by screening processes are also used for better separation of Si-minerals in bauxite ores. However, these methods are not efficient due to higher loss of alumina in fines. Therefore, floatation & other techniques are employed for recovery of Al-minerals from these fines reject. Thus, the current bauxite beneficiation processes are inefficient and need more number of unit operations to improve the Al-recovery.
[005] The conventional beneficiation techniques as shown in both Fig. 1(a) and Fig. 1(b) used commercially in bauxite mines are a) manual breaking, dressing & sorting; b) mechanical crushing & dry screening; and c) wet screening or desliming.
[006] In the mechanical crushing of bauxite, both Al-minerals and gangue minerals crush/fracture to fine sizes due to complex association and similar mineralogical properties. This results in loss of Al-minerals in fines along with gangue minerals during screening process. Therefore, wet magnetic separation (WHIMS) and / or floatation techniques are used for recovery of Al-minerals from fines fractions. Various process variants have been developed and tried with different combination of wet & dry unit operations but no beneficiation process is used except dry screening due to techno-economic feasibility with respect to Si reduction & Al losses.
[007] The autogenous grinding technique are used for selective crushing of minerals with similar mechanical / crushing properties. In conventional grinding techniques, the hard steel balls/rods are used as a grinding media, which crush larger particles. In conventional ball mills, the steel balls are hard and break simultaneously the useful Al-minerals along with gangue silicate minerals. On the other hand, in autogenous grinding technique the large particles of ore & mill wall surface act as media and crush the particles with limited impact / energy. This helps in selective breakage / separation of the softer minerals (weak bonds) from harder minerals. However, this process is not used for bauxite minerals due to sticking of fine silicate clay particles to the large bauxite particles. To overcome this, wet grinding is used. The major challenges for use of wet beneficiation techniques is the higher moisture content in final product. The higher moisture in bauxite dilutes the alkali liquor in Bayer process which need higher energy to remove it during spent liquor evaporation stage. This wet process also generates large quantity of effluent which needs to be processed to remove the fines for recycle of water. The storage or handling of wet fine waste (slime ponds) increases process cost.
[008] Therefore, there is a need of a process and an apparatus that solves some of the problems of the prior art.
SUMMARY OF THE INVENTION
[009] In an aspect, the present invention provides an apparatus for simultaneous grinding and screening of feed material comprising:
a cylindrically elongated mill shell defining a chamber with an inner wall, an outer wall, an inlet port to the chamber, an outlet port from the chamber;
a first portion in the mill shell comprising of a first set of pores of predetermined size extending through the inner wall and outer wall;
a second portion in the mill shell comprising second set of pores of predetermined size extending through the inner wall and outer wall;
a third portion in the mill shell comprising of a third set of pores of predetermined size extending through the inner wall and outer wall;
a shaft extending through the housing from inlet port to outlet port;
said inner wall comprising of a mill liner;
a plurality of mill lifter (116) arranged on the inner wall of the chamber at predetermined interval;
a rotative means for rotation of the mill shell; and
a means for collecting fed material exiting from the outlet port.
[010] In another aspect, the present invention provides a method for simultaneous grinding and screening (SGS) comprising;
(a) passing a feed material through the apparatus of the present invention for simultaneous grinding and screening of feed material;
(b) grinding and screening of the feed particles by rotating the mill shell of the apparatus;
(c) allowing grounded particles to exit through the first, second and third set of pores of the mill shell depending on particle size; and
(d) collecting the feed material from the outlet port and from the first, second and third portions of the mill shell;
[011] In an aspect, the process further comprises of reintroducing the grounded particles of step (d) in to the apparatus. The process of the present invention deliver coarser particles of the feed material pass through the outlet port without coating of fine or reject material.
BRIEF DESCRIPTION OF THE DRAWINGS
[012] Fig. 1(a) illustrates a process for beneficiation of bauxite, according to the prior art;
[013] Fig. 1(b) illustrates another process for beneficiation of bauxite, according to the prior art;
[014] Fig. 2 illustrates the single step process used in the present invention, according to the embodiments of the present invention;
[015] Fig. 3 (a) shows a front view of the apparatus used for carrying out the process of the present invention as depicted in Figure 2, according to the embodiments of the present invention; and
[016] Fig. 3 (b) shows a side view of the apparatus used for carrying out the process of the present invention as depicted in Figure 2, according to the embodiments of the present invention
[017] Fig 4(a) a graph showing the effect of grinding time on bauxite 1 grade
[018] Fig 4(b) a graph showing the effect of grinding time on bauxite 1 recovery
[019] Fig 5(a) a graph showing the effect of grinding time on bauxite 2 grade
[020] Fig 5(b) a graph showing the effect of grinding time on bauxite 1 recovery

BRIEF DESCRIPTION OF THE EMBODIMENTS
[021] The embodiments of the present invention eliminate or reduces the aforementioned problems of the prior art by providing a single step process that provides an integrated approach for simultaneous grinding and screening (SGS) unit operations. The present invention also provides a flexible apparatus that consumes lower energy while carrying out the said process. In other words, the present process and apparatus enhances selective grinding & separation of soft clay minerals from bauxite ores.
[022] According to the embodiments of the present invention, there is provided a process as shown in Figure 2, wherein the size reduction and mineral liberation of feed happens due to the abrasion/shearing, rolling and slow fracture forces acting on the bauxite particles throughout the mill length. Also, simultaneous separation of different size particles occurs throughout the length of the mill shell due to screens. At the end, the product with coarse particles free from gangue fines is collected at the discharge end. During the initial period, abrasion/shearing and/or chipping (reduction of sharp edges) action takes place on the particles which helps in the removal of silica impurities present as coatings over the bauxite ore particles. At the later period of operation, slow fracture leads to liberation of silica associated in the veins/grain boundaries of the ore particle and further enriches the bauxite quality in the coarser sizes.
[023] As the present invention is based on the integrated design/method for simultaneous grinding and screening, the different size fractions get separated as soon as they are generated in the process through different size holes in the mill liner shell. Therefore, the present process leads to continuous quality improvement throughout the operating/residence time and significant amount of softer gangue can be removed along with the fines.
[024] The present process involves designing the blend of feed to achieve desired feed size distribution. This is based on the physico-chemical properties of the ores in the feed blends. This is very important step to achieved desired forces for selective grinding and liberation of gangue minerals from useful minerals. In an embodiment, the media / balls can also be used to achieve desired breakage and liberation of gangue particles.
[025] In an embodiment, the present invention avoids the steep decrease of magnitude of interactive forces between particle-wall and particle-particle interaction during the entire period of autogenous grinding (fracturing). This effectively increases the selective liberation and classifies the bauxite ore with relatively lesser grinding time and energy. Further, simultaneous grinding (fracturing) cum screening avoids the fines agglomeration over the coarser size particles due to the simultaneous screening of coarse particles and fines.
[026] In an embodiment, the different feed size ratios as per bond work index and type of bauxite are designed to selectively separate / crush the silicate clay minerals from bauxite minerals with minimum damage to Al-minerals. This helps to reduce the loss of Al-minerals in fines fractions during screening. Moreover, fines are removed simultaneously during autogenous grinding process. The present process not only helps to separate the fines but also enhances the efficiency of autogenous grinding process as simultaneous separation of fines helps interaction of large particles with wall and other large particles.
[027] The present invention that is based on simultaneous grinding cum screening technique selectively allows the bauxite ore to liberate impurities through mechanisms like abrasion/shearing, attrition, rolling and slow fracture of both particle-wall and particle-particle interaction along with the screening.
[028] In an embodiment, the present process uses hot air to facilitate easy separation of fines from product. The hot air is employed for bauxite with very high moisture content and for sticky fines. The hot air not only helps to separate the fines but also to reduce the moisture content in the final product, which is desired for Bayer process. In an embodiment, the present process also works efficiently when water is used for faster desliming for ores with different texture and mineralogy.
[029] In an embodiment, the present process works efficiently in batch and continuous mode. Preferably, batch or continuous option is selected as per the operational requirements, feed quality, space availability, etc.
[030] In an embodiment, the simultaneous grinding (fracturing) cum screening operation is an intensified dry beneficiation technique that effectively improves the grade & recovery with minimum consumption of energy & time required for beneficiation.
[031] According to another embodiment of the present invention, there is provided a simultaneous grinding and screening apparatus, as more clearly shown in Figure 3 (a) & (b). The principle components of the apparatus are a) material handling (feeding & discharging), b) driving system consisting of electric motor with VFD or gear system for setting desired rpm, c) structural components including screens, lifters, etc. Through hopper or conveyor, the feed enters into the mill shell and then it starts centrifuging with the mill for the given residence time. Along with rotational motion, the feed also experiences motion along the length from one side to other. The motion along the length is mainly due to the inclination provided by the mill shell. During the motion of feed along the length, fines are separated through the holes provided on the mill surface and gets collected in bottom bin. Finally, the coarser product moves along the mill shell and gets collected as a heap or in storage bin.
[032] In an embodiment, the apparatus comprises of a mill shell (114) that acts as a main working zone of the equipment where feed experiences different type of forces and mineral liberation & separation takes place; an inlet port (101) through which the feed materials are introduced into the mill shell; a first set of pores (110), a second set of pores (111) and a third set of pores (109) for removal of finer size fractions during the process; a mill liner (115) whose main role is to protect the cylinder from the grinding body and material direct impact and friction; a shaft (108) that provides support to the mill shell and also helps in mill rotation; ; clamps (not shown) that provides support to the mill shell and also can be adjusted for varying the mill inclination; a motor (107) that is used for providing the necessary torque to the mill shell; mill lifters (116) which are small assemblies on the mill shell and provides necessary push/force needed to the particles for the achieving the desired trajectory/motion; fines collection system that are trays with discharge system are placed at the bottom of the mill shell to collect the fines.
[033] Depending on the process requirements the trays can have continuous or intermittent discharge system to evaluate the fines from the system. In an embodiment, the apparatus further comprises of media spraying system which has hot air and water nozzles at different locations on top and sides for effective separation of moisture & moist fines; In an embodiment, the apparatus further comprises of exhaust and dust collection system wherein gas succession port is provided on the top side at discharge end for collection of dust and gases, which is connected to conventional / standard dust separation system.
[034] In an embodiment, the type of mill shell is decided based on the hardness and abrasion resistance of the feed materials. The size & shape of the pores is selected as per the requirement of product grade & recovery. In another embodiment, the wall thickness of mill shell varies from 0.1mm to 200mm. In another embodiment, the shape of the pores includes regular (circle, star, elliptical, oval, triangle, square, rectangle, multiple sides) and/or irregular shapes. Preferably, the number of screens to be used in process varies from single to more than one as per process requirements. In an embodiment, the type of mill shell can be woven, punched or drilled.
[035] Preferably, the operating parameters of the mill such as mill rpm, feed rate, feed size, angle of inclination are decided based on the ore properties and requirements of final quality & productivity. The operating parameters can also vary as per batch or continuous operations.
WORKING EXAMPLES
[0036] According to the embodiments of the present invention, the SGS (Simultaneous Grinding cum screening) is depicted in Figure 3(a) & 3(b), wherein the size reduction and mineral liberation of feed happens due to slow fracture/impact, abrasion/shearing and rolling forces acting on the bauxite particles throughout the mill length.
BENEFICIATION OF BAUXITE
[037] The apparatus was tested using two types of bauxite i.e. bauxite 1 with higher silica (7.8%) content and bauxite 2 with relatively lower silica (3%) content. The beneficiated coarser product moves along the mill shell and gets collected as a heap or in a storage bin. The results of the trials are presented in Figures 4(a) & 4(b) for bauxite 1 and figures 5 (a) & 5(b) for bauxite 2 respectively. Figures 4(a) & 5(a) shows graphs of the grade of final product, that is, increase in valuable percentage alumina (Al2O3) and reduction in silica (SiO2) impurities with respect to grinding time whereas figures 4(b) & 5(b) shows graphs of the recovery of final product with respect to grinding time.
[038] During the initial period up to 20 min, the rate of abrasion and/or shearing was steep due to impact and rotational movements that helped higher removal of silica impurities which were present as coatings over the bauxite ore particles. In conventional grinding and milling processes, these fine particles remain inside the mill and hence they form a coating layer over the coarser particles. Nevertheless, in this novel SGC process, these fines are separated from coarse bauxite particles as soon as they are generated. At the later period of operation (> 20 min), relatively slow & selective fracture takes place during autogenous grinding that leads to liberation of silica associated in the veins/grain boundaries of the ore particle and further enriches the bauxite quality with respect to alumina in the coarser sizes. Due to the absence of hard grinding media, minimum fracturing/breakage of alumina minerals occurred that helped to control alumina losses in reject fines. The silica levels in the beneficiated coarser particles reduced by 35% and 50% for the bauxite 1 and bauxite 2 respectively as shown in
[039] The results in figures 4(a) & 5(a) clearly show the effect of bauxite mineralogy on selective separation of silica and alumina. In case of Bauxite 2 with lower silica (3%), approximately 50% of silica was removed with only ~2% alumina losses in the final product along with the recovery of ~58% after 200 minutes of processing. On the other hand, Bauxite 1 with high silica (~7.75%) experienced higher alumina loss of ~4.5% and silica removal was on 35% and recovery was ~61.5% after 120 minutes of processing. Higher losses of alumina and lower product recovery in case of bauxite 2 in shorter processing time were due to its lower work index.
[040] On an average, it shows that almost 60% of the recovery is feasible by this novel SGC method for both type of bauxites along with significant reduction in silica percentage. Further, the grinding time varies depending upon the association/liberation of different minerals in the feed. This is mainly associated due to the complex association of mineral phases and work index of the ore. Therefore, depending upon the quality of bauxite (Composition/associations of alumina, iron and silica), feed bauxite size, cut sizes, grinding time and shell speed are optimised to achieve desired beneficiated bauxite quality.

ADVANTAGES
[041] The major advantages of present invention are as follows:
(a) Flexibility to process / beneficiate wide range of bauxites & other softer ores. This increases mine life, increase of bauxite source, reduce the generation of hazardous bauxite residue at the end of Bayer process.
(b) Reduced energy consumption due to no or reduced use of separate grinding media and also there is no additional energy required for fines screening as compared separate screening unit step required in conventional processes.
(c) Achieved high quality product with reduced number of unit operations and footprint
(d) Process provides additional flexibility to operate wet ore feed in monsoon as compared to conventional dry beneficiation processes
(e) Process is environment friendly as integrated gas and dust collection system reduces the particulate matter mission to environment
[042] The foregoing description of specific embodiments of the present invention has been presented for purposes of description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obvious modifications and variations are possible in light of the above teaching.
[043] The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application thereby enabling 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.
Claims
,CLAIMS:We Claim:
1) An apparatus for simultaneous grinding and screening (SGS) of feed material comprising:
a cylindrically or polygonal elongated mill shell defining a chamber with an inner wall, an outer wall,
an inlet port to the chamber, an outlet port from the chamber;
at least one first portion in the mill shell comprising of a first set of pores of predetermined size extending through the inner wall and outer wall;
at least one second portion in the mill shell comprising second set of pores of predetermined size extending through the inner wall and outer wall;
at least one third portion in the mill shell comprising of a third set of pores of predetermined size extending through the inner wall and outer wall;
a shaft extending through the housing from inlet port to outlet port;
said inner wall comprising of a mill liner;
a plurality of mill lifter (116) arranged on the inner wall of the chamber at predetermined interval;
a rotative means for rotation of the mill shell; and
a means for collecting fed material exiting from the outlet port.

2) The apparatus as claimed in claim 1, wherein said mill shell comprises of a plurality of said first, second and third portions.
3) The apparatus as claimed in claim 1, wherein the diameter of first set of pores is smaller than diameter of second and third set of pores.
4) The apparatus as claimed in claim 1 and 2, wherein the diameter of first set of pores is in the range of 1mm to 300 mm
5) The apparatus as claimed in claim 1, wherein the diameter of second set of pores is larger than diameter of first set of pores and smaller than diameter third set of pores.
6) The apparatus as claimed in claim 1 and 2, wherein the diameter of second set of pores is in the range of 1mm to 300 mm
7) The apparatus as claimed in claim 1, wherein the diameter of third set of pores is larger than diameter of first set of pores and second set of pores.
8) The apparatus as claimed in claim 1 and 2, wherein the diameter of third set of pores is in the range of 1mm to 300 mm
9) The apparatus as claimed in claim 1, further comprises of a means for collecting feed particles exiting the mill shell through first, second and third set of pores.
10) The apparatus as claimed in claim 1, further comprises of a media spraying means using air, water or combining both media.
11) The apparatus as claimed in claim 1, further comprises of an exhaust and dust collection system collection of dust and gases.
12) The apparatus as claimed in any of preceding claims, optionally comprises of a second mill shell positioned over the first mill shell.
13) A method for simultaneous grinding and screening (SGS) comprising;
(e) passing a feed material through an apparatus for simultaneous grinding and screening of feed material as claimed in claim 1;
(f) grinding and screening of the feed particles by rotating the mill shell of the apparatus;
(g) allowing grounded particles to exit through the first, second and third set of pores of the mill shell depending on particle size; and
(h) collecting the feed material from the outlet port and from the first, second and third portions of the mill shell;
14) The process as claimed in claim 12, wherein the process further comprises of reintroducing the grounded particles of step (d) in to the apparatus of claim 1.
15) The process as claimed in claim 12, wherein coarser particles of the feed material pass through the outlet port without coating of fine or reject material.