JiU-VLFu1ran rafeS AC .yyU - neguarjon J.
ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title An improved electrostatic plate separator for mineral separation based on the difference in electrical resistivity and size of minerals The following statement is a full description of this invention, including the best method of performing it known to me/us: 1 FIELD OF INVENTION: Mineral beneficiation industry including heavy mineral sands separation and all other applications, involving differential attraction or repulsion of charged particles under the influence of an electric field, such as separation of iron ore, cassiterite, tantalite, wolframite etc. BACKGROUND OF THE INVENTION: Electrostatic plate (ESP) separators are widely employed industrial equipment for separation of conducting and non-conducting minerals, especially for removal o42 fine conducting minerals from coarse non-conducting rich streams. They are generally used after particulate mixtures have passed through a high tension roll (HTR) separator, which works on ion bombardment principle. In HTR operation, coarse conducting particles collect in the conducting fraction, whereas fine conducting particles in the feed report in the middling and non-conducting fractions. The electrostatic element of an ESP separator consists of a tubular high voltage static electrode of ellipsoidal or circular cross-section attached to a large surface area curved plate. The feed gravitates down a grounded metal sliue plate placed in front of the static electrode plate. Conducting particles are polarized in the electric field near the electrode and lose the charges of the same sign as that of the electrode to the grounded metal feed plate, thereby acquiring a net charge opposite in sign to that of the electrode. As the charge is of opposite polarity to that of the electrode, the conducting particles are attracted towards the electrode i.e., the conducting particles experience a 'lifting effect', The non -2conducting particle is unaffected by the weak forces involved and it continues down the grounded metal slide plate under the influence of gravity only. Hence, the conducting and non-conducting particles follow different trajectory. The conducting particles are collected through a splitter type collection means below the plate's lower edge dividing the feed into a mainly conducting and a mainly non-conducting fraction. Because the lifting effect depends on the surface charge as well as the mass of the particle, fine conducting particles are effectively separated from non-conducting particles. The equipment may also have additionally static electrodes suitably placed with the aim of maximizing the lifting of any residual conducting* particles from the non-conducting particles stream moving down the grounded metal slide plate. The equipment may also include a conducting screen of suitable screen-opening size to allow easy passage of the largest particles being treated. Conducting particles capable of assuming an induced charge are attracted by the electrode and prevented from passing through the screen grid. Non-conducting particles pass through the screen unaffected. Plate construction is typically of carbon, chrome or stainless steel when treating granular material of minus 1 mm in size. Plate width ranges upto 2000 mm, while the plate length is upto 500 mm per pass in industrial electrostatic plate separators. Feeding is accomplished by vibratory, belt, rotary spline or gravity methods, depending on the particle size being treated. Above description of the mechanism describes a one-pass separation process. ESP separators typically incorporate 5 identical passes with upto 2 starts of individual feed streams being treated in one machine. Each new pass follows the last with material cascading from one stage to the next. Typically, conducting particles are removed from non-conducting particles which continue on to the next pass for retreatment. Each pass is similar to the first with feed chute, -3grounded plate, electrode and splitter system duplicated and arranged one above the other in a vertical configuration. Adjustment of the splitters, electrode position and feed plate angle is typically done at each pass independently of other passes. In the treatment of mixtures of particles with a range of physical characteristics including conductivity, particle size and density, it is necessary to adjust not only the air gap between plate and electrode but also the slope and shape of the plate and splitter positions independently on each pass. Voltage and polarity is traditionally similar over all starts and passes on each machine bank as a single high voltage power supply is used for simplicity reasons. The voltage and polarity to be used also depends on the physical characteristics of the mixture of feed particles. Creating charge on the particle is the most critical step for an effective separation in an ESP separator. The electrical resistivity of conducting particles such as Ilmenite and rutile is of the order of about 104 Ofm, whereas electrical resistivity of non-conducting particles like zircon, Sillimanite & quartz ranges from 101 to 1014 Om. In an ESP separator, the conducting particles are charged by conductive induction. However, due to large mass flow rate, the particles are exposed to charging for a very short time and unless heated to about 50 - 80"C, the polarization and charge retention on the conducting particles is not effective. Also, as the non-conducting particles have a higher resistivity, they are unaffected by the weak induction forces and hence do not get charged. Therefore, the feed is generally heated to about 50 - 80*C to for charging of the conducting particles. -4- The equipment, as per the known art, shown in the Figure I is an ESP separator used to separate electrically conducting particles from non-conducting particles. A mixture of particulate material 1 is fed through a feeding means, such as a gate and a simple chute, onto the grounded metal plate 2. The particulate mixture 1 is pre-heated to a suitable temperature as required for effective separation. For example, for heavy mineral sand separation, the particulate mixture of heavy mineral sands is to be pre-heated to 80 0 C in a dryer. The particulate feeding mechanism may be varied in order to suit the nature of the feed material and other operating parameters, as would be well understood by the person experienced in the art. The equipment includes a static high voltage electrode 3. A high voltage DC power supply is connected to the electrode. The static electrode is placed opposite to the grounded metal plate 2 at a distance of about 50 mm from it. The conducting particles 4 acquire a net charge opposite in sign to that of the electrode. The conducting particles are then lifted off the feed plate due to the physical attraction of the oppositely charged electrode, while the non-conducting particles 5 continue to move forward. The conducting particles are collected through a splitter type collection means 6 below the plate's lower edge dividing the feed into a mainly conducting fraction 7 and a mainly non-conducting fraction 8. The performance of ESP separator depends on the size and specific mass of the particles, slope of the grounded metal plate, dimension and relative position of the high voltage static electrode, the level of the high voltage applied to the electrode in a manner which will be well understood by the person skilled in the art. Drawbacks connected with hitherto known Process/ device: a) ESP separators are best suited for separation of fine conducting particles from coarse non-conducting rich streams. However, the conducting particles collected in the conducting fraction are contaminated with a considerable amount of non conducting particles and thus require additional stages of cleaning operation. -5b) In mineral separation, ESP separators are exclusively used for separating conducting from non-conducting minerals. For example, in the heavy minerals application, ESP separators are used for separation of the conducting fraction i.e., Ilmenite and rutile from the non-conducting fraction namely, Sillimanite, zircon, quartz and garnet. ESP separators are not recommended for separation of non-conducting from other non-conducting minerals. Such applications are universally achieved by wet gravity separation methods and particularly froth flotation. OBJECTS OF THE INVENTION: An object of this invention is to propose an improved electrostatic plate separator for mineral separation based on the difference in electrical resistivity and size of minerals; Another object of this invention is to propose an improved electrostatic plate separator to minimize the contamination of the conducting fraction with non conducting particles; Still another object of this invention is to propose an electrostatic plate (ESP) separator to optimize the ESP separator operation to achieve separation of non conducting from other non-conducting minerals; Further, object of this invention is to propose an ESP separator which is capable of higher recovery of conducting particles. -6- BRIEF DESCRIPTION OF THE INVENTION: According to this invention there is provided an equipment for electrostatic mineral separation based on the difference in electrical resistivity and size of mineral particles; equipment comprises of: a static electrode of circular, ellipsoidal or any other suitable cross-section; a grounded slide plate on which the feed gravitates down; means for controlled feeding of mineral particles onto the grounded slide plate; a suitably placed ionizing electrode for imparting charge on mineral particles; suitably placed splitters for collection of separated fractions. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS: Fig 1: The equipment, as per the known art, shown in the Figure 1 is an ESP separator used to separate electrically conducting particles from non-conducting particles. Fig 2: it will be seen that an ionizing electrode 9 with a corona wire is introduced. DETAILED DESCRIPTION OF THE INVENTION: The present invention provides a means for charging non-conducting particles with charge of the same sign as of the static electrode, thereby causing repulsion of non-conducting particles by the static electrode, which helps in minimizing the lifting of non-conducting particles to the conducting fraction. Introduction of an ionizing electrode in an ESP separator, with its position so located such as to ensure charge deposition on the particle surface before its entry onto grounded plate of the ESP separator results in an efficient separation. -7- The polarity of the ionizing electrode has to be same as that of the static electrode. The DC voltage to the ionizing electrode has to be selected at a level which will not result in pining of non-conducting particles on the grounded plate but at the same time it has to be ensured that the particles are charged adequately. The charged conducting particles will lose the acquired charge to the grounded metal plate and. will only have a net charge resulting from the conductive induction effect of the static electrode. On the other hand, the non conducting particles will not be able to lose the acquired charge to the grounded plate and resultantly will be repelled by the static electrode. This repulsion force prevents lifting of non-conducting particles and thus minimizes contamination of conducting fraction. Further, it has also been found that ESP separator modified as per the invention mentioned herein can be used advantageously for applications involving separation of fine non-conducting particles from other non-conducting particles. Low DC voltages such as 3 kV to the ionizing electrode favour charging of particles with relatively lower resistivity and size such as fine Sillimanite, whereas charging of particles with high resistivity such as zircon & quartz is difficult. As a result, fine Sillimanite particles are repelled strongly by the static electrode of similar polarity, while other non-conducting particles such as zircon & quartz are repelled weakly. By optimum setting of splitters, this differential charging and resultant differential repelling forces can be made to result in a clear collection of a Sillimanite rich stream and another stream rich in other non-conducting particles. Referring now to the figure 11, it will be seen that an ionizing electrode 9 with a corona wire is introduced. This ionizing electrode is placed near and opposite to feed entry into the ESP separator. This ionizing electrode is spaced at a distance -8of about upto 2 inches from the feed plate. The particulate mixture is now subjected to being charged by the ionizing electrode. Thus, the charged particles enter the ESP separator making contact with the grounded metal plate. The conducting particles will lose their acquired charge quickly to the grounded plate and will be subject to conductive induction forces of the static electrode. Conducting particles thus acquire charge opposite to that of the electrode and are attracted towards the electrode i.e., the conducting particles experience a 'lifting effect'. In an ESP separator without incorporation of ionizing electrode detailed as per this invention, the non-conducting particles entrapped between conducting particles tend to get lifted and are carried over to the conducting fraction. Also, coated fine non-conducting particles also tend to get lifted into the conducting fraction. With incorporation of an ionizing electrode in an ESP separator detailed as per this invention, it is found that lifting of non-conducting particles is reduced by an order of magnitude, thus resulting in a cleaner conducting fraction. Non-conducting particles in the feed are charged due to ionization by the ionizing electrode and are repelled by the static electrode operating under similar polarity. The examples listed below reveal that on account of the invention, the particulate mixture can be efficiently separated resulting in a higher grade as well as higher recovery of the product. EXAMPLE: Examples of the invention are given below by way of illustration and not by way of limitation. -9- Examples 1 to 6 The middling from the high tension roll separator for treating magnetic fraction output from WHIMS (wet high intensity magnetic separator) used for primary magnetic separation of the heavy mineral concentrate at the Chavara beach sand mineral separation plant of IREL was used as the feed to the MDL make ESP separator of bench model - MKS. This feed to ESP separator contained about 72 to 75% llmenite and the remaining being other minerals such as garnet, monazite, rutile, leucoxene, zircon, Sillimanite and quartz. In the plant level operation of 1800 mm wide plate ESP separator with this feed, the operating conditions are 75 0 C temperature, 20 kV DC supply to static electrode and 28 kg/h/inch feed rate. Keeping all other parameters unchanged, the experiments were conducted for studying the effect of introducing the ionizing electrode in accordance with the described details of the invention. Bxpt Static Ionizing Feed Cond Non-Cond Cond Ratio of electrode electrode fraction fraction Recovery Cond/NC voltage & voltage & % in product ___Polarity Polarity _____& feed Ratio of Ratio of Ratio of Cond/NC Cond/NC Cond/NC 1) 20, +ve 0 3.1 5.94 0.43 92.86 1.92 2) 20, +ve 1, +ve 3.07 6.57 0.46 91.45 2.14 3) 20, +ve 2, +ve 3.11 7.4 0.3 94.21 2.38 4) 20, +ve 3, +ve 3.25 12.51 0.23 94.57 3.85 5) 20,+ve 4,+ve 3.35 8.17 0.4 92.57 2.44 6) 2,+ve 5,t+ve 3.47 7.47 0.39 93.7 2.15 -10- Example 7 The Sillimanite concentrate obtained by wet tabling of the overflow from the hindered settling classification of the non-conducting non-magnetic fraction spiral concentrate at the Manavalakurichi beach sand mineral separation plant of IREL was used as the feed to the ESP separator used in the above experiments. This feed contained about 73% Sillimanite and 20% quartz with zircon, kyanite, shell and a small fraction of conducting minerals Ilmenite, rutile and leucoxene. MDL make ESP separator of bench model - MKS was used with this feed, the operating- conditions were optimized to 75 0 C temperature, 22 kV DC supply to static electrode, 3 kV DC supply to ionizing electrode and 28 kg/h/inch feed rate. Expt. Temp 0 C Static Ionizing Sillimanit Sillimanite Sillimanite Electrode electrode e grade in grade% in the recovery % voltage & voltage & input non-conducting Polarity Polarity output (after I stage) 7) 75 22, +ve 3, +ve 73.82 93.3 92.5 The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. -11-