Ball Mill with Collection Device for Lead Oxide

A V e n t u r e o f C a p t . A n i l K . V e r m a

The matter was conceived and written by Capt. Anil K. Verma.

Ball Mill alongwith Collection Device

Grey Oxide Plant consists of the following parts which have been described below:

Ÿ Preparation UnitŸ Ball MillŸ Collection Unit

Ball Mill is a basic device in the modern world of metallurgy and is extensively used in a variety of applications. Among others, it's application in the processing of Non-Ferrous metals is one of the most important. In the field of Lead processing, Dry type Ball Mills are used for conversion of Lead pellets into fine Lead Oxide Powder (Lead Oxide + Free Lead), also commonly called Grey Oxide due to its appearance. The Ball Mill with its preparatory and collecting equipments is therefore also called a Grey Oxide Plant.

About Ball Mill

Lead is melt in a Melting Pot and cast into pellets (hemispherical or cylindrical shape) at a Ball Making Machine and fed into the Ball Mill. The Ball Mill is rotated on its own axis at low speeds. The rotation of the mill imparts tumbling & cascading action to the balls fed (in this case the Lead pellets). The rotation is adjusted so that the raw material inside does not reach critical speed. Below the critical speed (at around 65 - 75%), the balls which are carried along the walls of the Mill fall back to the centre. This continuous action leads to friction and generation of heat which induces the reaction of Lead (Pb) with Oxygen (O ) present in air and generation of Lead Oxide (2PbO.Pb). This reaction is an exothermic reaction which leads to 2

generation of heat and this sets of a chain reaction of heat generation and accelerated oxidation process. Draft of air through the mill carries small sized or light particles outside the mill to the collection system. Temperature within the Mill is controlled with the help of water spray outside the barrel of the Mill. Several crucial characteristics of this produce such as Free Lead percentage, Retention percentage, Apparent Density are set as per requirements of the Client.

Typical specifications for Lead Oxide produced in a Ball mill are:

Apparent Density : 1.40 – 1.60Retention on 240 mesh size : 0.1 (% max)Free Lead : 28 – 32 (% max)Water Absorption : 100 – 120 mg/gmAcid Absorption : 150 – 200 mg/gm

Description of Process

Preparation Unit

Description of Plant

This unit consists of a Melting Pot. Here Lead ingots are melt in a metal pot which is heated with a diesel burner. Molten Lead is then cast into balls (pellets, hemispherical or cylindrical shape) of pre-defined sizes at a Ball Making Machine (or a Continuous Caster). These Balls are either manually transferred to the Ball Mill or are fed to it using a bucket conveyor arrangement.

Ball Mill

A Ball Mill is a hollow cylindrical tube sealed from both the sides, rotating on its own axis. The tube is generally in mild steel construction with or without lining. This tube has inlet and outlet pieces attached to it on both ends. Lead pellets are fed into the tube from the inlet side and powder is sucked from the outlet end. These inlet and outlet pieces also double up as shafts

on which heavy duty bearings are fixed for the purpose of rotation of the mill. Inward spirals are provided on the inlet piece whereas reverse screw is provided with the outlet piece for increasing efficiency of the unit. The tube as a whole, after the inlet and outlet pieces have been fitted, is turned for alignment. A girth gear is fitted on the tube of the mill. This is provided motion with the help of a pinion gear fitted on a shaft which is driven by a motor. Depending upon the size of the mill, desired rotations of the unit are attained through reduction by pulley & belt arrangements or by use of gear boxes. The motor used is a standard TEFC squirrel cage induction motor, provided with a Variable Frequency Drive (VFD) for fine tuning the rotations of the mill. The mill is seated on a heavy duty structural frame which is properly balanced to enhance stability of the equipment. An automated water pump with a spray mechanism is provided outside the tube to control the temperatures within the Mill.

Typical specifications of a 5.0 TpD Ball Mill are:

Tube Dia : 1400mmTube Length : 2500mmInlet Dia : 250mmOutlet Dia : 250mmStructural Frame : ISMC 250 x 80 & ISMB 250 x 125Drive Motor : 50.0 HP x 1440 RPMDrive Reduction arrangement : Pulley Belt TypeMill rotational speed : 35-38 rpm

A Collection unit is also called Classifier unit, this is an integral part of the Grey Oxide Plant. The raw material which is fine grinded in the Ball Mill is collected in this part. The collection unit works on the principle of negative suction and entrapment. While negative suction in the system is provided by the Induced Draft Fan (ID fan), the collection unit itself consists of Cyclone (pre-filtration device) and a Bag Filter Unit (fine-filtration device).

Lead Oxide is sucked from the discharge end of the Ball Mill through the negative suction created by the ID Fan in the system. Air carrying Lead Oxide powder passes through an involute single tube Cyclone. Here particles of larger dimension are settled at the bottom. Thereafter, this air stream passes through a fine-filtration device which is a pulse jet based continuously cleaned bag filter system. Major collection of powdered particles is done here. It is fitted with a screw conveyor and an air lock rotary valve. They continuously discharge the collected powder at the bottom. It is received either in open trays, plastic sacks or into conveyors to carry them to silos, as per arrangement in individual units. ID Fan is placed at the end of the system.

Typical Specifications of a Collection Unit suitable for a 5.0 TpD Ball Mill are:

Cyclone Dia : 750mm Bag House Dimensions : 2350mm x 2360mmNo. of Bags : 100

2Total Filtration area : 140.0 mID Fan capacity : 2500 cmh

ID Fan Motor : 7.5 HP x 1440 RPM

Collection Unit

Note: Due to continuous up-gradation in technology, specified & projected values may differ in actual products

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