GB2094286A - Preparation of lead/lead oxide particulate material - Google Patents

Abstract

A method of making lead/lead oxide particles (for use in making active materials for lead-acid batteries) from molten lead droplets by an otherwise conventional reaction pot process includes the introduction into the reaction pot of atomised water having a pH of from 8 to 12 together with the air for oxidation of the lead droplets.

Description

SPECIFICATION Preparation of lead/lead oxide particulate material This invention is concerned with a method of making a lead/lead oxide particulate material for use in the production of active materials for lead-acid batteries.

The active materials in lead-acid batteries are prepared, as is well known, from a mixture of lead and lead oxide in the form of finely divided lead oxide particles with a residual core of lead.

Two principal processes are currently used commercially for the production of such lead/lead oxide particulate materials. The first utilizes an attrition mill in which blocks of solid lead are ground by self-attrition in a rotating steel drum. The resulting lead powder is partially oxidized by air, and the powder is then removed from the drum either by air or through screens. The powder is then generally classified and the oversize particles are returned for further milling, while the other particles are collected for use.

In the second process, molten lead is fed into a reaction vessel, for example a Barton pot, and is agitated by a rotating paddle. This breaks the molten lead into droplets and exposes a large surface area to incoming air for oxidation. The resulting powder is conveyed from the reaction pot by air flow and is then classified or further ground. The use of reaction pots is generally thought to have advantages over attrition mills in terms of lower capital and operating costs per unit output of lead/lead oxide particles. The material produced typically has a different particle shape from that obtained by the use of attrition mills, that is the particles are generally spherical as opposed to relatively flat or leaf-like, and have a somewhat higher lead oxide content.

However, processes using reaction pots tend to be more difficult to control than those using attrition mills in terms of the ability to produce particles of a consistent quality.

Thus, in the reaction pot process, the temperature is basically controlled by the rate at which lead is introduced into the reaction pot. The air throughput, together with the residence time in the reaction pot, control the amount of free lead in the product as well as the average particle size. Even when care is taken to control the temperature, it is sometimes difficult in practice to avoid undesirable "hot spots" in localized areas which can adversely affect product consistency, for example with respect to the orthorhombic lead oxide content which may be undesirably high.

Further, while longer than normally used residence times can reduce the average product particle size somewhat, this may also give rise to undesirably low lead contents. Typically, it is desirable to retain a lead content of 18 to 25% by weight so as to avoid undesirably low exothermic characteristics which hamper further processing, for example the curing of active material pastes formed from the lead/lead oxide particles.

It has been suggested that the manufacture of lead/lead oxide particles can be improved by the addition of water to the reaction pot. However, these efforts have been plagued by explosion and blowback problems. These difficulties appear to have arisen from the build-up of moist oxide in the upper part of the reaction pot which then drops back into the pot and causes steam expansion or the like, which then spews the lead/lead oxide particles out of the reaction pot and into the immediate area. To the best of our belief, prior attempts to use water injection in processes for making lead/lead oxide particles have been singularly unsuccessful.

It has also been suggested to incorporate various additives into the active materials of leadacid batteries to enhance various performance characteristics. Materials suggested include various lead compounds such as Pb(OH)2, PbO.H20 and Pb(GO3)2. In addition, it has been suggested to add to a battery a variety of metallic sulphates, such as sodium or potassium sulphate, in various ways for a multitude of purposes.

We have now developed an improved reaction pot method of making lead/lead oxide particles which enables higher production rates to be achieved and also gives a product with a lower average particle size and with a higher proportion of tetragonal lead oxide to orthorhombic lead oxide than conventional reaction pot processes.

According to the present invention, there is provided a method of making a lead/lead oxide particulate material for use in making active materials for lead-acid batteries, which comprises supplying molten lead to a reaction pot, dispersing the molten droplets into droplets, partially oxidising the lead droplets in the reaction pot by contact with a stream of air containing atomised water droplets, the water having a pH of from 8 to 1 2, removing the resulting partially oxidized lead particles from the reaction pot in the air stream, and then collecting them.

In order that the invention may be more fully understood, preferred embodiments of the method and typical apparatus for carrying it out will now be described, by way of example, with reference to the single Figure of the accompanying drawing, which is a diagrammatic elevation, partly in section, of such apparatus.

Referring to the drawing, apparatus for carrying out the method according to the invention comprises a pig feeder 10 which introduces lead pigs into a melting pot 12 provided with an exhaust vent 14. The molten lead produced is then conveyed by a pump 16 through an inclined trough 18 into a Barton pot 20. Alternatively, and more typically, molten lead may be fed into the Barton pot by gravity feed using air as a displacement medium, as is known.

The Barton pot includes means for dispersing the molten lead into droplets which are then partially oxidized by incoming air. Typically, and as shown, the Barton pot 20 is provided with a rotating paddle 22 driven by motor 24. Ambient air for oxidizing the lead droplets is drawn in through intake 26. The resulting product exits from the Barton pot 20 through exit port 28.

The conditions used in commercial Barton pots can vary widely. In principle, the amount of air drawn into the reaction pot should be more than sufficient both to carry out the partial oxidation of the lead and to serve as a carrier for the oxidized lead droplets into whatever collecting or classifying means are employed. Typically, for example, from 1600 to 2200 or 2300 pounds of molten lead per hour are introduced into the Barton pot with an air throughput of 1500 to 2500 cubic feet per minute.

For the purpose of the present invention, the Barton pot is provided with means for introducing atomized water together with the incoming air. To this end, water is provided from a source not shown through a rotameter 30 and a conventional pneumatic spray nozzle 32, which is positioned within the air intake 26.

The desired pH can be achieved by adding sodium or potassium hydroxide to the water by any suitable means, such as (not shown) a conventional displacement pump. The amounts needed will, of course, depend upon the pH of the untreated water, which typically will be about 5 or 6. It should be appreciated that while sodium or potassium hydroxide are preferred, any other base can be used so long as it does not result in the presence of materials that would significantlty adversely affect either the further processing of the particles or the resulting battery performance.

It has been found that the addition of water having a pH as specified enhances production rates per unit weight of lead in relation to conventional processes such as to offer substantial economic benefits.

The lead/lead oxide particles exit through exit port 28 and are then collected by any appropriate means. As illustrated, this can suitably comprise a settling chamber 34, a cyclone precipitator 36 and a bag filter 38. The product from each can be transported via a screw conveyor 40 for storage, shipment or the like. Air throughout in the system may be provided by a fan 42 which draws ambient air into the Barton pot and then through the system, as is known.

Oversize particles from the collection means, particularly from the settling chamber where at least half of the product will generally be obtained, can be recycled to the reaction pot, if desired. Moreover, if further reduction in the average particle size of the particles is desirable, this can be effected by using a hammer-mill or the like, as is often used in commercial practice.

However, as will be discussed in greater detail below, the products obtained by this invention typically have a relatively small average particle size.

The lead/lead oxide particles obtained by means of the present invention have characteristics which are, in some respects, quite different from those obtained in conventional reaction pot processes. It should be appreciated, of course, that the particular process parameters used may well affect the specific product parameters actually achieved. However, typically, the product obtained by the present invention will usually have a relatively smaller particle size than that obtained in conventional processes. Thus, the average particle size of products obtained in accordance with the present invention is typically from 1.5 to 17 microns (prior to any hammermilling or the like). Further, the products of this invention tend to have smaller amounts of orthorhombic lead oxide in comparison to the level obtained in conventional Barton pot processes.

The product of this invention also has somewhat higher acid absorption characteristics than conventional products. It is thought that this may be due to the product of this invention containing minor amounts of other compounds, for example 0.5 to 1 % or even more by weight of Pb(OH)2. The controlled pH used is believed to enhance the weight gain obtained over that obtained by the use of untreated water (that is having an unadjusted pH), presumably by increasing the amount of Pb(OH)2 or other compounds present. Regardless of the mechanism involved, it appears that improved oxide efficiencies are obtained, possibly due to improved acid absorption characteristics.

The lead/lead oxide product of the present invention may be used in conventional active material paste preparation without any necessity for change in formulation or processing techniques. Similarly, pasted grids for both positive and negative electrodes can then be formed from the active materials by any of the presently known techniques.

When the products of the present invention are used to prepare active materials for lead-acid batteries, no adverse effects have been seen. Indeed, it appears that for some reason the use of these products results in somewhat higher oxide efficiency. in the sense that the reserve capacity of batteries incorporating such products appears to be slightly enhanced in relation to reserve capacities obtained with active material formed from materials made by conventional reaction pot processes.

The following Example is given by way of illustration only.

Example To illustrate the performance of products made according to the present invention, test cells were made incorporating active materials made from a product cell having active materials made from a product made in a conventional reaction pot process.

Two cells were made using lead/lead oxide particles in which controlled amounts of water were introduced in atomized form into a Barton pot as generally shown in the accompanying Figure, the amount of water being about 1.5% by weight based upon the weight of molten lead being introduced and the pH #of the water being adjusted to 8 by addition of sodium hydroxide.

As a control, a cell was made in which the active materials were prepared using a similar product from a Barton pot process, except that no water addition was used. Each cell had six positive and seven negative electrodes. Each grid for the electrodes was pasted with approximately the same amount of active material and the pasted grids were similarly cured. The cells were then assembled and subjected to the same formation technique.

Each cell was then subjected to a discharge-charge regime for 32 cycles. The discharge regime involved a current discharge of 20 amps until a cut-off voltage of 1.75 was reached. The charge regime involved a recharge to provide from 110 to 11 5% of the Ampere-Hour capactiy removed in the discharge regime.

The time in minutes before the voltage cut-off was reached is set forth in the Table below: Control Cell Test Cells Cycle Number 1 2 3 1 142 146 144 2 151 153 155 3 160 163 167 4 171 173 175 5 176 174 179 6 174 179 181 7 179 182 184 8 180 186 188 9 179 182 184 10 179 183 186 11 177 183 185 12 172 177 179 13 171 176 178 14 167 178 180 15 173 180 182 16 167 173 177 17 157 167 169 18 162 167 170 19 159 165 169 20 155 162 167 21 163 170 176 22 159 165 171 23 165 167 174 24 158 162 170 25 153 155 161 26 149 152 159 27 153 159 165 28 149 155 163 29 147 151 159 30 145 149 153 31 160 162 165 32 165 167 172 As can be seen from the Table, the performance of the test cells using the product of the present invention consistently provided somewhat improved product performance.

Whilst we have particularly described the invention in terms of the use of a Barton pot as the reaction pot, it will be understood that other types of reaction pot can equally be used.

Claims (7)

1. A method of making a lead/lead oxide particulate material for use in making active materials for lead-acid batteries, which comprises supplying molten lead to a reaction pot, dispersing the molten lead into droplets, partially oxidizing the lead droplets in the reaction pot by contact with a stream of air containing atomised water droplets, the water having a pH of from 8 to 12, removing the resulting partially oxidized lead particles from the reaction pot in the air stream, and then collecting them.

2. A method according to claim 1, in which the water contains sufficient sodium or potassium hydroxide to provide a pH within the specified range.

3. A method according to claim 1 or 2, in which the amount of water introduced into the reaction pot is about 1.5% by weight based on the weight of molten lead introduced.

4. A method according to claim 1, substantially as herein described with reference to the accompanying drawing.

5. Lead/lead oxide particulate material when made by the method claimed in any of the preceding claims.

6. Active materials for lead-acid batteries made from a particulate material as claimed in claim 5.

7. A lead-acid battery incorporating an active material as claimed in claim 6.