Discussion on quality control of adding main and auxiliary materials in battery paste mixing process

Abstract:In this paper, the main control points of adding main auxiliary materials in the process of lead-acid battery paste mixing are mainly described, and the influences of the quantity and quality of lead powder controlled, the way of adding additives controlled, the way and amount of adding water controlled, the way of adding sulfuric acid and the stirring time are analyzed in detail.

Key words：Lead paste; Add method; Add time; Additives; Expander

The manufacturing quality of the lead paste plays a key role in the manufacture of the battery plate and the formation of the electrical performance of the plate. First, the prepared lead paste should have good malleability, so that the lead paste can be evenly and fully pasted in the grid when the plate is in pasting process. Second, it should have a certain strength. The lead paste should not be concave and fall off during rolling, mechanical vibration and impact when the plate is pasted.

As the production of lead paste is formed by mechanical mixing of Lead powder, sulfuric acid, DM water and additives with the paste mixing machine, the lead paste has certain physical and chemical characteristics. In this process, it is difficult to determine some physical and chemical characteristics of the lead paste formation through measurement and inspection, such as the homogeneity of additives in the lead paste, the proportion and distribution of various basic lead sulfate, etc., so the lead paste manufacturing process is a special process of lead-acid batteries, and the quality control of the entire manufacturing process is very important.

Quantity and quality control of materials used in lead paste manufacturing

The materials used in the production of lead paste include Lead powder, sulfuric acid, DM water and various additives. The quantity and quality of these materials will directly affect the quality of lead paste.

1. Quantity and quality control of Lead powder

The quantity of Lead powder refers to the total weight of Lead powder entering the mixing machine. At present, the paste mixing machine used by all production enterprises are mostly closed automatic feeding method. For the automatic control closed paste mixing machine, due to the error of the weighing system, the flexibility of the oxide conveying system and the vibration system, and the influence of many factors such as the residual Lead powder and sticky residual Lead powder in the weighing hopper, the amount of Lead powder entering the paste mixing machine is difficult to accurately reach the specified value; Due to the accuracy of the weighing instrument and the sense of responsibility of the weighing operator, it often happens that the amount of Lead powder in each cylinder does not reach the standard of 1000kg. In addition, there is a part of Lead powder left in the cylinder when feeding. The difference is a few kilograms at least, and tens of kilograms at most. In this way, the use proportion of additives with 1000kg Lead powder ratio in the paste process formula is wrong, which directly affects the quality of the paste.

The following work should be done to control the amount of Lead powder introduced:

1.1 In the research and formulation of lead paste process formula, the tolerance range of Lead powder amount should be given.

1.2 For the automatic control lead paste mixing machine, the accuracy of the weighing system should be calibrated frequently, the dynamic operation of the oxide conveying system and the vibration system should be checked, and the residual and sticky Lead powder in the weighing hopper should be removed in time.

The quality control of the Lead powder mainly refers to the inspection and confirmation of the oxidation degree of the Lead powder before it is put into paste mixing machine. Generally, the oxidation degree and apparent density of Lead powder are sampled and tested at the powder outlet of the ball mill during the Lead powder manufacturing process. The Lead powder that meets the process requirements is transported to the oxide silo or container for a 3 to 15 day storage period. Due to the influence of ambient temperature and humidity during the storage of Lead powder, its oxidation degree will rise, especially the temperature rise of the Lead powder stored in the oxide silo will be greater, When the paste mixing process formula is unchanged, if the oxidation degree of the Lead powder increases, the apparent density of the prepared lead paste will be lower, and the paste will be harder, which will affect the pasting, curing, formation and the electrical performance of the plate. Therefore, before the use of Lead powder, it is necessary to retest and confirm the oxidation index of Lead powder. For Lead powder with large changes in oxidation, it can only be fed after technical treatment or adjustment of process formula.

2. Control of additive(Expander) adding mode

The way of adding additives in the lead paste manufacturing determines the degree of uniform distribution of additives in the lead paste. If the additives are not evenly distributed in the lead paste, the quality of the produced plates will be uneven. Generally, all kinds of additives are weighed in advance according to the requirements of the process formula, packaged with containers or bag, and automatically controlled by the paste mixing machine to add the additives to the paste mixing machine through the material supply system. The manual feeding paste mixing machine will add the additives to the paste mixing machine through the person at the feeding port. Generally, after the Lead powder and additives are added, the paste machine starts to dry mix, and the dry mixing time is generally 2~3min, Due to the high density of Lead powder, dry mixing time is short, combined with the mechanical structure of paste mixing machine, it is difficult to fully mix evenly the oxide and additives that concentrated adding in a certain place of the paste mixing machine, after adding DM water, acid and proceeding wet mixing it is still difficult to achieve comprehensive Lead powder and additives fully mixed evenly, Finally leads to uneven additives in lead paste. Because the distribution of additives in the lead paste is difficult to test, it is easy to cause many quality problems such as local passivation of plate, bending of plate, shedding of active substances, etc.

Therefore, the following aspects should be carried out to control the additive adding method in the lead paste preparation process:

2.1 The additive adding method shall be clearly specified in the paste mixing process document.

2.2 When the Lead powder is transferred to the Lead powder weighing system, the first weighing is 500kg, the additives are   poured into the Lead powder bucket, and the second feeding is 500kg. When the weight of the Lead powder reaches the standard, the conveying system will automatically shut down and stop feeding oxide.

2.3 The additive and short fiber shall not contact with water, or even be affected with damp, otherwise, it is easy to agglomerate, resulting in poor plate strength.

2.4 Quality records shall be made for the addition of additives.

3. DM Water addition mode and amount control

The purpose of adding DM water in the lead paste manufacturing process is to further oxidize the metal lead in the Lead powder, and to adjust the consistency of the lead paste. After the dry Lead powder is added with DM water, on the one hand, the water enters the gap between the Lead powder particles and sticks to the particle surface. When the amount of DM water reaches a certain level, a water layer is formed between the Lead powder particles. On the other hand, because the PbO in the Lead powder is a kind of alkaline oxide, the water added and mixed with PbO generates an alkaline colloidal substance Pb (OH) 2, that is, PbO ＋ H2O ＝ Pb (OH) 2 (colloidal substance), which makes the water layer produce adhesion, Make the Lead powder particles in the separated state contact each other through the water layer with adhesive force on the surface to form a loose and dense system, which can be deformed under the action of external force, that is, the Lead powder becomes a malleable lead paste.

In the process of mixing the Lead powder from dispersed granules to loose and thick system with certain adhesion by adding water, the amount and speed of adding water have a very important influence on the formation of the malleability of the lead paste (including the water in dilute sulfuric acid). If the water is excessive, the water layer between the Lead powder particles is too large, the consistency is reduced, and the particles begin to float with fluidity, then the lead paste will lose its malleability and become thinner, This kind of lead paste is easy to be pasted with a low amount of paste, and the uncharged plate is easy to suffer from plate surface collapse, paste drop, and no clear and neat cloth pattern on the surface of the plate. If the amount of water is less, the water layer between the Lead powder particles is too small, the consistency of the lead paste is increased, the lead paste is hard, the pressure of the lead paste on the grid is too large during pasting process, which makes the plate easy to deform and the uneven amount of paste leads to poor flatness of the plate.

In addition, when the amount of Lead powder is fixed, the more the water content of the lead paste, the lower the apparent density of the lead paste. After the uncharged plate is curing and dried, most of this water will evaporate and be consumed, forming a large number of pores in the dry lead paste of the uncharged plate. Obviously, if the apparent density of the lead paste is low, there will be many pores formed in the paste of uncharged plate, and the porosity of the plate is large, which is conducive to the diffusion of sulfuric acid during the charging and discharging of the battery. The reaction area of the plate will increase relatively, and the capacity will increase. However, if the water amount is too much, the porosity of the plate will be high, which will easily lead to the weak combination between the active substances and between the active substances and the grid, making the lead paste easy to fall off during the charging and discharging of the battery, leading to the shortening of the battery life. On the contrary, if the amount of water is too small and the apparent density of lead paste is too high, it is easy to reduce the porosity of the plate and the capacity, but the charging and discharging life of the battery is relatively good.

Therefore, the following aspects should be carried out to control the amount and time of adding water during the production of lead paste:

3.1 The water quantity, time and method of adding water shall be clearly specified in the paste mixing process document.

3.2 Before adding water, confirm the amount of water added, and record the time and method of adding water.

3.3 Generally, it is required to add water quickly. The specific time for adding water should be determined according to the oxidation degree of Lead powder and the ambient temperature. When the oxidation degree of Lead powder is high and the ambient temperature is high, the time for adding water can be shorter. When the oxidation degree of Lead powder is low and the ambient temperature is low, the time for adding water can be longer. However, it is generally required that the time for adding water should be about 1min, and the mixing time should be about 6min. If the time of adding water or stirring is too long, a large amount of metal lead in the Lead powder will be oxidized to PbO, or even further oxidized to Pb3O4. The oxidation process of metal Pb in lead powder should be completed in a long time during paste mixing, curing and drying to ensure the good properties and mechanical strength of the plate. Therefore, a large amount of oxidation of metal Pb in lead powder during paste mixing will lead to a series of problems in the performance and mechanical strength of the plate.

4. During the paste mixing process, dilute sulfuric acid with a certain density shall be added after adding water and stirring. The purpose of adding dilute sulfuric acid is to form a malleable lead paste mainly composed of basic lead sulfate. Because the basic lead sulfate is needle shaped crystals or rod shaped crystals, the crystals are interlaced with each other to form a certain mechanical strength, which is convenient for the pasting and is conducive to the curing and formation of the plate.

4.1 Influence of sulfuric acid addition mode

The method of adding sulfuric acid mainly refers to the method and time of adding the prepared dilute sulfuric acid into paste mixing machine. Theoretically, the addition of sulfuric acid should enable the lead powder of unit area to obtain the same amount of sulfuric acid at the same time in unit time. However, in practice, the theoretical requirements are difficult to achieve due to the structural problems of the paste mixing machine and the heat dissipation problem generated from the reaction of sulfuric acid and lead powder, This requires that the method and time of adding sulfuric acid should be as scientific as possible in the process of paste mixing. The traditional pipe injection method and container direct injection method will make sulfuric acid enter local lead powder too intensively, resulting in too much acid in local lead powder and too high temperature rise. The formation state of alkaline lead sulfate is quite different. Due to the structural problems of the paste mixing machine, The lead powder group that obtains sulfuric acid first is difficult to fully infiltrate with the lead powder group in other parts in a short time, and the uneven phenomenon of lead paste alkaline lead sulfate often occurs. After the use of such lead paste, the quality difference between the plates is easy to occur. The acid adding time of "sand type paste" is generally controlled to be about 20min. If the acid adding time is too short, the temperature of the lead paste will rise too fast and too high, and the lead paste will become hard. If the acid addition time is too long, the lead paste becomes very soft, and it is difficult to apply the plate for pasting. The lead paste after the plate curing is brittle, and the active substance after the plate formation is in powder form.

4.2 Influence of mixing time

After the addition of sulfuric acid, the lead paste shall be stirred for a period of time. The purpose is to make the composition of the lead paste fully uniform, but the stirring time has an impact on the quality of the lead paste. Generally, it is better to carry out about 20 minutes after the addition of sulfuric acid. If the time is short, the composition of the lead paste cannot be fully uniform; If the time is long, due to the thermal effect of the paste mixing process, 4PbO·PbSO4 will be generated too much, making the lead paste hard.

The amount of sulfuric acid added has a great influence on the apparent density and consistency of the lead paste. Under a certain amount of water, the more dilute sulfuric acid is added, and the lower the apparent density of the lead paste is due to the increase of the total liquid amount. The consistency of the lead paste increases linearly with the increase of the amount of sulfuric acid in the lead paste.