Curing process pasted positive plates

Curing is the process of exposing plates pasted positive and negative to a regime of (a) controlled time (minimum 32h), (b) temperature (30-35°C), and (c) relative humidity (>90%). This process converts the free lead into lead oxide, using oxygen from the surrounding air. The plates are allowed to cure for a minimum of 32 h. Care is also taken to ensure that the maximum temperature of the plate does not exceed 60°C. The cured plates are then parted. 

Positive and negative active-mass formation. The cured pastes of both positive and negative plates comprise identical mixtures of bivalent lead compounds (3BS, 4BS, PbO), which cannot create electromotive forces when the pasted plates are assembled into cells. The purpose of the formation step is to convert the cured pastes into electrochemically active porous materials — Pb02 in the positive plates and Pb in the negative plates — which are connected mechanically and electrically to the grids. The process of formation can be conducted via two basic schemes, as shown in Fig. 3.1. 

Zonal processes are slow processes. They are responsible for the long duration of plate formation. Hence, it is important to find methods to by-pass the zonal processes. In order to accelerate the formation of positive plates, some conductive additives have been added to the positive pastes [28]. These additives increase the conductivity of the cured paste and formation proceeds almost uniformly throughout the whole plate volume as the electric current flows along the conductive additive network. The additives should be chemically stable in H2SO4 solution. Data have been reported about successful attempts to reduce the duration of the formation procedure to 8 h. 

Positive plates need much more time to form than negatives. The reason for this is the dielectric behaviour of the cured positive paste. Oxidation of the bivalent lead compounds in the paste and formation of the Pb02 positive active mass passes through a number of chemical reactions, some of which proceed at a low rate, which retards the technological process of formation of the positive plate. In an attempt to accelerate the formation process, additives to the positive paste have been looked for, which are characterised by electro-conductive properties and stability in sulfuric acid. These additives create an electro-conductive network in the paste and the process of oxidation proceeds simultaneously within a large paste volume, thus accelerating plate formation. 

With the first curing method water wfll evaporate from the paste most quickly and the curing process will stop at a very early stage. The degree of active mass formation and the capacity of the positive plates will be low (Table 8.3). In the second method of curing, with increase of temperature to 55 °C and RH = 80%, water evaporation from the plates wfll be slowed down... 

Paste that has been insufficiently cured is easily broken. The uncured paste is pale in color and soft. If the plate has been insufficiently cured, it can be rewetted and reheated to force the paste to cure. Another way is to dip the partially cured paste into dilute sulfuric acid, which is called pickling. The pickling curing process is also used for curing of powder-filled tubular positive plates. 

Sulfate salts have been introduced into the positive paste of VRLA cells during the paste-mixing process. In a recent study [65], sodium sulfate was added to positive pastes at 0, 0.01, 0.05, 1, and 2M levels (note, the authors did not explain how the molarity levels were calculated). The plates were cured at 85°C to form tetrabasic lead sulfate and a-PbO. The tetrabasic lead sulfate crystals had sizes of... 

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