Our client Azor

Car batteries recovery Car batteries wastewater treatment by forced circulation evaporation

 
Evaporator TC 60000 FFZoom

Installation at Azor premises

This report describes how treatment of waste from car batteries recovery by means of LED forced circulation evaporation provided:

  • an environmental benefit from recycle of sulphuric acid
  • a reduction in demand of main water due to re-use
  • a big saving in disposal cost due to zero liquid discharge
  • a return on investment of less than two years

Introduction

Since 1948 Azor - Murcia, Spain - is a secondary lead smelting plant that produces mainly lead ingots of 25 kg for automotive car batteries, lead alloys and lead shot for hunting and sport shooting. Azor has a recovery plant for lead-acid batteries, capable of separating the different components of batteries by wet system, with neutralization of electrolyte sulphuric acid. While the segregated lead can be used as feed for the melting furnaces and re-used to build new batteries the sulphuric acid needs a specific treatment plant to be recycled as sodium sulphate. The plant is run in accordance with ISO-90021 System Assessment of Quality. The total production of wastewater generated by batteries recovery operations on site was over 15.000 tons per year, waste that the company had to disposed of, off site.

In order to reduce the ever increasing disposal costs, the company implemented a project in 2004, based around the principle of reducing waste volumes by vacuum evaporation. The disposal costs saved by this project provided a return on investment of less than two years.

Vacuum evaporation provided a thermal separation of the wastewater into two streams, a distillate and a concentrate. The distillate representing nearly 60% of the inlet volume, was in compliance with the regulation in force and was re-cycled back to process as rinsing water. The concentrate which was around 40% of the initial volume, was a solution over the solubility level, having a big quantity of sodium sulphate that was segregated as wet salt (as in the picture). The implementation of the water treatment based on evaporation gave the site the ability to attain the "zero liquid discharge" status, which is becoming more and more significant giving the trend of environmental legislation. The reduction in wastewater disposal volumes was the key factor in the economic return.

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Why evaporation?

The waste water coming from Azor batteries recovery operations was an aqueous solution having a strong acidic pH (<1,0) with high sulphate content, and a high concentration of heavy metals such as lead and iron. This particular kind of wastewater, provided an excellent feed for the evaporation technique. Furthermore, the evaporation process required only pH correction and a settling tank as pre-treatment. In particular, respect the physical-chemical treatment, that is the most common alternative method, the evaporation is not time consuming and does not produce any waste to dispose. The choice of the right construction material was very important to guaranty the necessary corrosion resistance..

The evaporation process

Evaporation is a thermal treatment that can be applied directly to waste waters with little or no pre-treatment, and allows separation of the water from the waste because of the different volatility of the compounds contained within the water. Evaporation provides advantages over traditional water treatment techniques, such as full automation, very low chemical consumption and high tolerance of variability to concentrations of pollutants. Typically, it provides excellent separation of metals and ions; oils and surfactants; COD and BOD.

The evaporation equipment

The evaporator at the heart of the waste water treatment system chosen by Azor is a TC 60000 FF model, one of a standard range of skid mounted modular designs, produced by LED Italia. The machine is classified as mechanical vapour recompression (MVR), forced circulation, evaporator and is able to produce up to 60.000 litres of distillate per day. This model is designed for low maintenance and continuous, unattended 24h, 7day/week working operations. The machine is constructed from SAF 2507, a type of Super Duplex Stainless Steel, which resists the corrosive effects of chemical compounds concentrated by the continuous water evaporation. The working temperature is around 90°C and the pressure in the boiling chamber is around 70kPa.

The liquid to be treated is preheated by the discharged distillate. The inflow is controlled automatically by a level sensor which acts on the loading pump handled by an inverter and monitored by flow transmitter. Within the boiling chamber the pressure is 70 kPa and the temperature is around 90°C. The water is than pumped through the primary heat exchanger where it receives the heat before going back to the boiling chamber, where immediately boils (flash evaporation). The produced vapor pass through a packed bed in order to eliminate suspended liquid drops and improve the separation. A blower compresses the vapor and sends it to the heat exchanger where it releases the latent heat to the process liquid. The distillate and the concentrate are than collected in tanks, the concentrate being discharged automatically according to a pre set determined at the commissioning stage. The Led TC 60000 FF evaporator requires only 1/10 of the energy needed by an atmospheric gas fired evaporator.

Performance

The distillation yield of the installed equipment has shown a consistent performance: 60% distillate having low hardness and low conductivity; 40% concentrate from which it was possible to recover the salt. This allowed Azor to re-use the distillate as rinsing water, thus reducing the site demand for fresh mains water and leading to further savings. 

Conclusions

The experience that Azor have gained with their evaporation systems have demonstrated the suitability of the process to treat wastes generated by car batteries recovery operations. The company has reduced dramatically the costs of waste disposal and has been able to demonstrate to the local authorities and the local community that they have a pro-active and responsible attitude to their environmental obligations.

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Process scheme

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