Monitoring pickling baths in the steel industry

Steel-pickling involves removing impurities resulting from previous production steps - for example the Scale 1 produced during rolling, or any rust already present - then preparing the surface for subsequent process steps.

At the same time, interfering annealing colours are removed, the surface being passivated by the formation of a protective layer and in this way protected against further corrosion.

Only after the pickling process can steel be formed or its surfaces treated and, for example, used for the production of automobile or metal-construction components.

The pickling baths used are made up of diluted acids, for example hydrochloric or sulphuric acid, or are mixtures of acids such as HNO3/HF or H2SO4/H3PO4/HF.

Accelerants and other auxiliary agents are often added to these mixtures to optimise and speed-up the process.

While pickling removes impurities, the acids used also attack the steel surface and partially dissolve it.

This is why it is important that process-relevant parameters such as dwell time, bath temperature and bath composition are accurately controlled and maintained.

Physical parameters such as dwell time and bath temperature are easy to monitor, whereas the analysis of bath constituents must be carried out in an analytical laboratory.

The Metrohm Processlab system removes the need for qualified laboratory personnel to analyse bath constituents twenty-four hours a day.

Processlab can be installed directly in the process area.

This enables on-site process analysis around the clock and results in more direct and precise process control.

The Processlab system automatically determines parameters that are important for the smooth running of a pickling bath: free acid and total acid as well as the Fe(II) and Fe(III) concentrations.

The system is installed in the vicinity of the process to be monitored and allows the rapid on-site analysis of various pickling baths.

A Processlab analysis system always consists of one or several analytical modules adapted to the particular application and a TFT operating unit.

The operating unit and the analysis module are contained in a robust, splash-protected housing and are suitable for use in harsh conditions.

The single Processlab housing includes: a titration vessel with magnetic stirrer; variable and automatic sample metering using a sample loop; an 800 Dosino; and two 800 Dosinos for the exact addition of the two titrants: Ce(SO4)2 and NaOH.

The peristaltic pumps are used for the automatic addition of reagents and auxiliary agents and for rinsing the titration vessel.

The user only has to take a sample from the bath and bring it to the system.

The analysis sequence for the automatic determination of all four parameters is started by pressing a single button.

The sample is metered automatically, transferred to the titration vessel and the concentrations of the analytes determined by titration.

All the necessary process information is available within only a few minutes, without the bath samples having to be taken to the laboratory for analysis.

To determine the free and total acid, a 2mL sample is metered fully automatically, treated with 20mL potassium fluoride solution (c(KF) = 3 mol/L) to mask the iron, demineralised water is added and the solution then titrated with NaOH solution to pH 4.2.

The NaOH consumption corresponds to the amount of free acid in the bath.

A further aliquot of the sample is titrated with NaOH solution to pH 8.6; in this case the total amount of NaOH consumed corresponds to the total acid concentration in the bath.

To determine the Fe(II) and Fe(III) concentrations, demineralised water is placed in the titration vessel and a sample aliquot of 2mL is added to it automatically.

Using a Pt electrode, titration is performed with cerium (IV) sulphate solution (c(Ce(SO4)2) = 0.1 mol/L) to the first endpoint.

The Ce(IV) consumption corresponds to the Fe(II) concentration in the pickling bath.

The Processlab system determines all the relevant bath parameters and collects important process information.

The results are available at any time for the subsequent traceability of the process and for making audits easier.

The system can also transmit status signals, for example if a predefined limit is infringed or if a fault occurs in the system.

Information about the bath content can also be easily transmitted in the form of an analogue 4-20mA signal.

Information is immediately made available to the personnel in the operations centre or the process-monitoring system.

This makes it possible to react directly to variations and, if necessary, to take countermeasures.

If a variation does occur, the correct amount of fresh acid or auxiliary agent to be added can be calculated with the acid of the included tiamo for Processlab software; the addition can be carried out fully automatically.

The rapid availability of the analytical data allows the pickling process to be carried out under optimal conditions.

This improves the quality of the end product; reducing the use of chemicals results in lower running costs.

The more efficient bath control and higher quality end-products often means the Processlab system will pay for itself in fewer than three years.