Acid pickling operation

In a steel product manufacturing plant involving acid pickling operation, disposal of hazardous metal sludge is the most expensive engineering task, and treatment of pickling liquor and rinse water is the second most expensive engineering task. 

Acid pickling This can be done under the following operating conditions, either with sulphuric acid (H2SO4), or hydrochloric acid (HCl). H2SO4 releases a lot of fumes and is ineffective under cold conditions. It forms iron sulphate, which forms a hard deposit at the bottom of the tank and is difficult to remove (see table on next page). 

Sheet steel is normally prepared for application of enamel by a sequence of operations including thorough degreasing, acid pickling and neutralisation. A nickel dip stage is often included to deposit a thin, porous layer of nickel applied at about 1 g/m especially when conventional groundcoat is not used (see Section 13.7). 

Parker Amchem. Rodine Acid Inhibitors for Industrial Cleaning and Steel Pickling Operations. Technical and marketing document. Parker Amchem, Inc. (Henkel), USA. 

Acid pickling. Steel products are immersed in heated acid solutions to remove surface scale during pickling operations. This generates wastewater from three sources ... 

Water rinses are used in surface preparation operations such as acid pickling, alkaline cleaning, and nickel deposition to remove any process solution film left from the previous bath. A water rinse may also follow the neutralization step. Another common water use is in the ball milling process, which uses water as the vehicle for the enamel ingredients, as a cooling medium, and for cleaning the equipment. Coating application processes normally use wet spray booths to capture oversprayed enamel particles. Water wash spray booths use a water curtain into which the enamel particles are blown and captured. 

Acid recovery systems are used to recover the free acid in the WPL. They are not employed in larger facilities because they recover only 2-4% free HC1 from the spent acid, but leave the FeCl2 in the solution that must be processed or disposed of separately. These acid recovery systems are generally closed-loop processes that do not emit HC1. In their survey, U.S. EPA compiled data from different types of pickling operations and their estimated emissions.5 This information is reproduced in Table 28.11. 

In order to estimate emissions from pickling facilities, U.S. EPA developed 17 model plants to represent five types of pickling operations and one acid regeneration process.12 The model plants include one or more size variation for each process model. The model plants were developed from information obtained from a survey of steel pickling operations and control technologies. U.S. EPA estimated emission rates for model facilities. Using these emission rates and the production and hours of operation for the model pickling plants, emission factors were calculated. These appear in Table 28.12. 

Raw materials used by the plant include coil and sheet metal stock, solvent-based marking ink, and protective plastic end caps. The two major operations in this plant, pipe and fitting formation and acid pickling, are described in this section. 

The basic unit operations/processes required for treating the acid pickling wastewater are (a) neutralization with NaOH and/or lime to increase the pH and (b) physicochemical methods, such as chemical coagulation, precipitation, clarification (sedimentation or DAF), and filtration to remove BOD5, COD, and iron. 

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