Monday, December 7, 2009

Rod cleaning and preparation prior to drawing

That there are many ways to skin a cat is certainly true when it comes to removal of mill scale and surface oxide.The scale is the Fe3o4 blue oxide that forms on the wire as it cools following the rolling process.

Fe2O3 is the red surface oxide or rust that forms during shipping and storage.

Both oxides need to be removed prior to drawing.

You can do this by 4 basically different methods.

  • Acid pickling
  • Mechanical de-scaling
  • Abrasive Blasting or Sanding
  • Shaving.

Acid pickling in turn can be divided up in 2 major sub methods

1. Chemical acid pickling

and

2. Electrolytic pickling

Conventional chemical acid pickling is done by submerging the rod bundles in sulfuric or hydrochloric acid.

The iron ions will then go into solution as hydrogen gas is formed on the steel surface "blasting" off the iron oxide.

Following the acid dip, the rod bundle is submerged in a rinse water tank followed by a lime or borax tank.

The rod is then placed in a baker where the crystal water in the lime or borax is removed.

The wire rod is then ready for processing in the wire mill.


Continous Pickling Line

View of Line through Tunnel Hood

Manually operated crane pickling line

Electrolytic cleaning or bipolar cleaning. (see sketch )

Electric current will always take the pass where the electrical resistance is the lowest. In an electrolytic bipolar cleaning tank, most of the current will flow from the cathode to through the electrolyte to the nearest point of the wire. The current will then pass through the wire to the point where the wire is closest to the anode, where it will exit the wire and again pass through the electrolyte on to the anode.

At the point where the current enters the wire Oxygen gas will form on the steel surface and where the current exits the wire Hydrogen gas will form on the steel surface breaking off the scale.

In order to reduce the amount of acid and electric energy needed, it is common to first mechanically de-scale (see below) the rod before it enters the bipolar cleaning process.

Mechanical de-scaling.

In this process the wire rod is pulled through a set pulleys (normally 5) 2 guide wheels and 3 breaker wheels of approximately 3 inch diameter with the axis of rotation offset 90 degrees.

As the wire is bent over the breaker wheels as much as 80 % of the mill scale is removed.


Scale breaker unit

View of Combination Line with scale breaker followed by steel brushes.

Most (often all) of the remaining undesirable scale residue can then be removed by a water/air jet cleaner using a combination of a water wash/air dry sequence.

or

The wire rod can be passed through an electrolytic bipolar cleaning process.


Electrolytic Bi-polar cleaning unit


Mechanical descaling
followed by electrolytic cleaning
borax coating and pull through capstan

or

The wire rod can be passed through a cavity cleaner Paraorbital unit where water at high pressure causes the wire rod to vibrate hence removing residual scale.

It is also common in certain applications like bed spring wire to let the wire rod pass through a set of abrasive belt sanders that polishes the rod surface while removing any remaining scale residue.

Abrasive Blasting or Sanding

Yet another method used is so called abrasive blast cleaning where abrasive material is "blasted" on the wire rod surface.


Sand Blasting Unit

Shaving.

Shaving is a method by which the rod is pulled through a shaving die that removes or shaves off surface imperfections at the same time as the mill scale is removed. This method has predominantly been used for stainless steel wire rods.

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