Answer to Question #309415 in Chemical Engineering for Gurus

Question

Describe the composition , physical properties and mettods of fabrication , and uses of acid resistant brick

Answer 1

Traditionally, acid reistant brick in combination with an acid resistant membrane has been used to protect carbon steel equipment from corrosion by sulphuric acid.  The brick itself is not totally acid resistant since a portion of the brick is acid soluble.  The brick merely serves to reduce the rate at which the acid migrates to the membrane and shell.  The other function of the brick lining is to reduce the temperature at the shell to a point where the acid resistant membrane used will not be damaged by high temperatures.

Two types of bricks are commonly used in the sulphuric acid industry; Red Shale and Clay Bricks.

Bricks are classified under three general types:

Type I - For use where low absorption and high acid resistance are not major factors.  Formerly designated as Type "H".

Type II - For use where lower absorption and higher acid resistance are required.

Type III - For use where minimum absorption and maximum acid resistance are required.  Formerly designated as Type "L".

The standard does not attempt to control the composition of the brick as it will differ depending on the source of or raw materials used to manufacture the brick.  The standard does define certain physical properties to which the brick must conform.  The physical properties are:

• Strength
• Water Absoption
• Sizes
• Warpage
• Surface Texture

Typical physical properties of acid brick are as follows:

The porosity of a brick is a function of the material (fireclay or red shale) and the method of manufacture.  A higher porosity implies that it is easier for liquid to penetrate the brick.  Thus it is generally desireable to have a brick with the lowest porosity possible to minimize migration of acid through the brick to the shell of the vessel. 

There are some cases where higher porosity is desireable such as in high temperature applications.  In this situation, brick porosity helps a brick to resist spalling due to thermal shock by allowing it to absorb dimensional changes more easily.

The iron content of the brick is an important factored to be considered when selecting the type of brick.  Acid will be absorbed into the brick and will react with any iron present to form iron sulphate.  The volume of iron sulphate is 6-7 times that of iron.  The result is spalling or flaking off of the brick surface as illustrated in the photo of a pump tank lined with red shale brick.  The typical iron content of red shale brick can be as high as 7.5% compared to an iron content of only 1.5% for clay brick.

Spalling of brick can be the result of severe temperature cycling and/or chemical attack.  In this case the brick work is in a pump tank so the likelihood of high temperature cycles is minimal.  Instead the brick work has been subjected to chemical attack.  The bleaching or whitening of the surface of the brick indicates that the iron has been leached out of the brick. 

If fluorides are present, they contribute to the weakening of the brick leading to spalling of the brick surface.

Acid brick installed in the bottom of an absorber tower is generally exposed to high gas temperatures in the range of 160 to 250°C.  Thermal shock of the brick can occur when a dry area of the brick work is exposed to the high temperature gas and is suddenly cooled by contact with the relatively colder acid.  The brick will eventually spall when exposed to these conditions over a long period of time.  A typical example of high temperature brick spalling is shown in the photo taken in bottom of an absorber tower.  The surface of the brick on the support arch shows the results of thermal shock on the brick.