Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
Red Shale vs. Fireclay
High Temperature Installation
Thermal Shock Resistance
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.
The specification of acid resistant brick can be based on the requirements set forth in ASTM C279 'Standard Specification for Chemical-Resistant Masonary Units' and other reference specifications. It covers both shale and clay type 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:
- Water Absoption
- Surface Texture
Further specification of acid resistant brick can be based on physical properties as set forth in the following specifications:
|ASTM C20||Standard Test Methods for Apparent Porosity, Water Absorption, Apparent Specific Gravity, and Bulk Density of Burned Refractory Brick and Shapes by Boiling Water|
|ASTM C133||Standard Test Methods for Cold Crushing Strength and Modulus of Rupture of Refractories|
|ASTM C279||Standard Specification for Chemical-Resistant Masonary Units|
|ASTM C373||Standard Test Method for Water Absorption, Bulk Density, Apparent Porosity and Apparent Specific Gravity of Fired Whiteware Products|
The specification of acid resistant brick can be based on the requirements set forth in the following British Standards
|BS 1902||Methods of Testing Refractory Materials|
|BS 3921||Specification for Clay Bricks|
|BS EN 993-16||Methods of Test for Dense Shaped Refractory Products|
The specification of acid resistant brick can be based on the requirements set forth in the following DIN specifications.
|DIN 28062||Building Materials for Brick Lining; Classification, Properties, Testing|
|DIN EN 993-1||Determination of Bulk Density, Apparent Porosity and True Porosity|
|DIN 51067||Determination of the Crushing Strength at Room Temperature (CCS) on Refractory Materials|
|DIN 51068 Part 1||Determination of the Resistance Against Thermal Shock|
|DIN 51102 Part 2||Determination of the Resistance Against Sulphuric Acid|
The specification of acid resistant brick can be based on the requirements set forth in Indian Specification 4860-1968.
Tests 1st Class 2nd Class Water Absorption 2% 4% Flexural Strength 100 kg/cm 70 kg/cm Compression Strength 700 kg/cm 500 kg/cm Acid Loss 1.5% 4% Resistance to Wear 2.0 mm - Warpage 2.5 mm 2.5 mm Tolerances - Length ± 3.5 mm ± 3.5 mm - Width ± 2.0 mm ± 2.0 mm - Thickness ± 1.0 mm ± 1.0 mm
Length Height Width 230 mm (9.06 in) 115 mm (4.53 in) 38 mm (1.50 in) 230 mm (9.06 in) 115 mm (4.53 in) 64 mm (2.52 in) 230 mm (9.06 in) 115 mm (4.53 in) 75 mm (2.95 in)
Typical physical properties of acid brick are as follows:
|Red Shale||Fireclay||Test Method|
|Density (g/cm³)||2.45||2.15||ASTM C20|
|Water Absorption (%)||2||5||ASTM C20|
|Apparent Porosity (%)||5||11||ASTM C20|
|Cold Crushing Strength (N/mm²)||>150||>80||ASTM C133|
|Acid Solubility (%)||7||6||ASTM C279|
|Thermal Spalling (cycles 450°C)||5||10||DIN 51068|
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.
Red shale brick typically has an iron content of 6.5% compared to fireclay which has only about 2.5% iron. Since iron and other elements will leach out of the brick, red shale should not be used in applications where product purity must be maintained at high levels.
Fireclay brick are generally manufactured to higher tolerances which results in dimensional stability, less warping, better overall shape, etc. Fireclay bricks will generally result in a higher quality installation than red shale brick. Regardless of the brick used, the experience and ability of the installer is the primarily factor in the quality of the installation.
Fireclay bricks are preferred in areas exposed to high temperatures such as gas inlets on absorber towers. Fireclay bricks are less brittle and has better thermal shock resistance than red shale bricks. The alumina content of fireclay brick is higher which gives it more refractory properties.
Bricks come in standard sizes to enable mass production of the bricks and reduce costs. There will generally be a difference between brick purchased from European suppliers and North American suppliers. Suppliers will offer a variety of custon shapes for specific applications such as nozzles, arches, curved surfaces, etc. The larger the selection of shapes and sizes, the fewer field cuts required to lined a vessel. The overall result should be a better quality lining.
|240 mm (9.45 in)||115 mm (4.53 in)||40 mm (1.57 in)|
|240 mm (9.45 in)||115 mm (4.53 in)||50 mm (1.97 in)|
|240 mm (9.45 in)||115 mm (4.53 in)||65 mm (2.56 in)|
|240 mm (9.45 in)||115 mm (4.53 in)||80 mm (3.15 in)|
|240 mm (9.45 in)||115 mm (4.53 in)||100 mm (3.94 in)|
North American - Fire Clay
|2" Split||9" (228.6 mm)||4 1/2" (114.3 mm)||2" (50.8 mm)|
|Straight||9" (228.6 mm)||4 1/2" (114.3 mm)||2 1/2" (63.5 mm)|
North American - Red Shale
|Single||8" (203.2 mm)||3 3/4" (95.25 mm)||2 1/4" (57.15 mm)|
|Double||8" (203.2 mm)||3 3/4" (95.25 mm)||4 1/2" (114.3 mm)|
|Split||8" (203.2 mm)||3 3/4" (95.25 mm)||1 1/8" (28.58 mm)|
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.
Thermal Shock Resistance
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.