Mill scale, often called scale, is basically Iron Oxide and is the flaky surface of hot rolled steel. It is formed on the outer surfaces of plates, sheets or other profiles during the production process, when these are produced by rolling red hot iron or steel billets in rolling mill.
In steel production process of hot rolling and casting large amounts of water are used depending on the scale of the operation. As steel leaves the casting operations, cooling and cutting with water produces mill scale (iron oxides).
This hot water from the mills is transported to Scale pit for the first stage of clarification. This partly clarified water is pumped to settling tank (Second stage of clarification) for removal of suspended solids as well as oil and grease. In the final stage, special HIGH RATE DEPTH FILTER for the removal of suspended solids and oil contents are used.
"Process Details and Design Criteria"
The conventional definition of filtration is a mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Mill scale filtration process in addition to separation of mill scale particles also remove free oil particles (droplets) from water.
There are two types of filters for separating particulate solids from fluids – Surface filters (sieves), where separation occurs at a single perforated layer (a sieve), and Deep Bed (or Depth) filters, where the granular particulate media (multilayer lattice) retains those particles that are unable to follow the tortuous channels of the filter media.
High Rate Depth Filter works by providing the particulate solids with many opportunities to be captured on the surface of the media particles. As fluid flows through the porous media along a tortuous route, the particulates come close to media grains. They can be captured by one of the following mechanisms
With these variations on mechanisms for removal there are many different factors involved in design of High Rate Depth Filters. Like any other process equipment, filters need to be designed so that they are efficient, economical, easy to operate and maintain. Understanding the relationship between filter performance, filtration rate and filter media properties is the key in successful design.
High Rate Depth filtration is accomplished by attachment of the particles to the media, which is a completely different mechanism than that of Sieves as the Depth Filters do not primarily work by straining or size exclusion.
After a filtration cycle has progressed for a period of time, the bed becomes loaded with dirt and contaminants and begins to lose its effectiveness as a filter. It may also begin to clog preventing fluid from passing there across. As a remedy, the particle bed is periodically backwashed to remove the dirt and contamination from the filter media and flush the same away from filtration system for disposal and reclamation. Contaminant loading causes a larger pressure drop across the bed and unless the bed is backwashed to remove the trapped materials a fracture will form allowing water to bypass the filter bed matrix. It is the backwash method which predominantly distinguishes the good filters from the bad ones.
The backwash system in a High Rate Depth Filter is a nozzle less system. The under drain design is the T-beam, constructed of compact corrosion-protected sectional steel beams arranged side by side on a flat steel ring. Media selection is critical as the steel particles that need to be filtered out are dense. At the same time they need to be removed during the backwashing process, or otherwise after sometime the Filter bed gets clogged. This requires specially graded filter media and perfectly designed internal arrangement. Special Silica sand media with a specific higher bed depth is used.
The process steps in removal of the particles use a combination of Air and Water in a specific sequence to enable removal of the filtered particles. The higher bed depth of filtration also enables larger hold up of the filtered particles in the bed thus enabling the filter to take up a higher suspendedsolids load. Proper Water and Air distribution during service and also especially during thebackwashing operation is important. This is ensured by having a uniform gap between the bottomlaterals. The Laterals are designed in such a manner so that there are uniform distribution points notonly for the water but also an Air bed is generated below these laterals by means of a speciallydesigned Air distribution Header and Lateral arrangement. The generation of this Air bed ensuresuniform Air distribution which is essential for proper backwashing.
| Description | Inlet Water Quality | Outlet Water Quality |
|---|---|---|
| Suspended Solids | 50 to 75 ppm | 5 ppm |
| Total Oil & Grease | 15 to 20 ppm | 3 ppm |
| Flow Rate | As per customer process tailor made cost effective system is offered. |