Sanitaryware is a commonly used term in the ceramics industry referring to sanitary appliances found in toilets and bathrooms. Traditionally, WC bowls, cisterns, bidets, urinals and washbasins are made from porcelain (a ceramic material made from clay that might be described as ‘vitreous china’ when coated with enamel). This might be referred to as ceramic, porcelain or china sanitary ware, and is often white.
In the modern day, most sanitaryware products are white or have a very pale colouring. Any defect is easily visible with the affected product being rejected prior to release from the manufacturing plant. This is costly and time-consuming, with rejected materials being re-crushed and fed back into the process.
One of the most common causes of defects is iron, either as free iron or as an iron bearing mineral. Iron contamination occurs as black spots on the surface of the sanitaryware. It also can affect the colour of the glaze, reducing the whiteness.
There are several key stages within a sanitaryware manufacturing process including:
- Body preparation;
- Glaze preparation;
- Moulding and Mould preparation;
- Control and Spraying;
Within this process, there are key areas to locate and position magnetic separators.
The ‘Body’ is the material mix used to form the core structure of the sanitaryware item. Body is produced by mixing the raw materials Ball Clay, Silica, China Clay and Feldspar.
In most cases, raw material suppliers have already removed a large proportion if not most of the free iron and iron bearing minerals. However, tramp metal is often accidently introduced during transportation. If tramp metal enters the Ball Mill it can cause significant damage or be reduced and spread extensively through the body mix, resulting in a high defect rate.
To remove the tramp metal, a Magnetic Separator is installed prior to the Ball Mill. There are several options included:
- Overband Magnets – suspended over the feed conveyor to automatically lift and remove tramp metal;
- Plate Magnets – again suspended over the conveyor belt although these tend to be manually cleaned;
- Grate Magnets – material cascades from the end of the conveyor into a Grate Magnet. The Grate Magnet has rows of Tube Magnets that capture and hold any ferrous metal contamination;
- Pulley Magnets – the head pulley is replaced with a magnetic pulley to automatically attract and remove tramp metal from conveyed raw material;
- Drum Magnets – these offer the best level of separation. Material is fed onto a rotating non-magnetic shell where tramp metal is attracted to the stationary magnetic arc and then discharged into a separate collection chute away from the cleaned product;
The mixed and milled raw materials then form the ‘Body’. As it is discharged from the Ball Mill, the body slurry is often passed through Electro Magnetic Filters to capture any iron or iron bearing minerals that were liberated during the milling process.
The Body is then poured into a mould and cast. In the manufacture of sanitaryware, high pressure casting is used. The casted ware is left to dry and then released from the mould. This ‘Green Ware’ is allowed to dry in atmospheric temperature for one to three days before sent to drier.
Glaze Preparation and Application
Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a ceramic body through firing. Through history, it has been used to cover and disguise iron-induced and other defects.
Glaze is produced by mixing zircon, feldspar, quartz, calcite, china clay and zinc oxide. The raw material mix is grinded in ball mill.
Ensuring the Glaze is iron-free is more critical than with the Body. Any iron present in the Glaze will be visible on the surface of the sanitaryware product. As the Glaze is discharged from the Ball Mill, it is commonly fed through an Electro Magnetic Filter to remove even the weakest and smallest iron particles.
The Glaze is then taken to a Glazing Station in a container. Here the Glaze is sprayed onto the cast sanitaryware using either manually or robotic techniques. The Glaze thickness is between 0.8mm and 1.6mm, depending on the manufacturer’s specification. Glaze run-off returns to the container.
Mounted between the Glaze container and the sprayer are high intensity permanent Rare Earth Magnets, commonly in the form of Tubes. The Glaze passes over the Tube Magnets, which attract and hold any iron particles. This Magnetic Separation stage is critical as it is the final opportunity to remove iron prior before the sanitaryware body is glazed.
The sprayed ware is then fed into a kiln for firing. The fired sanitaryware then passes through inspection to search for defects such as cracks or spots (from iron or other materials). Rejected products will either go for reworking or be crushed and blended back into the raw material feed.
Crushed rejects are passed over a Drum Magnet or Rare Earth Roll Magnetic Separator to remove iron and ensure that contaminated product is not reintroduced to the process.
Prevention Better Than Rejection
High intensity magnetic separators used at key locations with a sanitaryware manufacturing process will significantly reduce end product reject rates.
For further information on magnetic separators used to cleanse ceramic body and glaze of iron contamination or to discuss a specific project, please contact us via:
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