Tablet Press Salty Compression
Salt tablets are used in various home care or chemical applications, like washing machines in households, hotels and restaurants or laundries. It can also be used for softening of drinking water, disinfecting swimming pools and also as nutritional supplements for animal nutrition.
We are often approached by customers looking for new machines to expand their production of salt tablets.
In this kind of application, the customer does not really care about tablet shape and size, his goal is to maximise machine output in terms of compressed salt (in tonnes) in the unit time. However, the process is extremely challenging because of the technical characteristics of the product and of the process itself, so to achieve the goal, specific technical solutions had to be developed.
Process Criticalities in Salt Compression
To start, the product is extremely abrasive and aggressive; it can wear parts both within the process area and in the mechanical area, in case the product migrates to the lower compartment.
Salt tends to be hygroscopic and high water content affects powder flowability, forcing a reduction in machine speed and therefore in the output.
Additionally, the process needs high compression force due to the crystal nature of salt and high weight of the tablet itself, but lamination appears above 70 kN.
The shape and size of the tablet, with high height of the band, causes the tablet to stick to the die walls and high force is needed to eject the tablet from the die. On a standard press and with standard dies, the sticking of the tablet causes the die to be lifted from its seat in the die table.
This is particularly critical also because tablets, immediately after being formed, have a low hardness and can be easily damaged, both during ejection from the die and during subsequent handling, and then to the conveying belt followed by the secondary packaging station. After 10-15 minutes, the tablets become very hard and can then be packed without any problems in 25 kg bags.
Technical Solutions for Salt Compression
Some of the main features of Prexima are particularly useful for this application: the complete separation between the process area and lower mechanical compartment, together with the overpressure of the mechanical area, avoid product penetration in the lower compartment. Thus, cams and rollers are kept in a protected environment.
In the process area, specific materials have been selected for parts which will be in contact with the salt: V-Ring, bellows, turret and die tables have been chosen with appropriate materials and coatings in order to enhance its life against abrasion and chemical aggression.
Process parameters have been carefully studied together during trials with our technologist at the Ima Active Laboratory:
• To enhance powder flowability and guarantee a smooth filling of the product to the dies, before being fed to the tablet press machine, salt is sieved and then dried in the oven to reduce water content.
• Compression force has been set at 60 kN, increasing the dwell time through the use of 441 punches with a large punch head.
• Upper punch penetration during compression was set very low, to form the tablet near the upper surface of the die, improving air ejection and reducing extraction forces.
The old machines that the customer was using were not fitted with load cells and self-adjustment feedback loop for die filling, however these features are standard on Prexima and the customer can appreciate how it offers a much more stable process, less variations in weight and compression force, and thus less stress for the machine itself.
To minimise the ejection force and preserve tablet integrity, Ima’s technologist together with the technical department suggested increasing the tablet diameter from 25 to 28 mm, thus decreasing the height of the tablet. Additionally, the machine was fitted with a special ejection cam with a dedicated lubrication system. The use of tapered dies with a lower collar ensures that the dies are not lifted during the ejection of the tablets.
Finally, the ejection blade and exit chute have been modified for gently handling the tablets at the machine’s exit, until the natural hardening process takes place.
The a.m. technical solutions allowed Prexima 800 to reach the customer’s goal of manufacturing 14.5 gr tablets, round in shape with a diameter of 28 mm and a band height of 12 mm. At 40 rpm, corresponding to an output of 4,240 tablets/minute, Prexima 800 achieved the required production of 3.7 tonnes/h.
Four Prexima 800 machines have already been installed at different customer sites for salt compression, and several other projects are ongoing.