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Absolutely reliable

Pinch valves for titanium dioxide production
Absolutely reliable

Pinch valves offer crucial advantages when it comes to shutting off and regulating bulky or powdery product flows as well as crystallising liquids and abrasive sludges containing coarse-grained particles. They allow full, straight passage in the open position and are absolutely gas-tight. These rugged control valves are ideal for titanium dioxide production, even in the harshest environments.

Dipl.-Ing. Raimo Lavikainen

Titanium dioxide (TiO2) is the world’s most commonly used white pigment. Its primary function is to guarantee the gloss and hiding power of white products, such as paints, coatings, plastics, paper, fibres, foodstuffs and cosmetics. Kemira Pigments in Finland, one of the leading manufacturers of titanium dioxide, produces this pigment with the aid of a multi-stage sulphate process. The starting materials are ilmenite, titanium slag and rutile. The first step in the Kemira process is to mix the dried, ground ilmenite with sulphuric acid.
No more problems with clogging, leaks or maintenance
Highly aggressive chemicals and abrasive scrap chips are needed for this ilmenite reduction process. The process conditions are tough and the control valves have to be designed accordingly. Each reducing tank has two main valves, one overflow valve and one drain valve. In the past, rubberised slide valves were used for these tasks. Under the harsh conditions that prevail during the process, however, they were not sufficiently reliable. Nowadays, relatively simple pinch valves are installed instead.
The valves at the outlets are only operated twice a day. Any malfunctions would soon cause operating and maintenance costs to escalate. Initial tests with pinch valves revealed them to be exceptionally durable, even under extreme process conditions. On the basis of these tests, Kemira replaced the old valves with pinch valves at 24 different points.
As a result, all problems with clogging, leaks and maintenance were eliminated. Today, the only form of maintenance is the scheduled inspections that take place every two to three years, when the valve sleeves are replaced as a routine measure.
Emergency shut-down must be safe
In order to ensure the required high product quality of the white pigment, the TiO2 particles are reduced to a very fine powder in a wet grinding process. Under normal condi-tions, the wet grinding mill operates continuously 24 hours a day. The outlet valves usually remain open permanently. Only in the event of unforeseen interruptions to operation must they be closed quickly and reliably, to prevent the extremely small grinding media from being discharged together with the fine titanium dioxide sludge.
Following its positive experience with pinch valves for handling abrasive media, Kemira also decided to test the valves at these critical points. Since they allow free passage in the open position, these valves suffer practically no wear. The costs for maintenance and spare parts at these outlet points were drastically reduced as a result. All the outlet points on the wet grinding mills were subsequently fitted with pinch valves. As no permanent incrustations are able to form on the elastic sleeves, reliable closing is guaranteed even if operation is interrupted due to a breakdown. A preloaded spring in the pneumatic actuators of the valves ensures that the emergency closing mechanism still functions reliably, even if the compressor for the compressed air supply has failed.
Abrasiveness as a challenge
The post-treatment of the titanium dioxide determines the final quality of the pigment. Owing to the exceptionally abrasive nature of the process, this step represents a challenge for the design and functioning of the control valves. In the past, special ball valves were used by Kemira in this section of the plant. Unfortunately, though, leaks were a regular occurrence. At such points, however, even the minutest of leaks is intolerable, since the already fully treated media and the as yet untreated sludges must be effectively prevented from mixing and thus impairing the quality of the product. Once again, the pinch valves demonstrated their absolute reliability throughout an 18-month test period. Following this endurance test, they were removed and examined for wear jointly with the valve manufacturer. No traces of wear whatsoever could be found. As a result, all the shut-off points in this extremely abrasive environment were also fitted with pinch valves. The first pinch valves, originally installed for test purposes, have in the meantime been working perfectly, without any kind of leakage or other malfunction, for more than three years. No sleeves have had to be replaced during this period. Pinch valves are not only used for Kemira’s titanium production however, but also for waste water treatment. Owing to the stuffing-box packings that are used, and to the relatively open sealing zones, metal valves are extremely prone to malfunctions when lime sludge is pumped. With pinch valves, on the other hand, crystallisa-tion represents not even the slightest problem. Kemira, the Finnish pigment manufacturer, has therefore meanwhile installed more than 30 pinch valves in its waste water treatment cycle.
Adaptable to any application
The pinch valves, which are purchased from a Finnish supplier, are available in sizes from DN 25 to DN 1000 for system pressures ranging from a vacuum to 100 bar. The valve sleeves can be supplied with opening straps. They ensure that the valves still open reliably, even at high temperatures or low pressures, as well as in applications where they are required to remain closed for prolonged periods of time. Special sleeve designs are available for regulating or throttling tasks and for vacuum applications. A new double-layer sleeve has been developed for the special material combinations that are essential for particularly abrasive and aggressive applications.
Larox
Fax: ++49/358/5/6688588
Further information cpp-205
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