August 2011 industriAl minerAls 49

Zircon Processing

The use of zircon (ZrSiO4) as an opacifier glaze is now common in the sanitaryware and tile industries, with ceramics account-ing for 56% of the 1.3m. tpa

global zircon market.To achieve the best results in opacity and

whiteness in the glaze it is important that the size and shape of the zircon particles are closely controlled. Zircon flour, typically at a particle size below 45µm, as well as micronised zircon sand at a fineness below 6µm, are today used as opacifiers. However, to meet today’s market demands micronised grades are by far the preferred grades as they can be used directly in the glaze.

For all the micronised grades the average particle size, known as the D50 value, is the determining factor for the quality and grades from D50 = 0.8µm to 1.5µm are commercially available and in use today.

In general we can say that the lower the D50 value the better the opacity result. However the D50 value itself is not the only quality determining factor as the overall particle size distribution of the powder – which can be of a narrow or wide distribution, depending on the processing technology used

for micronisation – also plays a part.In the past wet processing followed by spray

drying was used, as dry processing technology was technically not available for achieving such fine grades, however over recent years the dry processing technology has been developed.

The availability of air separators which can achieve the fine grades up to D50 = 0.8µm now means that the dry process can not only obtain the very fine grades, but also the desired narrow particle size distribution which provides, based on the same D50 value of the wet processed one, a better opacity and a smoother surface of the glaze.

Dry zircon processingDuring dry processing it is crucial to avoid contamination by metal and even aluminium oxide which is used as wear protection for the air-separators and milling equipment. As nowadays comparatively affordable aluminium oxide mosaic plates or sintered parts are available as efficient wear protection of processing equipment, it is no longer such a problem to eliminate metal contamination.

Careful selection of the quality of the aluminium oxide wear protection is important however, as the wrong choice can result in an

unacceptably high level of contamination. Typically iron contamination in a dry processing plant for zircon opacifier must be kept below an increase of 0.01 % and the aluminium oxide contamination should be kept well below an addition of 0.8 %.

The dry process in use today uses the closed circuit ball mill in conjunction with a high efficiency air classifier, which can directly produce grades up to D50 = 0.8µm average particle size.

Earlier ball mill processes were targeting the flour production in a closed ball-mill classifying circuit of minus 45µm only and used an additional post classification process to produce relatively small amounts of the 6µm grades. However due to the high demand of micronised opacifier the closed ball-mill classifying process, which directly achieves the target product, without additional external classification, is the preferred process today.

For this reason both the milling and the classifying processes have been developed with the objective of achieving fine grades with a narrow particle size distribution and with the lowest contamination levels, down to grades of D50 = 0.8 to 1.5µm, as the following flow sheet (Figure 1) shows.

The zircon sand, typically at a feed size of below 200-300µm, is accurately metered to the ball mill, which is lined with aluminium oxide bricks and aluminium oxide grinding media in the form of cylpebs to grind the zircon sand.

The selection of the grinding media size mix is very important in order to achieve efficient results in terms of fineness and specific grinding energy. During this ultrafine grinding a lot of heat is generated, which must be kept at a level which the ball mill design and the glue of the aluminium oxide lining can tolerate.

Hosokawa Alpine’s dietmar Alber outlines the optimal conditions for dry processing of widely-used ceramic opacifier, zircon sand

Zircon sand: milling matters

Alpine Turboplex multi-wheel classifier 315/6 ATP/GS for zircon opacifier

Wear protected Alpine Turboplex head multi-wheel classifier 200/4 ATP/GS with sintered classifier wheels made from aluminium oxide

50 industriAl minerAls August 2011

Processing Zircon

When grinding to such fine levels, electrostatic forces between the fine particles become very strong and are responsible for forming agglomerates of powder in the mill. These cause a very low grinding efficiency and create problems for the subsequent air separator as it is very difficult to disperse them and to separate them as fine powder. Therefore a suitable grinding aid must be found and used to avoid this heavy agglomeration effect in order to make the process efficient. Additionally further precautions for the heat development in the ball mill must be taken.

Mill specificationsTo achieve efficient results during grinding in the ball mill it is important to select the most appropriate L/D ratio of the ball mill drum. The filling level of the mill and the ratio of the grinding media to zircon sand in the ball mill are important factors in achieving stable conditions during grinding, and to achieve this balance the whole ball mill is put on load cells where the actual filling level can be controlled exactly by weighing. The result is then a very stable output of the ground zircon sand.

The micronised product leaves the ball mill into the classifying air stream to the classifier, where the fully wear protected classifier separates the fine particles at the desired grade of D50 = 0.8 to 1.5µm. The classifier itself is

completely wear protected by aluminium oxide and even the classifier wheels, which are running at very high tip speeds, are made of sintered aluminium oxide parts, which can withstand the high forces.

In order to achieve the high capacities required in commercial production, multi-wheel classifier arrangements are used, as bigger single classifier wheels cannot achieve such high fineness and cannot be manufactured as sintered wheels in such large sizes to withstand the necessary high circumferential speed of the wheel and its forces.

The final product is collected and separated from the processing air in an automatic reverse jet filter where the opacifier is discharged for bagging. In order to keep the process as constant as possible, permanent airflow measurements and control for the classifying air are used, as even temperature differences during seasons or day and night can influence the stable quality of the final product.

A typical plant set up from Hosokawa Alpine, based in Augsburg, Germany, is a Superfine Ball Mill SO-SF with 450kW drive in a closed circuit with a multi-wheel classifier Turboplex 315/6 ATP/GS with a typical capacity of 850 kg/h for opacifier at a D50 = 1.2µm or 700 kg/h for D50 = 0.8µm.

When dry processing it is interesting to note

that besides the advantage of getting a more narrow particle size distribution, the total specific energy consumption compared to the wet process is about 580 kWh/tonne for the D50 = 1.2µm opacifier, whereas the wet process including the drying consumes in total energy equivalent to 850 kWh/tonne. This is a significant energy saving.

SummaryToday ultrafine micronisation of zircon sand up to the highest opacifier standard fineness of D50 = 0.8-1.5µm is, from the standpoint of product quality and energy efficiency, very likely to be done on dry processing equipment.

By working closely with processing equipment suppliers such as Hosokawa Alpine, minerals producers can achieve the required end product fineness, specification satisfaction and the most cost effective operation possible.

Contributor: Dipl.-Kfm. Dietmar Alber, Min-erals and Metals Division, Hosokawa Alpine AG, Augsburg, Germany. minerals@alpine.hosokawa.com

More onlinewww.indmin.com/zirconIM May 2011: Zircon substitution – myth or reality?

Source: Hosokawa Alpine AG

Figure 1: Screenshot of zircon opacifier plant with a ball mill on load cells and a multi-wheel classifier