The invention relates to a method according to the preamble of claim 1 for preparing precoat on the surface of the filter medium of a polishing filter. The invention also relates to polishing filter equipment according to claim 8 and to the use according to claim 14.

Various filtering methods are well known. One of the most demanding methods is polishing filtration. There the basic principle is to reduce the solids content of process liquids down to an extremely low level, in some applications even down to fractions of a billionth. Polishing filters can be utilized in many different applications, for example in chemical and process industry. Polishing filters can also be utilized in metallurgy plants that use electrolytic concentra- tion processes for removing impurities. Gold and zinc plants are well able to make use of polishing filters.

The purpose of polishing filtration is to separate, as completely as possible, the extremely small solid par- tides contained in a liquid. The size of the particles varies. Typically the size is smaller than 1,000 nm, and by average within the range 40 - 400 nm. Generally particles of this size are in English called submicrons, i.e. particles having a size of order of less than one micrometer. In this invention, they are called nanosize particles, which refers to the same thing. Different types of equipment can be used for measuring the size of particles, for example the Hanna Instruments HI 93703 turbidity meter. The device is based on an 825 nm light source, and the measurement is carried out as a reflection of 90 degrees for readings below 40 FTU.

When the solids content of process liquids is reduced down to a very low level, and the particle size remains as small as this, there are several problems in the filtering process. Especially these nanosize particles, i.e. particles with a size of less than one micrometer, tend to block the filter medium in a short time. A particular problem is created by the layer impermeable to liquid that is formed on the surface and pores of the filter medium.

Thus the filter medium, such as a filter cloth or a ceramic medium, is rapidly blocked, which directly affects the filtering efficiency. This in turn increases the filtering costs, such as costs caused directly by the filter material, as well as maintenance and service costs. Moreover, aid medium costs as well as waste and post-processing volumes increase. These are extremely significant aspects considering that in process industry, particularly efficiency and all possible cost benefits are welcome.

According to the invention, an arrangement has been devised where the above enlisted problems can be solved. In the arrangement according to the invention, it is essential to make use of an autogenous material created in the process as a filter aid medium compris- ing a precoat . Here the term ^precoat' refers to a filter aid medium that is formed essentially on the surface of the filter medium, or formed to be in connection with said medium. This kind of second filtering layer formed on the filter, such as the filter cloth, substantially improves the filtering efficien- cy. At the same time, better use can be made of the whole process.

In an arrangement according to the invention, polish- ing filtration is utilized more efficiently than in the prior art, and at the same time, the operation of the whole equipment is optimized. The inventive nature of the filtering process according to the invention is particularly based on the fact that the process in question is polishing filtration where an autogenous precoat is made use of. Here the term 'autogenous' means that the precoat is formed of essentially the same material as the material to be filtered. In practice, only the particle size of the material is dif- ferent, either completely or partly. Thus it is particularly essential for the invention to make use of the underflow created in the process, with a particle size larger than in the overflow, of which underflow there is formed, by a rapid filtration, a precoat on the surface of the filter medium, to serve as a filter aid medium. Said underflow, serving as a filter aid medium, improves the separation by filtration of the solids contained in the process overflow and having a particle size smaller than the underflow, without blocking the filter medium. The filter aid medium serves as a kind of porous filter in which the solids fed at a low feed rate are filtered, possibly also precipitated, adsorbed or crystallized.

The object of the present invention is to achieve a completely new arrangement for enhancing the filtration by a polishing filter, so that the filtering efficiency is substantially improved. Different types of filters can be employed as the polishing filter. A particularly advantageous filter is a pressurized polishing filter, for instance the Outotec Larox Polish- ing Filter, i.e. LSF filter, or the Outotec Larox Vertical Polishing Filter, i.e. VPF filter, where the operation is based on a pressure vessel and on cloth- coated frames provided therein, in which frames the filtering process is carried out. Apart from LSF or VPF filters, also other types of filters can be used, for example precoat filters, precoat drum filters, chamber filters, pressure leaf filters or candle-type ceramic or bag polishing filters. The essential fea- ture in the invention is the method for forming a precoat layer on the filter medium surface, and equipment for utilizing said method. More precisely, the invention is characterized by what is set forth in the appended claims.

The invention is described in more detail below, with reference to the appended drawings, where

Figure 1 is a simplified illustration of a method and equipment according to an embodiment of the invention.

Figure 2 is a simplified illustration of how the precoat surface is formed on top of the filter medium.

Figure 1 illustrates an embodiment of the method according to the invention, where a precoat layer 3 is formed on the surface of the filter medium 2 of a pressurized polishing filter 1. Said precoat layer 3 is formed either completely or partially of an under- flow, crystals or precipitated solids obtained from a separation process, and thus the precoat 3 serves as an autogenous filter aid medium in the method. According to step I of Figure 2, the precoat 3 is advantageously fed to an empty polishing filter 1, including a filter element 4, a filter medium 2, such as filter cloth, and a filtrate conduit 5. When the pre- coat 3 is fed to the filter 1, it forms an even layer on the surface of the filter medium 2, such as a filter cloth. According to step II, liquid is filtered through the cloth into the filtrate conduit 5. After forming the precoat layer 3, the solids to be fil- tered, such as an overflow 6, are fed to the filter 1. Filtering is carried out by means of the precoat layer 3 and the filter medium 2, according to step III, so that the small, nanoparticle-size crystals 7 are filtered by the precoat 3 serving as the filter aid medi- urn, or by the surface thereof. Thus the filter aid medium comprising the precoat 3 serves as a kind of porous filter in which the solids 7 fed at a low feed rate are filtered, possibly also precipitated, adsorbed or crystallized.

The use of an autogenous material, i.e. material obtained from the process, as the filter aid medium forming the precoat 3, is stable in the process and does neither cause impurity residues in the process liquid nor increase mass flows in the plant. Thus said use reduces the process byproduct volumes and valuable material losses. The autogenous precoat and the filtered solids can also be returned back to the process. Moreover, the method enables the filtering of nanopar- ticle-size crystals 7, i.e. solids with a particle size of roughly 40 - 400 nm by average, from the process liquid. Consequently, by means of an arrangement according to the invention, it is possible to increase filtering efficiency, reduce process residues or pos- sible waste and minimize the filtering costs. An advantageous example of a method utilizing the in ^¬ vention is the polishing filtration of calcium sulfate overflow (CaSC ) making use of polishing filtration with an autogenous precoat. In an exemplary filtra- tion that was carried out, the employed filter aid medium forming the precoat was underflow from a gypsum thickener .

The arrangement according to the invention was reached on the basis of various experiments and test., runs, because from earlier experience it was found problematic that in the course of time, gypsum was crystallized in the filter medium of the polishing filter, thus causing it to be blocked. Therefore the overflow of the CaSC>4 clarifier cannot, in most cases, be directly polishing filtered. The overflow rapidly crystallizes a layer impermeable to liquid on the surface of the filter medium. By means of an arrangement according to the invention, this can be avoided by using the under- flow of the CaS0 ₄ clarifier as precoat material on the surface of the filter medium. The method is made even more effective when the used precoat is recycled back to polishing filtration, to be reused as filter aid medium. Advantageously the polishing filter used in the method is an LSF filter, by means of which the filtering of the CaSC>4 overflow is realized.

Another advantageous example of a method utilizing the invention is manganese recovery for instance from the overflow of finishing neutralization by applying polishing filtration utilizing an autogenous precoat. In ^■ a performed exemplary filtration, the autogenous precoat serving as the filter aid medium was formed by utilizing the overflow from finishing neutralization and the underflow from the gypsum thickener, which is mainly calcium sulfate (CaSCU) . The aim was to make manganese sulfate, MnSCU, to ei ^¬ ther crystallize in the precoat layer or to retain on the precoat layer serving as the filter aid medium. First there was formed a precoat layer on the surface of the polishing filter cloth, and the filtering of the overflow from the finishing neutralization was continued without interruptions after forming the autogenous precoat. On the basis of analysis results from a test run realized at a low flow rate, it was found that the retention of manganese sulfate was higher than 99.8%. The method is made even more effective when the used precoat is recycled back to the process, to be reused as an autogenous filter aid medium. Advantageously the polishing filter used in the method is an LSF filter, by means of which the filtering of the overflow containing MnSC>4 is realized.

According to the invention, it is essential that by applying the new autogenous precoat system, there is achieved an advantageous precoat that is formed of the same material as the material to be filtered, or of material obtained from the same process as the material to be filtered. The invention makes the separation filtering of solids more effective and at the same time improves the recovery of nanoparticles . A saturated solution precipitates/crystallizes nearly all of the calcium in the precoat layer or on the surface thereof according to the first exemplary embodiment, or manganese according to the second exemplary embodi- ment, without, however, in any way interfering with the polishing filtration process.

A person skilled in the art appreciates that the invention is not restricted to the above described em- bodiments, but the embodiments may vary within the scope of the appended claims.