METHOD FOR TREATING MATERIALS, PARTICULARLY

STONY MATERIALS, BY PULSATING FLUID JETS AND

APPARATUS FOR CARRYING OUT SAID METHOD

The present invention relates to a method for treating materials, particularly stony materials, by pulsating fluid jets and apparatus for carrying out said method.

More specifically, the invention concerns a method that can use the pulsating pressure high speed fluid jet technology, generated by nozzles having different configuration, for treatment and modification of the aesthetic surface and rugosity features of stony material surfaces.

As it is well known, different methods exist for finishing and surface treatment of stony materials, in order to obtain a set aesthetic result or to increase roughtness of its surfaces and/or reduce its slittering. Presently available and properly applied techniques are listed and shortly described in the following:

Chiselling: it is a shock treatment comprising hitting stone by scalpels having different dimensions determining on the surface an alternance of lowered and relief zones. Bush-hammering: rustic shock treatment on surface of stony materials, carried out by hush-hammer, a hammer with thick pyramid shaped tips. This technique is mainly adopted for products to be used outdoor, such as sculprures, stairs, kerbs, pavings. This working permits conferring to the treated surface a particular graved, rough and relief aspect (so called "buccia d'arancia").

Hammering: it is a working, which is analogous to the bush- hammering, but it is carried out by a hammer permitting realization of less uniform surfaces,

Scratching (or chiselling): it is a kind of surface finishing suitable for raw, smoothed, polished or even already bush-hammered or flamed manufactured products, carried hand by hand, empoloying a hammer or chisel (or "subia tip").

Stepping: it is a modification of the scratching technique, the aesthetic result of which is a surface having a more thick and irregular streaks.

Sand blasting: it provides delivering an abrasive mixture comprised of water and sand, or other hard material such as corundum or glass balls, under high pressure and speed; it is rough, delicate and

elegant working that can be carried out on every kind of material, even with very thin thickness (8 mm).

Flamming: thermal treatment, which is obtained by delivering a high temperature, flame, obtained by single or multiple low-pipes, which are usually at 45° and usually supplied with oxygen (comburent) and propane (comburant).

Rubbing: it is an old cleaning and finishing technique of surfaces obtained by rubbing surface of two rocks (stone against stone).

Grinding: it is a "levelling" working, i.e. it is obtained finishing a material by abrasive tools such as mole, plates, rolls, ecc.

Honing: it is a finishing" working, comprising flattening surface irregularities on the sawed material, carried out by a series of abrasive mole having an always more fine granulometry.

Polishing: working giving to the treated surface brightness, specularity and an optimum planarity. It confers to the material a lower attackability both by chemical and wheather outer agents, acting as an impermeabilization of the surface.

The above techniques have each one different drawbacks reducing the interest of the market. In fact, as far as surface finishing techniques are concerned which are based on the use of mechanical tools, such as bush-hammering, sand blasting and flamming, main drawbacks are due to the modification of the surface features of the treated material under a mechanical point of view, due to the mechanical or thermal shock actions, and under a chromatic point of view, due to the presence of microfractures or to fusion of surface crystals, determining a reduction of colours brightness and chromatic differences, causing a material opacization effect not always acceptable to the product users.

Instead, as far as polishing process, in which chromatic and micromechanical aspects are saved, final product has slipperiness features, which, mainly if used for paving, are a remarkable drawback.

In order to overcome these problems, it is available since many time a treatment by which working of surface of stony material is carried out by high-speed water jets generated by uniform pressure. Said technique is usually known as water-jet technology. However, stony material treatment by water-jet technology obviates to the above drawbacks of the known techniques but reduces productivity and increases cost of the same treatment. In fact, according to

this method, water (costant) pressure varies between 200 Mpa and 400 MPa. Problems connected with this technology arise from the needing of generating very high-pressure water jets and consequently with very low flow rate, reducing jet impact area and thus limiting productivity of treatment, with the consequent increase of costs.

It is well evident that this procedure is expensive.

Another known technology, which has never been applied to the surface finishing, is the one based on pulsating fluid (usually water) jets. International patent application WO 2006/097887 for example describes an apparatus that can generate pulsating water jets by an acoustic system, making it particularly efficient the jet obtained. In any case, different technologies exist for realizing apparatuses generating pulsating jets, based, for example, on rotating mechanical devices integrated within pulsating water jet outlet nozzle, such as rotors, or devices modifying water jet outlet section.

Pulsating jet technology permits reducing pressure necessary to working of more than 80%, overcomning efficiency limits of water-jet technology.

However, water-jet systems and/or pulsating jet systems, even if permitting working of stony material surfaces, do not permit nowadays obtaining particular aesthetic effects. Therefore, their use, notwithstanding possible economic convenience, is limited due to impossibility of using them properly in order to carry out workings.

In view of the above, it is an object of the present invention that of suggesting a method based on pulsating jet technology, particularly stony material, having all the advantages of the fluid jet technology, but also permitting realizing all workings necessary to obtain the wished surface effects.

It is further object of the present invention that of obtaining, even by suitable solutions, a high working efficiency.

It is therefore specific object of the present invention a method for treating materials, particularly stony materials, by pulsating fluid jets comprising the following steps: (a) providing a nozzle-holding head, the position of which can be adjusted by first positioning parameters and suitable to house one or more nozzles suitable to emit pulsating fluid jets that can be adjusted by second fluid-dynamic parameters, said one or more nozzles being movable with respect to said nozzle-holding head and

with respect to the surface by third motion parameters; (b) placing said nozzle-holding head in correspondence of a surface of a piece of material to be subjected to treatment; (c) setting said first positioning parameters of said nozzle-holding head with respect to said surface to be subjected to treatment; (d) setting said second fluid-dynamic parameters of said pulsating fluid jets; (e) setting said third motion parameters of said nozzles; and (T) activating and carrying out an at least partial scanning of said surface by said pulsating fluid jets.

Always according to the invention, said first positioning parameters can comprise distance between said first nozzles and said surface of the material to be subjected to treatment, measured along the jet axis, said distance being include within the range between 20 mm and 80 mm.

Still according to the invention, said first parameters can comprise the inclination angle (y) of the axis of said pulsating fluid jets with respect to the surface to be subjected to treatment, so as to adjust extension of the impact area, so that reducing said inclination angle a higher impact area is obtained and a lower digging effect is obtained, with an extended but not homogeneous surface working, said inclination angle being included within the range between 30° and 90°.

Furthermore, according to the invention, said second parameters can comprise average pressure, the use of a low average pressure being suitable for scratching or stepping workings, or to obtain sandng effects, while a higher average pressure being suitable for more marked effects, similar to those obtained by flamming/bush-hammering, said average pressure being included within the range between 30 and 30 MPa.

Advantageously, according to the invention, said second parameters can comprise pulsation frequency of pressure that can be between 10 and 40 kHz, preferably, for stony materials, 20 kHz, and it is set by said pulsation generation unit.

Always according to the invention, said second parameters can comprise pressure pulsation amplitude, that can be between 25% and

100% of average pressure, wherein, for homogeneous surface workings, with effects like flamming/bush-hammering and sand blasting, it is suitable operating with lower amplitudes, while higher pulsation amplitudes are

suitable for reproducing scratching or stepping effects being preferably higher amplitudes and pulsations.

Still according to the invention, said second parameters can comprise diameter equivalent to the one of said nozzles so as to determine, with the same average pressure, flow rate of pulsating fluid jets, and defining area of surface subjected to treatment, said equivalent diameter can be included between 1 and 3 mm.

Furthermore, according to the invention, said second parameters can comprise: - fluid flow rate; or

- hydraulic power; said flow rate and said hydraulic power being secondary parameters obtained combining average pressure and equivalent diameter, and being preferably respectively included between 10 and 90 l/min and between 5 and 75 kW.

Advantageously, according to the invention, said third parameters can include translation speed of nozzle-holding head with respect to the surface to be subjected to treatment, said translation speed being included within the range between 0.1 and 30.0 m/min. Always according to the invention, said nozzles can be movable with respect to said nozzle-holding head carrying out rotation and/or oscillation movements and said third parameters comprising rotation speed, oscillation angle and oscillation frequency of said nozzles, said rotation speed of said nozzles being included within the range between 10 and 1200 rpm, oscillation angle of said nozzles being included between 5° and 45° and said oscillation frequency of said nozzles being included within the range between 1 and 30 Hz, so as to increase speed of said pulsating fluid jets with respect to the surface, so as to obtain a flamming/bush-hammering or sand blasting type working. Still according to the invention, said third parameters can comprise translation speed of said pulsating fluid jets with respect to the surface, said translation speed varying between 0.1 and about 30 m/min, in case of fixed pulsating jets, and between 10 m/min and about 400 m/min in case of rotating or oscillating pulsating fluid jets. Furthermore, according to the invention, said step (f) can comprise sub-step of displacing said pulsating fluid jets with respect to

said material to be subjected to treatment on the basis of linear and/or curvilinear and/or parallel and/or crossed passages.

Advantageosly according to the invention said nozzles can have circular section so as to obtain jets with a higher digging capacity for scratching or stepping workings, or an elliptical section, so as to obtain jets suitable to cover a wide area of said surface to be subjected to the treatment in order to obtain homogeneous surface effects such as those of flamming/bush-hammering or of sand blasting.

Always according to the invention, said fluid can be water or a mixture of water and different additives and/or an abrasive suspension comprised of water and sand of different abrading materials.

Still according to the invention, said nozzle-holding head can house a plurality of nozzles, that can be maintained fixed, particularly for scratching or stepping workings. It is further object of the present invention an apparatus for treatment of materials, particularly stony materials, for carrying out the above method, comprising a base on which a piece of material to be subjected to treatment can be placed, first motion means for said piece of material to be subjected to treatment, which are connected with said base; a nozzle-holding head, housing one or more nozzles for emitting one or more pulsating fluid jets, for working said surface to be subjected to treatment; second motion means for said nozzle-holding head; a pulsation generation unit, connected with said nozzle-holding head for generation and adjustment of said pulsating fluid jets; and a unit for pressurizing fluid connected with said pulsation generation unit for adjustment of outlet pressure of said pulsating fluid jets; said apparatus being characterised in that it is suitable to permit the adjustment of: first positioning parameters of said nozzle-holding head of controlling said second motion means; second fluid-dynamic parameters of said pulsating fluid jets for controlling said pulsation generation unit and said fluid pressurization unit; and third motion parameters of said nozzles with respect to said nozzle-holding head for controlling said nozzle-holding head. Preferably, according to the invention, said piece of material to be subjected to treatment can have the shape of a plate.

Always according to the invention, said first positioning parameters can comprise comprise distance between said first nozzles and said surface of the material to be subjected to treatment, measured along the jet axis; the inclination angle (y) of the axis of said pulsating fluid jets with respect to the surface to be subjected to treatment; said second parameters can comprise average pressure; pressure pulsation amplitude; diameter equivalent of said nozzles; fluid flow rate; hydraulic power; said third parameters can include translation speed of nozzle-holding head with respect to the surface; rotation speed of said nozzles with respect to the nozzle-holding head; oscillation angle and frequency angle of said nozzles with respect to the nozzle-holding head.

The present invention will be now described, for illustrative but not limitative purposes, according to its preferred embodiments, with particular reference to the figure of the enclosed drawing, wherein it is shown an apparatus for treatment of stony materials by pulsating fluid jets.

Making reference to the figure, it is observed an apparatus 1 for carrying out the method according to the invention, comprising a support base 2, on which a piece of material 3 to be subjected to treatment is placed, having the shape of a plate, with the surface to be subjected to treatmet 3' faced upward.

Apparatus 1 further comprises a nozzle-holding head 4 on which one or more nozzles 5 are placed for exit of pulsating fluid jets 6.

Apparatus 1 is provided with a pulsation generation unit 7 and with a fluid pressurization unit 8 connected with the pulsation generation unit 7 for controlling the pulsating fluid jet 6.

Movement of piece 3 of material to be subjected to treatment, in the figure a plate, with respect to the nozzle-holding head 4 occurs by a motion unit 9 or by the same base 2.

Nozzle-holding head 4 permits orientation of pulsating fluid jet 6. Apparatus permits adjustment of a plurality of parameters of the position of surface 3' of piece 3 of material to be subjected to treatment with respect to the direction from where pulsating fluid jet 6 arrives and with respect to the same pulsating fluid jet 6, the combination of which permits carrying out different surface working, as it will be better explained in the following.

More specifically, once material plate 3 to be subjected to treatment is placed within the area under said nozzle-holding head 4, the following main steps have been carried out:

- setting positioning parameters of said nozzle-holding head 4 with respect to said piece 3 of material to be subjected to treatment;

- setting fluid-dynamic parameters of said pulsating fluid jets 6; and

- setting motion parameters of said pulsating fluid jets 6 with respect to said piece 3 of material to be subjected to treatment. Pulsating jet technology causes dynamic stress actions on target material due to fluid jet trains, impacting on surface with different speeds.

Impact pressure generated in area interested by pulsating jet 6 is about ten times higher than static pressure generated by a fixed jet having the same pressure, dynamic action of the pulsating jet, besides producing an increase of the maximum value of pressure exerted on material, causes a particular stress distribution, characterised by surface zones subjected to traction stresses, that can vary with a periodicity connected with the pulsation frequency. Thus, breakage of material occurs by phenomena connected both with passing compression or traction resistance limit value of material (in function of the peak value of the impact pressure) and for fatigue fractures, with reference to the natural frequency of the material.

As already said, the above technology is particularly efficient for stony material, having a general structure characterised by a discontinuity with a sub-millimetric discontinuity, made up of contacts between crystalline grains and infra-crystalline discontinuities (such as crystals cleavage planes) which are typical of the different mineral species comprising the rock. In this kind of structure, effect of the compression, but mainly of traction, stress wavesare amplified just for presence of these discontinuities, mainly when they are also a separation elements for mineral species having different mechanical features (this is the case of stony materials such as granites, mainly comprised of quartz, felspar, plagioclase and mica). Breakage usually starts in correspondence of the discontinuity and also develops due to a kind of wedge effect generated by the pressurised fluid.

Capability offered by pulsating jets of operating at lower pressure with respect to fixed jets implies possibility of increasing fluid flow rate values, without reaching high hydraulic power values, also increasing the impact surface and thus its productivity. Frequency and pulsation amplitude of pressure are adjustment parameters of apparatus 1 and can be varied according to the material subjected to treatment.

As to the pulsation frequency, tests carried out have indicated that in case of stony materials optimum values, which are correlated with natural frequencies, are about 20 kHz, as it will be better described in the following.

Other variable operative parameters are average pressure, fluid flow rate, diameter and kind of nozzles 5, number of nozzles 5, possible motion of nozzles 5 and their inclination with respect to surface 3' to be subjected to treatment, distance between nozzle 5 and surface 3'.

Present method provides generation of one or more pulsating jets 6 with a circular or elliptical section (fan-shaped or plan jets), which are fixed, rotating or oscillating, incising the stony surface to be subjected to treatment. Scanning of jet(s) 6 on surface 3' occurs by moving piece 3 to be subjected to treatment and of the same jet(s) 6. Distance between surface subjected to treatment and nozzle can be varied on the basis of the result to be obtained.

High number of adjustment parameters of the system is one of the advantages of the suggested technology. In fact, thanks to the combination of adjustments is possible obtaining different kinds of surface working for stony material.

A description will be given in the following of the different kind of nozzles 5 and of the main adjustments that can be carried out of the above parameters by apparatus 1 according to the invention, as well as variation intervals of said parameters and the corresponding effects of working.

Nozzles 5 can be of different kind or can emit a jet with different sections, e.g. circular or elliptical section (fan-shaped jets). In the first case, with the same equivalent diameter, a jet is obtained with a lower impact area with a higher digging capability, thus also suitable for scratching or stepping workings. In the second case, a fan-shaped jet is

obtained, able covering a larger impact area, but a slightly lower digging capability, in order to reproduce homogeneous surface effects similar to those of the flamming/bush-hammering or of sand blasting.

As already said, nozzle-holding heads 4 can house one or more nozzles 5, thus obtaining fixed, rotating or oscillating single or multiple jets. The kinematiks of jet(s) is a function of the nozzle-holding head 4. Nozzle 5 can move integrally with nozzle-holding head 4 (fixed jets) or they can make movements with respect to the same, usually rotation or oscillation movements. Furthermore, in case of fixed jets, their motion with respect to head 4 is not provided, and path from incidence point of jet(s) on surface 3' of the material 3 to be subjected to treatment is determined by motion of nozzle-holding head with respect to the same surface 3.

These fixed jets configuration are suitable both for making scratching or stepping workings, particularly if circular section jets are employed, and for homogeneous workings (mainly using fan-shaped jets) a higher number of jets determines possibility of making a scanning of a larger area, thus increasing productivity of the system.

In case of rotating or oscillating jets, it is provided a motion of pulsating fluid jets 6 involving a continuous rotation or a partial oscillation (of a set angle) about the nozzle holding head 4. Effect obtained is that of increasing jet speed with respect to the target surface (speed that is the combination of the rotation or oscillation motion of nozzles 5 with respect to head 4 and motion of head 4 with respect to surface 3' subjected to treatment), also in this case increasing productivity with respect to fixed jets, but reducing digging performances. The above heads 4 are suitable to homogeneous surface workings (flamming/bush-hammering or sand blasting).

Another parameter that can be controlled before working surface 3' is distance between nozzle 5 and the same surface, said distance, measured along the pulsating fluid jet 6 tipically varying between 20 and 80 mm.

For pulsating jets 6 it has been noted that an optimum range exists for distance between nozzle 5 and target surface 3' (known as stand-off distance). This range has an extension and distance from nozzle varying on the basis of adjustment of other parameters such as pressure, frequency and pressure oscillation amplitude.

Optimum range, within which pulsation effects are efficient, usually extend for about ten millimetres and it is included within said range within which it is necessary working. Generally speaking, with the same adjustment of the other parameters, when increasing stand-off distance, pulsating fluid jet 6 digging depth decreases and produces a more uniform effect (it is distributed on a larger area by the effect of the lose of jet cohesion), suitable for working similar to flamming/bush-hammering or sand blasting, while lower stand-off distances are preferred in order to obtain a scratching or stepping effect. Another very important parameter is average pressure, modifying the same being it possible obtaining different aesthetic effects.

In fact, with the same other parameters, when average pressure rises, digging depth increases, but a more irregular surface is obtained. In the same way, an increase of average pressure also causes an increase of distance from nozzle 5 and of extension of optimum range of stand-off distance.

To make lighter, less deep, workings, i.e. to obtain both scratching or stepping effects, or for sand blasting effects (which are less evident with respect to flamming/bush-hammering effects), it is suitable operating with relatively low pressures (about 30 MPa), while for obtaining more evident effects it is necessary providing higher pressures. A typical range for average pressure is 30 - 70 MPa.

Pressure pulsation frequency, which is usually included between 10 and 40 kHz, is determined by pulsation generation unit 7. Frequency employed for stony materials, also function of the natural frequency of the material, is about 20 kHz.

Main effect of variation of frequency is linked to the optimum range of stand-off distance. Increasing pulsation frequency, it is reduced distance from nozzle 5 of the optimum range of stand-off distance and at the same time jet impact area. In any case, optimum frequency is set by the kind of material to be subjected to treatment.

Pressure pulsation is characterized, besides by frequency, by amplitude. Said pulsation amplitude is usually included between 25% and 100% of average pressure. If pulsation pressure increases, jet 6 "loss of coherence" effect increases, and thus distance between nozzle 5 and optimum stand-off distance range is reduced.

For homogeneous surface workings (with effects like flamming/bush-hammering and sand blasting) it is suitable operating with lower amplitudes, while for reproducing scratching or stepping effects it is more convenient operating with wider pulsation amplitude. Another very important parameter is nozzle 5 equivalent diameter, determining, with the same pressure, jet fluid flow rate, thus defining impact area of surface subjected to treatment. In order to increase said area, and thus to obtain more homogeneous surface treatments (such as flamming/bush-hammering and sand blasting), it is better working with, bigger diameters. On the contrary, in order to create scratching or stepping effects it is usually better adopting lower diameters. Typically, equivalent diameter of nozzles is 1 - 3 mm.

Flow rate and hydraulic power are determined by chosen pressure value and nozzle diameter 5 and lower and upper limit of their variation range respectively correspond to combination of minimum and maximum pressure and nozzle diameter 5. As to the hydraulic power, its value determines power of employed system and thus influences, along with productivity, working costs. Preferred flow rates are between 10 and 90 l/min, while hydraulic power is preferably comprised between 5 and 75 kW.

Axis inclination of jet(s) 6 with respect to surface 3' determines, with the same other parameters, extension of impact area. This area is minimum (A _m j _n ) in case of a jet perpendicular to surface (angle y=90°) while it increases when angle y decreases (A=A _m j _π /sen y). In the last case, impact area does not have a pressure distribution with symmetry similar to the symmetry of nozzle 5. Pressures are in fact higher in the jet part 6 incising on surface at a distance lower than the nozzle 5. When y decreases, a higher impact area and a lower digging effect are obtained, with a wide but not homogeneous surface working, that can have a particular aesthetic value. Typically, angle y variation range is between 30° and 90°.

A further parameter for surface working is translation speed of pulsating fluid jet(s) 6 with respect to surface subjected to treatment. Said parameter can vary between about 0.1 and 30 m/min, in case of jets fixed with respect to the nozzle-holding head, and between 10 and about 400 m/min in case of jets rotating and oscillating with respect to said head if translation speed increases, digging depth diminishes, thus obtaining a

more homogeneous working (less grooves) and, within set limits, "aerial" working speed increases, thus increasing productivity.

System adjustment procedure provides definition of combination of values of different parameters in order to reach the required result, in order to maximise translation speed, and thus optimising productivity.

As anticipated in the above, productivity in fact has a remarkable weight not only under an economic point of view, since it has a proportional influence on production costs, but also under a technical point of view, due to capacity of manufacturing on the basis of market requests.

Scanning of surface 3' to be subjected to treatment of stony material 3 can be uniform, so as to create a surface having uniform rugosity and chromatic features, or by linear or curvilinear, parallel or crossed, lines characterised by a distance that generally is longer than the treatment thickness of the single pass, so as to create aesthetic effects on the surface subjected to treatment.

Product obtained has particularly valuable chromatic features combined with a rugosity of the surface that can be modulated according to the set application and needs of the user. Method according to the invention, is moreover suitable for surface finishing of materials, such as those basalt and pyroclastic roks, having such features to make it difficult treatment by traditional technologies.

In the following, some examples of workings for obtaining some effects.

First Example

Method for surface treatment of plates "Pearl Grey" granite is described in the following.

Said treatment has been obtained by a linear scanning of a pulsating jet generated by an elliptical section nozzle (so called flat jet).

Adjustment parameters of the system are the following:

Average pressure: 50 Mpa;

Nozzle equivalent diameter: 3.00 mm;

Water flow rate: 88.5 l/min; Hydraulic power: 74 kW;

Pressure pulsation frequency: 20 kHz;

Pressure pulsation amplitude: about 35 MPa;

Distance between nozzle and surface subjected to treatment: 50mm.

Angle between jet axis and surface subjected to treatment: 90° Linear movement speed of the jet on surface subjected to treatment: 12 m/min.

Results obtained are the following:

Thickness of the treatment zone for each single pass: 19 mm; Treatment aerial speed: 13.68 m ² /h; Kind of treatment: uniform rough working; Average rugosity Ra = 43.7 μm;

Maximum rugosity Rmax = 287.9 μm; Ondulation WT = 257.7 μm;

Chromatic appearance: can be compared with the polished surface. Second Example

The application describes surface treatment of plates of "Sardinian Basalt".

Treatment has been obtained by a linear scanning of a pulsating jet generated by an elliptical section nozzle (so called flat jet). Adjustment parameters of the system are the following:

Average pressure: 30 Mpa; Nozzle equivalent diameter: 2.05 mm; Water flow rate: 32 l/min; Hydraulic power: 16 kW; Pressure pulsation frequency: 20 kHz;

Pressure pulsation amplitude: about 20 MPa; Distance between nozzle and surface subjected to treatment: 40 mm

Angle between jet axis and surface subjected to treatment: 90° Linear movement speed of the jet on surface subjected to treatment: 24 m/min.

Results obtained are the following:

Thickness of the treatment zone for each single pass: 19 mm; Treatment aerial speed: 27,36 m ² /h; Kind of treatment: uniform rough working;

Average rugosity Ra = 49.3 μm; Maximum rugosity Rmax = 318.0 μm;

Ondulation WT = 220.9 μm;

Chromatic appearance: can be compared with the polished surface.

Third Example It is described a surface treatment of plates of "Guatemala

Green" marble.

Treatment has been obtained by a linear scanning of a pulsating jet generated by an elliptical section nozzle (so called flat jet).

Adjustment parameters of the system are the following: Average pressure: 30 Mpa;

Nozzle equivalent diameter: 2.05 mm; Water flow rate: 32 l/min; Hydraulic power: 16 kW; Pressure pulsation frequency: 20 kHz; Pressure pulsation amplitude: about 20 MPa;

Distance between nozzle and surface subjected to treatment: 40 mm

Angle between jet axis and surface subjected to treatment: 90° Linear movement speed of the jet on surface subjected to treatment: 10 m/min.

Results obtained are the following:

Thickness of the treatment zone for each single pass: 19 mm; Treatment aerial speed: 11.40 m ² /h; Kind of treatment: uniform rough working; Average rugosity Ra = 190.6 μm;

Maximum rugosity Rmax = 189.5 μm; Ondulation WT = 189.5 μm;

Chromatic appearance: can be compared with the polished surface. An advantage of the present invention is the fact that, by the combined variation of suitable parameters, it is substantially possible obtaining all workings of the stony material surfaces.

A further advantage of the treatment method according to the invention is that by the pulsation generation unit 7 it is possible obtaining different results with average pressures tenths times lower than those employed in the waterjet treatment. Moreover, the use of said method

permits obtaining the same or higher productivity corresponding to those presently required by the market.

The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiments, but it is to be understood that modifications and/or changes can be introduced by those skilled in the art without departing from the relevant scope as defined in the enclosed claims.