The Montorfano and Baveno granite ore bodies are located in the Lake District (VCO-NE Piedmont). They were and are still quarried as dimension stones, with a consequent production of a huge volume of “waste.” In 1995, an Italian company (GMM S.p.A) decided to invest in the valorisation of granite quarry waste as a secondary raw material. An in situ geological prospecting was carried out in order to evaluate the characteristics of the material and the thickness and volume of the useful disposal sites that could be used (by means of geophysical surveys). As a consequence of the field work, the amount of quarry waste was estimated as nearly 2 Mm3. Chemical analysis was carried out on 75 granite samples, in order to individuate the Fe2O3 grade: from 1.321–2.593% of the original waste to 0.160–0.228% after the “dry process” treatment. Three different detailed maps that show the typology, the locations, and the quality distribution of the material in the dumping areas have been drawn up.
The main quarry disposal sites, the subject of the present research, are located in two different areas: the Sengio and Ciana Tane-Pilastretto dumps, which are located on the southern slope of the Montorfano Massif, while the Braghini site pertains to Monte Camoscio, on the right side of the Toce River (Ossola Valley) (Figure
Geographic context of the studied area. The main image shows the satellite orthophoto of a portion of Massiccio dei Laghi batholith. In the northern part of the map (Montorfano area), it is possible to individuate both the white granite (represented by grey stripes) and the green granite portion (green Mergozzo type, represented by green stripes). The orange area defines the Baveno-Mottarone pluton. Furthermore in the image the exploited quarry dumps (red points) are represented and the ancient quarry waste areas, potentially useful for a future valorisation (yellow points).
The two granitic plutons belong to the Hercynian Magmatic belt, which is located between the
The Mottarone-Baveno pluton is mainly formed by two different varieties of granite: a
Petrographic analysis of the Montorfano white granite and the Baveno pink granite.
Mineralogy | Chemical composition | |||||||
Al2O3 | Fe2O3 | TiO2 | CaO | MgO | K2O | Na2O | ||
Baveno pink granite | Quartz, K-feldspar, plagioclase, biotite, zircon, allanite and fluorite | 13,98 | 2,301 | 0,26 | 1,19 | 0,29 | 4,97 | 3,18 |
Montorfano white granite | Plagioclase, quartz, K-feldspar, biotite, apatite, zircon, allanite | 13,75 | 2,2895 | 0,25 | 1,25 | 0,265 | 4,875 | 3,225 |
The Montorfano pluton is formed by a medium-grained white granite and a small amount of a “green granite.” It is actually an episyenite consisting mainly of albite and chlorite. This so-called Verde Mergozzo occurs in the northern slope of the pluton, where the unconformity with the
The Montorfano massif is delimited by a complex structural fault system (Figure the Gravellona-Fondotoce-Bieno fault, WSW-ENE oriented, divides the Montorfano from Mottarone-Baveno massif, the Candoglia-Mergozzo-Fondotoce fault, NNW-SSE, corresponds to the major axes of the Mergozzo lake, the Pogallo-Lago d’Orta fault, in the N-S direction, is in the western part of the pluton.
Structural sketch map of the Montorfano pluton, showing the main directions of the principal fracture systems and the main faults in the granite massif. These directions depend on the regional faults system (Gravellona-Fondotoce-Bieno, Candoglia-Mergozzo-Fondotoce, and Pogallo-Lago d’Orta faults)
The studied granite resource was only quarried in the past for a local use: in the XVI century, production grew and the material was exported and used for the construction of important buildings and by manufactures in the neighbouring regions (above all the pink granite for Lazzaretto in Milan). The production increases even more during the XVII century, thanks to the modernization of the exploitation technology and innovative quarry techniques (such as the extraction of megalithic blocks for use as columns for historical palaces and important churches).
In the XVIII and XIX centuries, the granitic structures and manufactures could be found throughout the North of Italy and even reached Rome; in 1830, 82 columns of Montorfano White Granite were sent to Rome, by means of river transport, for the realization of the famous
The quarries were exploited using explosives (large charge rock blasting (large charge rock blasting, fired in small adits or tunnels, driven in the face of the quarry (at the level of the floor). Coyote shooting),
The volumes of quarry waste are a clear example of the problems connected to mining activities: the exploitation works in this territory have caused and are causing an evident hazard for the population, as well as significant environmental and landscape impacts on this rather touristic area (
Southern side of Montorfano massif.
Thanks to the evolutions in quarrying techniques, the quantity of waste has been reduced over the last few decades, but the problems connected to the management of such a kind of materials have remained unsolved. A possible way of solving the problem, with contemporary economic benefits, should be the new exploitation of the materials, stocked in dumps, for the production, by specific mineral dressing treatment, of secondary raw materials (SRMs) for industry (mainly for the production of Grès Porcelain stoneware) through specific mineral dressing treatments [
According to the regulations, the raw materials for the Grès Porcelain stoneware had to include pure quartz and feldspar; for this reason the price of the final product is expensive. The introduction of secondary raw materials from the exploitation of quarry waste from the Montorfano and Baveno granites, whose mineral composition (33% quartz, 62% feldspar) is quite similar to the desired mixture, represents the basis for a new waste valorisation which nowadays represents an interesting economic reality which has produced a subsequent decrease in the costs of the raw materials.
The feldspar market, as an industrial mineral, has increased year after year in Italy: from 200.000 t in 1975 to 2,5 Mt in recent years (2005 data); the imported natural resources (about 2,0 Mt) have to be added to these amounts. Some industrial mineral factories, such as Ecomin S.p.A. (Ecomin is one of the companies of
This treatment plant, which has operated in the field of dressing of raw material since 1995, has the concession to exploit the three granite quarry waste disposal sites: the Sengio and Ciana Tane-Pilastretto areas (white granite) and Braghini area (pink granite exploitation).
In order to guarantee the highest level of safety in the overhanging yards and the stability of the quarry fronts, the material layer is exploited from the top to the bottom of the whole volume: the quarried material is then loaded into dumpers and transported to the treatment plant. The large-sized blocks that are stocked in the areas are reduced using blasting and breaking techniques, or are used, as such, as riprap or armour stones. The total recovery of the granite waste has exposed the underlying bedrock, in order to minimize the hydrogeologic hazard in the area.
The ore, conveyed from the quarry waste areas to the plant, is treated by crushers, roller mills, and so forth, in order to reduce each grain size class and to obtain 1.25 mm as the maximum grain size dimension. It is then sieved to obtain different grain size materials and to separate the powder granite from the other products. Finally this material passes through electromagnetic separators, which select the ferromagnetic minerals from the final product, characterized by appropriate physical-chemical properties. The waste produced during the enrichment phase (powder granite, ferromagnetic minerals, mud) is also treated to obtain byproducts which are used in other applications.
In particular, the main product is commercially known as F60P (quartz feldspar mixture: 60% of feldspar, mostly K-feldspar), whose production is about
The importance of such a treatment is that, at first, it is possible to valorise quarry waste as secondary raw material, and then it is possible to achieve the goal of a zero-waste-volume production, with a consequent reduction ins costs for quarry enterprises and indisputable environmental advantages for the territory.
The Gruppo Minerali Maffei S.p.A has carried out different geo-mechanic stability controls on the slopes and has characterised the three exploited areas, in order to establish the possibility of rock detachment, both from the mountain faces and from the debris landfill (debris flow and/or rock collapse).
As far as the modelling of the potential gravitational dynamics is concerned, an ILA programme, which allows the safety factor (Fs) values to be calculated, on the basis of common stability models (Bishop, Jambu, etc.), was adopted. The parameters used to calculate the Fs are summarised in Table
Quarry dumps characteristics (granite substratum and debris landfill).
Sengio area | Ciana-Tane Pilastretto area | Braghini area | |||||
Granite substratum | Debris landfill | Granite substratum | Debris landfill | Granite substratum | Debris landfill | Eluvio-colluvial and fluvio-glacial deposits | |
Saturated volume weight (kN/m3) | 25 | 18.5 | 25 | 18.5 | 25 | 18.5 | 20 |
Not drained cohesion (kN/m3) | 0 | 0 | 0 | 0 | 0 | 0 | 0.1 |
Internal friction angle | 45° | 38° | 45° | 38° | 45° | 38° | 28° |
On the basis of the reported data, it was possible to calculate the Fs (December 2004) for the three areas and to foresee the Fs in 5 (December 2009) and 10 years (December 2014), see Table
Geotechnical stability checks.
Sengio area | Ciana-Tane Pilastretto area | Braghini area | ||||
Slope face within 290–350 m (sliding surfaces characterising the contact between debris and basement) | Slope face within 260–350 m (sliding surfaces characterising the contact between debris and basement) | General stability of the quarrying face (sliding surfaces passing through a point) | Local stability of the quarrying face (sliding surfaces passing through a point | Slope face within 590–680 m (sliding surfaces characterising the contact between debris and basement) | Slope face within 540–585 m (sliding surfaces characterising the contact between debris and basement) | |
Fs (December 2004) | 1.25–1.37 | 1.28–1.30 | 1.39–1.59 | 1.52–1.83 | 1.11 | 1.21–1.34 |
Fs (Evolution within 5 years, December 2009) | 1.33–1.67 | 1.36–1.41(1) | 2.27–3 | >3 | 1.45–1.86 | 1.32–2.01 |
Fs (Evolution within 10 years, December 2014) | > 2.10–3(2) | 1.7–1.95(3) | — | — | — | 2.04–3 |
(1)Circular surfaces tangent to the contact: rock basement-debris
(2)Slope face within 265–290 m (sliding surfaces characterising the contact between debris and basement)
(3)Slope face within 235–290 m (sliding surfaces characterising the contact between debris and basement).
The Fs value was obtained considering the different heights of the rock face (granitic basement), considering a hypothetical rock collapse from different altitudes, and also considering the “in-progress” mining activity (on the granite debris).
The evaluated Fs values, connected to geomechanic stability of the debris deposits, are very close to the equilibrium limit. Moreover, it is possible to underline that the Fs pertaining to the mining exploitation of the quarry waste deposits (in the studied areas) increases more and more over the years.
The data reported in Table
What emerges from the shown data (Table
The present study illustrates a detailed geological and geomineral survey of the three mining areas, which was focused mainly on the granite waste deposits and on quaternary formations (fluvial and glacial deposits), in order to delineate their geometric surface.
Furthermore, the research described the structural, granulometric, and stability properties of the waste deposits and, at the same time, conducted a petrologic characterization of the bedrock, with the purpose of underlining the general characteristics (petrology, size distribution, morphology of the bedrock, and stability of the slopes) of the three deposits. This data collection is useful to describe the general characteristic of the waste deposits, compared to the adjacent areas, in order to evaluate the volume of the ore bodies, using surface and thickness data (see Section
The granite waste (Figure
Granite quarry waste detail. The different, casual grain size distribution compromise the stability of the coarse volume.
The Sengio white granite deposit (Figure
The femic masses are frequently the cause of the lower quality of the dimension stone; therefore, a large quantity of waste was produced in the Sengio area during exploitation. When the orebody became uneconomic, because of the decrease in quality, the mine conductor abandoned the quarry and transferred the exploitation to the SE area.
The Ciana Tane-Pilastretto (Figure
The Ciana Tane-Pilastretto granitic waste is formed by metric blocks in the superficial layer and sandy to gravel materials in underlying volume, which can be observed from the road that cuts the middle sector of the dump. The granulometric fractioning is caused by the mobilization of the metric boulders that have been left on the superficial layer, above all from the top of the granite waste exploitation.
The Braghini dump (Figure
An important characteristic of the waste, when considering it as raw materials for ceramic industries, is the different weathering alteration of the sandy fractions, caused by the different steps of the granite waste during the centenarian activity: the vegetation is also composed of different plants because of the different periods of deposition. This factor is very important when deciding, projecting, and programming the best way to valorize the materials in a treatment plant.
Another goal of the study was to determine the volumes of the ore bodies: a geophysical survey—an electrical tomography investigation—was carried out to estimate the thickness of the waste deposits. Thanks to a processing phase, the collected field data (resistivity data) were processed to obtain a resistivity model of the investigated area and a 2D resistivity cross-section.
In the Sengio area (Figure
(a) Sengio area. (b) Tane Pilastretto area. (c) Braghini area.
The same configuration as that of the Sengio area was used in the Ciana Tane-Pilastretto (Figure
Finally, in the Braghini dump (Figure
It is possible to show the tomographic profiles of the three studied dumps (Figure
It is possible to estimate the deepness of the granitic bedrock in the cross section, thanks to the use of different colours, which correspond to different resistivity values. The resistivity difference can be related to the unconformity of the granitic waste and bedrock.
A comparison between the tomography cross sections and the geological surveys data made it possible to estimate the thickness of the waste deposits.
Elaborating the parameter reported in Table
Thickness and volumes for the investigate areas.
Quarry waste dumps | Average thickness calculated thanks to tomography cross-sections (m) | Ore body volume (m3) |
---|---|---|
Sengio | 20 m | 361.600 |
Ciana Tane-Pilastretto | 25 m | 1.489.000 |
Braghini | 15 m (tomography limit) | 158.000 |
It is important to underline that the waste amount in the old quarry dumps is periodically refilled by the flowing waste and that the research for “new ore bodies” is still in progress. Therefore a longer treatment plant activity can be hoped for.
After a geological survey a representative sampling on the debris deposit was carried out. The material characterised by a size distribution of 30 to 150 mm was sampled: 30 mm is the lower limit of the “stone chips” (debris rocks) which can be treated directly in the transformation plant (Ecomin).
Each area of the Sengio and Ciana-Tane Pilastretto quarry dumps was split into a square net of about 50 m per side, while the Braghini quarry dump was divided into a net of about 30 m per side.
78 samples were taken in the three studied dumps: 26 samples from the Sengio quarry dumps, 30 from the Ciana Tane-Pilastretto area, and 22 from the Braghini area. The samples were treated and analysed in the Minerali Industriali Lab. (Cacciano—Masserano, BI).
The laboratory treatment involved a simulation of the ore dressing pilot plant to what normally happens in the Ecomin treatment plant (Figure
Simplified flow-chart of Ecomin transformation plant.
Simplified flow-chart for the laboratory pilot plant.
Chemical analyses are fundamental to evaluate the Fe2O3 grade of each dumps; this parameter is necessary to calculate the quality of the expected products from the treatment plant. On the basis of the lab results, it was possible to draw up, for each investigated area, a Fe2O3 grade map in order to individuate the zones characterised by the best quality (Figure
(a) Ciana-Tane Pilastretto Area. It is possible to notice the service track, in the middle of the quarry dump (black line), and a stream in the upper left side of the map (blue line). (b) Braghini Area. It is possible to notice, in the middle of the map, a rockfall protection embankments (wire netting area), the Rio Cavallaccio stream just above the rockfall protection embankments (blue line) and the concession area borders (red lines). (c) Sengio Area. Just in the middle of the investigated area it is represented a service track (black line). The borders of the concession area are portrayed as red lines.
In Figure
The Fe2O3 grade was quite different (Table
Fe2O3 average grade for the three studied areas.
Quarry dumps | Average grade % Fe2O3 (TQ) | Average grade % Fe2O3 (2SM) |
---|---|---|
Sengio (white granite) | 2,317 | 0.205 |
Tane-Pilastretto (white granite) | 2,262 | 0,184 |
Braghini (pink granite) | 2,301 | 0.210 |
In the Sengio quarry dump, the presence of altered arsenopyrite concentrations in the rocks is probably the reason for the bare magnetic separation during the treatment. On the other hand, the decrease in the quality of the final products in the Braghini quarry dump is probably caused by the weathering alteration in the fine matrix of the granite waste deposit, which compromises the magnetic separation.
Furthermore, looking at the three maps, it is possible to see that there are different Fe2O3 grades, even in the same dump area, which depend on the progressive waste stockpiling in dumps over the years (from different exploited areas). The data regarding the Fe2O3 grade distribution in the three areas are essential to guarantee the integral exploitation of the “waste resources” and to assure a controlled and uniform feeding to the treatment plant.
The lab testing phase also regarded the search for the right method to decrease the Fe2O3 grade in the samples characterised by a very low quality; in these cases a deeper crushing and adding magnetic separations would be the correct ways to valorise the material. On the basis of the test results, it is possible to notice a quality increase in the quality of treated material, in particular in the 0.1–0.5 mm size distribution classes.
As far as the efficiency of the magnetic separation in a future experimental phase is concerned, it is possible to notice that the Fe2O3 grade does not decrease substantially after the passages in the separator (3-4 passages): in fact the grade tendency is close to an asymptote. Such a result, directly imputable to the liberation grade of the single grains, is closely connected to the size distribution of the final product.
The old dumps can be considered as “new mines”: they can be rehabilitated during the exploitation phase by the enterprise which owns the mining concession [
The valorisation of the material stocked in the dumps guarantees positive results, both for the company (ECOMIN) and the interests of the community at large. The systematic exploitation of the “waste” accumulated during the years and/or actually produced is the basic condition for the safety of the landfill areas, intervening with appropriate accommodation and environmental recovery, the impacts reduction (dust, etc.), due to the presence of incoherent material placed on the dump slopes, a positive economic return, due to the exploitation of the material placed in the dumps, which should not be quarried (with explosives and/or mechanical equipment), but only picked up and sent to the treatment plant.
Projects concerning “land use changes in areas characterised by hydrological constraints” have been approved, as far as the exploitation plans are concerned. As a result, the project envisages the exploitation of debris materials stocked on the slope, in order to reach the granite substrate (Figure
The uncovering of granite substrate thanks to mechanical equipments. The material is transported to the plant, located just below the slope (on the other side of the street).
Other well-known forced “bio-engineering” re-vegetation methods were used for the reclamation of the areas [
(a) Quarry dump rehabilitation (Braghini area). (b) Quarry dump rehabilitation after the exploitation of the pink granite waste, in order to obtain secondary raw material for ceramic industries. Scala dei Ratti yard (Baveno area), quarried by Mineral Baveno s.r.l.
The key elements, necessary to achieve the total safety conditions, seem to be the “walls” that have been placed at the base of the excavation, and which have been made using the less valuable material produced during the exploitation. These artificial hills are frequently useful to create a “screen” or a visual impediment to the outside world, in harmony with the surrounding nature. They are also important as protection elements for the adjacent and very busy SS34 (State highway, Figure
The frequent fall of large amounts of water, due to the heavy rainfall that periodically affects the area, constitutes a repeated and hard test for the stability of the faces, which are often bares and characterised by the presence of incoherent materials.
Therefore, it is possible to ask, what would have happen to the safety of roads, sides, and so forth, if the miner had not decided to intervene, and to exploit the Baveno and Montorfano “waste-deposits.” The exploitation of these materials is truly favourable for slopes stability and for the safety of the roads and infrastructure near the quarries.
A wise and planned mining activity is also important for the defence of the territory, as well as for the production of secondary raw materials (MPSs), which are profitable for the company and for the country.
The paper emphasizes that the interest of private companies for the systematic recovery of quarry waste should be interpreted as a signal of the good will of private and public bodies to guarantee environment and territory protection and the safety of the quarries.
The research points out that it is possible to ensure sustainable development for mining activities, guaranteeing, at the same time, profit for the virtuous companies involved in the exploitation, valorisation, and recovery of the “new ore-bodies.”
In particular, treating and valorising the waste stored in quarry sites, it is possible to guarantee fewer (with less impact) problems concerning environmental impact and hydrogeological problems connected to quarry dumps, greater slope stability, to bring to surface other parts of the granite ore body, which were hidden by waste piles, potential revenues from the treatment of MPS exploited from the dump/ore body, and so forth.
Granite quarry wastes represent, therefore, an important alternative (integrating) source, as a substitute to the exploitation of “virgin” material from the primary quartz and feldspar mines.
As already mentioned, the exploitation of the quarry waste often ensures a correct environment recovery and the safety of slopes affected by the dumps.
In order to fully exploit the granite resource from the dumps it is still essential to conduct a complete investigation in order to estimate the volumes, size distribution and chemical and mineralogical characteristics of the material.
The research should involve both a field investigation (detecting geostructural remote land sensing, geophysical surveys, etc.) and a weighty part of lab analysis (mineral-chemical characterization of the raw material as such and of the products obtained from the treatment, ore enrichment in a pilot plant, etc.).
Thanks to a detailed study of the calculate the quantity of material available for the treatment plant (2,9 Mtons), assess the quality of these materials in order to select what to send to the treatment plant and what to keep separate, because of their poorer quality. This unusable fraction should also be used for environmental recovery and the rehabilitation of the landfill morphology, evaluate the reserves available in dumps, added to a coming waste from the quarry, which will ensure the productivity of the treatment plant for not less than 15 years.
The authors would like to thank the Gruppo Minerali Maffei S.p.A., Ecomin s.r.l, and Minerali Industriali S.p.A. for their fundamental help during the field surveys and the laboratory phase; a special thanks are due to Eng. G. Bozzola, Dr. T. Mestriner, Eng. E. Salvaia, Dr. A. Lorenzi, and Dr. S. Vegis. Thanks are also due to Prof. A. Godio and the Geophysics Lab at DITAG-Politecnico di Torino, for their help during the geophysical processing. Finally, they would like to thank Rag. D. Marchetti (Giacomini S.p.A. enterprise) for the kindness during the visit to the Mineral Baveno quarrying area and treatment plant.