The Sicilian rock salt and potash mines.
With the advent of industrial society in the second half of the last century there was an enormous expansion in sulphur mining and, until the early years of the present century, Sicily became the most important sulphur producing region in the world. Notwithstanding various vicissitudes and the entry into the world market of other producers, Sicilian sulphur production continued at a high level until the end of the Korean War. Thereafter an irreversible decline set in -- the small low-grade Sicilian deposits could not compete with sulphur produced in the U.S. by the Frasch method, nor with by-product sulphur from oil refining. All the mines closed with the exception of the Cozzo Disi mine, in the province of Agrigento, which was transformed into a mining museum.
Until the 1950s rock salt mining, on the other hand, remained a marginal operation whose products were limited to meeting the alimentary needs of the local population.
The decade from the mid 1950s to mid-1960s was very important for Sicilian mining. Once it was understood that sulphur mining no longer had any chance of recovery, intensive prospecting for potassium minerals began, stimulated by the discovery of a deposit of kainite at S. Cataldo during earlier prospecting for sulphur and by the presence of chemical plants along the east coast of Sicily.
Prospecting activity was extensive, mainly carried out by drilling. Thousands of metres of rock were drilled, the holes being typically 400m to 600m deep. Depths of 1,000m were often reached, however, and the deepest holes went to 1,500m. Although the prospecting was not well co-ordinated, as a variety of companies took part, numerous deposits were discovered, such as Pasquasia, Racalmuto, Realmonte, Covillo, S. Caterina and Montedoro. Others, such as Milena and Cozzo Campana, were found later.
Rock salt was not a target of the prospecting activity as the deposits of Cattolica, Coffari, Muti and Pantanelle in the province of Agrigento, Petralia in the province of Palermo and Ranieri in the province of Caltanissetta had already been identified and were being exploited. It is worth noting, however, that these deposits were being worked as a series of small operations, each bordering on the other, with all the problems that this entails as regards safety, encroachments and mutual interference.
The Sicilian mining law of 1956 put order into the sector by promoting the formation of consortia and creating the conditions whereby, in the course of a few years, a single and more rational administration of the various mines emerged.
All the mines are situated in a strip of land in the central-southern part of Sicily, a corridor 50 km wide which lies in a north-easterly/south-westerly direction, stretching 120 km from Nicosia to the Mediterranean Sea. Lying along the margin of this strip there are a number of other salt deposits whose economic exploitation is not as yet considered viable.
The Sicilian rock salt deposits are found in the sulphur-bearing chalk rock formation, a succession of mainly evaporite sediments forming part of the Messinian layer of the early Miocene Period. The saline succession has been studied in a series of drill holes and in the working mines. The salts, which were most likely formed in a single evaporitic basin, at present constitute independent bodies and are characterised by an intense tectonic compression with diapiric folds, sometimes quite close together. For this reason the apparent width can, at times, exceed 1,000 m.
The stratigraphical succession of the soluble Sicilian salts is made up as follows, from top to bottom:
* A formation of rock salt made up of beds containing polyhalite and clay layers. Thickness, 50 m.
* A formation of rock salt beds (NaCl over 98%) from which salt is extracted in some mines for human consumption and for the chemical industry. Thickness, 50 m.
* A potassium-rich formation, made up of alternating layers of carnallite, sylvinite and rock salt. These potassium chlorides are not mined at present. Thickness, 50 m.
* Formations of potassium salts comprising alternate beds of kainite (KCl.MgS|O.sub.4~.3|H.sub.2~O) and rock salt. There are six kainite beds which can be correlated in all the deposits. The thickness of individual kainite beds varies from 5m to 10m and the potassium content, expressed as |K.sub.2~O, ranges from 12% to 16%. These are the source of the potash mined. The overall width of the formation, including the intervening rock salt, is 100 m.
* Rock salt with anhydritic nodules, the anhydrite becoming less towards the top of the formation, where the NaCl content exceeds 95%. Total thickness, 100 m.
The reserves of potassium salts amount to some millions of cubic metres, while the rock salt reserves exceed one billion |m.sup.3~.
Initial mining operations
The first two mines to be started up for potash mining were Santa Caterina and S. Cataldo, both of which yielded kainite.
The first was a small open-pit mine. In over 10 years some 1.5 Mt of kainite were extracted for use in the chemical plants which the mining company itself operated near Syracuse, for the production of fertilizers.
The S. Cataldo mine was more important. It had comparatively modern equipment including scrapers, and shafts equipped with Koepe hoists. The company constructed a pre-concentration plant at the mine and, as the water necessary for processing was not available at that site, transported the enriched kainite by cableway to a further plant at Campofranco, a distance of 30 km.
While the S. Cataldo deposit was being mined, another deposit called Palo was discovered 3 km away. There was no possibility of connecting the two mines underground, and at the new site two shafts were sunk. Both had Koepe hoists, one being for hoisting mineral and the other for people, in 8-man cages. In the new mine Joy mining units were introduced, initial models being powered at 380 V, later units operating at 500 V.
Both mines closed in the 1980s. Unfortunately, considerable problems of subsidence exist in the area above the former S. Cataldo section, brought about by the infiltration of water into the mines, causing gradual dissolution of the pillars and remaining sills. The whole zone has been closed off and the phenomenon monitored by means of periodic topographic surveys as well as by the regular reading of tiltmeters and piezometers installed at critical points.
Current mining operations
At present the administration of all the mines is entrusted to ITALKALI, whose head office is in Palermo. This is a company whose majority shareholder is the Ente Minerario Siciliano (the Sicilian Mining Authority), representing the Sicilian region, while the remaining shares are held by private organisations which run the mines. The decision of the regional authorities to entrust the running of the mines to a single company has led to a general reduction in costs, the re-establishment of stable production levels and the drawing up of medium- and long-term investment programmes.
Until the 1970s mining, especially of potash, was principally conducted using electric-powered Joy units, with consequent advantages for the environment. However, there were attendant problems, deriving from the lack of mobility of the machines with their electric cables. With the transfer of administration to ITALKALI, the first continuous miners were introduced. Together with diesel powered loaders and personnel carriers these were able to guarantee higher levels of production.
The gradual increase in the number of diesel-powered machines gave rise to problems with air quality in the mines, a matter in which the Italian legal requirements at the time were inadequate. However, with the co-operation of management and workforce, it was agreed to work to standards set in other European Community countries, and in particular the regulations applied in potash mines in Germany were used as a reference. New ventilation shafts were sunk to improve the volumes of air circulating, and the use of conveyor belts for transport was increased to reduce the number of diesel vehicles in circulation. Close attention was paid to vehicle maintenance and control of the use of filters. Improvements also arose, of course, through the introduction of new equipment and the phasing out of older vehicles.
The Petralia mine
This is located on the northern edge of the Sicilian saline basin. A very high quality of rock salt is extracted, of which 30% is used for human food and the remaining 70% for the chemical industry. The present production is about 3,500 t/d, obtained by a staff of 90 persons, including technicians and workers, who operate two shifts. The mine's capacity is about 700,000 t/y and, after 20 years of activity, the mine has reserves for approximately another 15 to 20 years, on the basis of the latest data available.
Extraction takes place by the room and pillar method on two levels, the galleries being 18 m wide by 7 m high. Production from these is supplemented by drilling and blasting out the intervening 10 m sill pillar, to give openings with final dimensions of 18 m wide by 24 m high. Pillars are 24m square.
Galleries are excavated using two Alpine Miner AM 100/058 machines, which can cut up to a height of 7 m. Blastholes are drilled using two twin-boom drill rigs. Broken rock is loaded out using five loaders -- International Harvester and Fiat Allis -- fitted with 2.5 and 3 |m.sup.3~ buckets respectively. Haulage is by seven Perlini 36 t capacity dump trucks powered by 420 hp GM engines.
The geological characteristics of the rock salt have enabled experimentation to be carried out with completely automated continuous miners. All the hardware problems have been resolved and only some software questions remain to be sorted out.
The mine has three ventilation shafts, of which the oldest is a historical rarity with 2 m x 2 m square section. The others are circular, with diameters of 2.5 m and 5 m.
The highly stable nature of the ground has made it possible to install a finished product packaging plant inside the mine. Salt is thus packed not only in sacks of 25 kg, 50 kg, 1,000 kg and 1,500 kg but also, with varying degrees of fineness according to market demand, in 1 kg packets. The whole crushing, milling and screening cycle is wholly automated, the plant having a capacity of about 700 t/d. The remaining production is stockpiled, and sold in bulk.
So far as is known, this is the only mine where the finished product is packaged underground. Not only does this give big cost savings, it also eliminates environmental problems -- the mine is situated in an area designated as a natural park, where it would not be possible to build an industrial plant on surface.
The Realmonte mine
This is the most recent mine to be developed in the Agrigento region. The large deposit is at shallow depth, with reserves which will certainly last more than 30 years: the deposit limits are open towards the south.
Access is by truck through a 36 |m.sup.2~ cross-section incline with a 13.5% gradient which, for the first 450 m is paved. At present the ramp, which goes from an elevation of +65 m to a depth of -125 m, is 1,100 m long overall.
Initially only rock-salt was extracted and used mainly for de-icing roads. Mining, by the room and pillar method, took place on various horizons at 15 m vertical intervals, from the -15 m level to the -105 m level. Pillar size increased with depth of working.
As mining progressed, seams of kainite were encountered, and Realmonte thus became the first Sicilian mine from which it is possible to extract either rock salt or kainite, according to demand. The rock salt is sent for shipment to Porto Empedocle, only a few kilometres away, whereas the kainite is sent to the Campofranco plant for enrichment and conversion into |K.sub.2~S|O.sub.4~.
Initially the concession area was very limited, but discovery of the kainite caused the company to seek an extension to the west. To gain access, development roads were driven, without blasting, under the residential area of Realmonte. Ground conditions were such that no particular problems of stability were encountered, except in a few stretches where roof-bolting was used as a precautionary measure. After a number of years, these bolts remain in good condition and the 1,830 m long conveyor gallery is very impressive.
A feeder breaker was installed underground serving levels -30 and -45, and a 600 t/h capacity conveyor installed which carries crushed material out of the mine to surface bins, for final dispatch. The whole of this plant is automated, with a single operator.
The mine possesses three Westfalia Wav 300 continuous miners, three Atlas Copco hydraulic drill jumbos, six power loaders (GHH and Fiat-Allis) with 3 |m.sup.3~ or 4 |m.sup.3~ buckets, and seven dump trucks. A special Pingon scaling machine is also used for dressing operations in the 7m high galleries. There is also a fleet of vehicles for the movement of men and materials. Maintenance of underground equipment is carried out in a workshop in a worked-out area on the -30 m level.
In this area it was noted that some of the pillars showed cracks running the full height of the pillar, reducing their effective areas. The matter was investigated by the University of Turin, and the problem has been solved by hooping the pillars with pre-stressed steel reinforcements. The zone has been exhaustively surveyed and the problem of cracking has practically stopped. Indeed, in places it is possible to observe recrystallisation of the salt and it is considered that pillar stability is no longer a problem.
A recent development has been the boring of a new 5 m diameter upcast, known as the Giglione shaft, which is 250 m deep.
The productive capacity of the mine is, on average, over 4,000 t/d and it employs 120 workers who operate on three shifts.
The Pantanelle mine
This is one of two mines located on the elliptically-shaped Racalmuto mineral field in the province of Agrigento.
Access to the mine is through a 22 |m.sup.2~ cross-section decline, lined in its near-surface length and 800 m long overall. For the first time in Sicily, Bernold sheet metal was used as the reinforcement for reinforced concrete.
Room and pillar extraction has progressed, with galleries 14 m wide x 7 m high and leaving 9 m sill pillars, on a number of horizons from the +270 m level to the +196 m level in the southeastern section and down to the +176 m level towards the northeast.
Pantanelle was conceived as a rock salt mine and worked as such until a few years ago, when prospecting towards the north led to the discovery of kainite
beds. These eventually proved to be an extension of beds worked at greater depths towards the east in the adjacent Racalmuto concession.
Pantanelle is, at the moment, the only mine in the group in which continuous miners are not used. Extraction is by drilling and blasting: two drill jumbos are employed, along with four loaders and a number of dump trucks. There is also a Pingon scaling machine.
The mine is currently producing about 700 t/d of rock salt and up to 500 t/d of kainite and its reserves are certainly sufficient for more than 30 years at current rates of extraction. The salt goes for domestic, animal husbandry and industrial uses, while the kainite is trucked to the Casteltermini plant for conversion to potassium sulphate.
The Racalmuto mine
This mine, opened only for the mining of kainite, has suffered problems with its main shaft.
The 5 m diameter circular shaft is about 1,000 m deep and is equipped with two 5 t capacity skips, driven by an ASEA-Siemens Koepe hoist using 48 mm diameter ropes. The skips travel on rigid guides at a hoisting speed of 16 m/s. The shaft has required continuous maintenance due to the very strong pressures acting along the east-west axis, which have damaged the rigid shaft lining. It was necessary to close the mine for some time in order to carry out maintenance work on the shaft and to re-align the guides.
As mining went to greater depths, the costs of shaft maintenance became excessive and an access ramp was designed. Part of this has been driven from the adjoining Pantanelle mine.
Currently the mine is on stand-by, and shaft maintenance is being confined to the upper 500m, as the zones at greater depths require excessive amounts of repair. Nevertheless, decisions on the future of this mine will have to be taken in the coming months in view of the major investment needed for the completion of the ramp, as well as the high costs of running such a deep mine with all the attendant problems of guaranteeing safe working conditions.
However, some millions of tonnes of potentially exploitable mineral have been identified, in addition to which there is considerable interest in further prospecting towards the south. In the coming years, it seems certain that everything possible will be done in order to bring this mine into a state of full and economically viable production.
The Pasquasia Mine
This is the most important of the operating Sicilian potash mines, whose equipment is among the most modern in Europe. There are very ambitious plans for the complete exploitation of the mineral extracted.
Situated in the Enna district in the centre of Sicily, this mine works the largest deposit hitherto found in the island. With its long axis trending northeast/southwest, the deposit is over 8 km long and about 3 km wide.
The mine has been in production for over 30 years, so its initial design was traditional, with access underground being through three shafts.
The No. 1 shaft is 5 m in diameter and 825 m deep. Rope-guided skips, travelling at 10 m/s and driven by a 4-cable tower-mounted Koepe hoist, have the capacity to raise 240 t/h. The No. 2 shaft, also 5 m diameter with a 4-cable tower-mounted Koepe hoist, is 757 m deep and is fitted with cages running on rigid guides, each capable of carrying 54 workers. The third shaft, an upcast, is 310 m deep.
Over the years, the working districts have moved further and further from the shafts, so a decline has been driven from surface to the current zone of activity. It is approximately 1,920 m long and has a gradient of 16.5%. A 500 t/h capacity conveyor belt has been installed, while the dimensions of the incline also allow for the passage of vehicles. Some problems arose in this ramp along a stretch which went through bischofite (Mg|Cl.sub.2.~6|H.sub.2~O) which, on account of this mineral's deliquescence, requires continual maintenance. One original measure which has been taken was the installation of a refrigeration plant which, in addition to lowering the temperature in the bischofite zone, also reduced the humidity. Furthermore, the incline was asphalted in order to eliminate dust in its upper section.
In addition, a new 290 m deep ventilation shaft has been sunk to improve ventilation in the present working areas, in particular to meet the needs of the diesel powered equipment.
The mine was originally conceived for the extraction of sylvanite and carnallite, but the working of these has now been suspended and kainite is the only mineral being extracted at present. There are five beds of kainite, each of which is of exploitable thickness and continuity. They lie in a regular syncline. The richest is the first, or topmost, bed while the most interesting commercially is the second bed, which has a thickness of 30 to 35 m.
Table 1: Major items of equipment used in the Pasquasia mine. Type Make and model Number Continuous Voest-Alpine AM 100 4 Miners Paurat E34 1 Marietta Miner 5012S 1 Joy 6PM 2AHY 1 Scaling Pingon 14 FB 4 Drilling machines Atlas-Copco Diamec 500 1 Montabert T21 2 Flotmann CB 20, CB 12 8 Loaders MAN GHH LF 12 (224 hp) 5 MAN GHH LF 7 (182 hp) 1 Shuttle car/ Joy 10 SC 25 2 rock loader Joy 10 SC 22-56D (25 t) 3 Joy 14 BU 10-11 D 2
Extraction of the second bed is by the room-and-pillar system, with pillars 30 m x 30 m. Sub-level development galleries are 8 m x 8 m, and are driven for a length determined by the deposit -- some are over 300 m long.
The high degree of |K.sub.2~O in the various beds, above all in the first, gave rise to notable problems regarding the safety of the working places and transit route. The problems were solved in the following ways:
* By carrying out extensive resin-bonded roof-bolting. During the past four years, over 20,000 roof-bolts have been put in place.
* By testing and then purchasing four Pingon scaling machines.
* By reducing the use of explosives and introducing the use of continuous miners (seven are now in use: see Table 1).
* By utilising remote controlled LHDs, where loading takes place in areas unsafe for personnel.
Production has now stabilised at around 6,000 to 6,500 t/d.
There are two plants in operation for processing kainite. One is at Pasquasia, downstream from the mine of that name, which is able to handle about 1.5 Mt/y of the mineral. The other, of slightly smaller capacity, is at Casteltermini, near Agrigento, and serves the Realmonte and Racalmuto mines.
At Pasquasia, the kainite is sent to flotation, the concentrates passing to a conversion plant for the production of potassium sulphate. In order to satisfy the needs for industrial water, the Villarosa dam was built nearby, which has a capacity of about 10 million |m.sup.3~.
The residual waste brine from the process is at present discarded. A number of studies have been carried out concerning possible uses for this brine, including the production of by-products such as magnesium oxide, sulphuric acid, hyper-fine sodium chloride, synthetic carnallite, magnesium chloride (thence metallic magnesium and chlorine), and other secondary products. However, market conditions do not justify the considerable investments which would be needed. Nevertheless, some minor plants are under construction whose end products could be sold without difficulty (hyper-pure salt, for example).
Rock salt, besides being handled in bulk from the various mines, is also packaged in the Petralia mine, at Porto Empedocle and in the Racalmuto mine plant.
The authors would like to thank the Italkali company for its assistance, as well as the mine managers, Messrs Burgio, Gambazza, Lo Celso and Simili.
Dr. Ing. S. Adamo is Chief Engineer for the Caltanissetta Mining District. The section on geology was contributed by Dr. Lamberto Ramberti, of the Geological Office of Helkeli.