Source: Engineering & Mining Journal
Roca Fosforica Mexicana SA de CV (Rofomex) started up one phosphate mine in Baha California Sur in 1981 and another in 1982. The combined production of these operations were expected to lift Mexico from a position of almost total dependence on phosphate rock imports in 1980 to self-sufficiency in 1985. In recent years, Mexico has imported about 1.7 million mt/yr of phosphate rock, mainly from Florida in the U.S. and Morocco. The Mexican government has placed high priority on the Rofomex mines as part of its program to improve productivity on Mexican farms.
The first of the new Rofomex mines is located at San Juan de la Costa on the Gulf of California. It produced its first concentrates in January 1980, and the first shipload of concentrates left the Rofomex dock bound for Lazaro Cardenas on Mexico’s Pacific Coast in April. Shiploadings are scheduled to take place every 12 days.
From Lazaro Cardenas, the phosphate concentrates will be shipped by rail to Fertimex plants in San Luis Potosi, Queretaro, and Guadalajara. Later, some shipments will move through the Panama Canal and the Gulf of Mexico to the Fertimex complex at the port of Pajaritos.
At capacity, the San Juan de la Costa mine will produce 730,000 mt/yr of concentrates grading about 31% P2O5 from combined open-pit and underground mining operations that will extract ores grading 18% P2O5. Monthly productions of concentrates totaled 1,200 mt in January, 12,800 mt in February, 12,000 mt in March, and 20,000 mt in April. Production was scheduled to build up to about 60,000 mt in July and to remain at about that level through the rest of the year.
At Santo Domingo, on the Pacific shore of the Baja California peninsula, Rofomex constructed a mine that will produce 1.5 million mt/yr of concentrates by dredge mining a beach sand deposit that grades about 4.5% P2O5 at a cutoff grade of 3% P2O5. Two Ellicott cutting head suction dredges feed ore to a barge-mounted primary flotation plant, which will float in the mines area behind the dredges.
Rofomex started up at Santo Domingo in mid 1982, and in 1984 the company’s two new mines generated a short-lived phosphate surplus in Mexico, with the excess available for export. In 1985 new Fertimex plants were scheduled to come into production, economic and population growth will have raised phosphate demand within Mexico, and a renewed phosphate shortage is predicted. In anticipation of this demand, Rofomex planned expansion of 4.5 million mt/yr on the drawing boards.
Over the long run, Mexico has a substantial phosphate resource to draw on in Baja California Sur, apart from the two deposits being developed. Rofomex geologists have reported other phosphate occurrences at San Hilario, Santa Rita, Tembabiche, La Purisima, San Jose de Castro, and San Roque. Of these, the deposit at San Hilario has been the most extensively explored, with drill indications of substantial rock tonnages grading 11-13% in weathered cap rock and 14-18% in unweathered rock under 30-80m of overburden. In the Santa Rita area, possible economic concentrations of phosphates occur in recent sediments under almost no overburden, but these remain to be more intensively explored.
Huge Santo Domingo Resource
The existence of phosphate sands in the area of Bahia de Magdalena on the Pacific Coast has been known since 1914, and in 1955 Hanna Mining and Minera Fornos conducted a feasibility study in the area. However, lack of infrastructure and Mexicanization of the mining industry caused Hanna to withdraw. In 1974, “Consejo de Recursos Minerals,” a government agency, took another look at the area and also decided that mining was not economically feasible. In 1978, government geologists conducted a drilling program, following the deposits up the coast with holes at 2-km intervals, and they found that the phosphates were present over a distance of more than 70 km. The actual limits were not found before the drilling program was ended. Fomento Minero took another look at the prospect, decided that an economic beneficiation process could be developed, and proceeded with mine development.
The western limit of the Santo Domingo deposit is the Pacific Ocean, and its width to the east is about 20 km. It is essentially flat-lying, with little or no soil cover, and its thickness averages 19 m. Relief is minor, with maximum elevations rarely exceeding 15 m above sea level. The ore body is of recent age, and the phosphate occurs as fine, rounded, granular sand, usually in the coarser fraction of the deposit. Minor amounts of magnetite, ferromagnesium, rutile, and sphene, as well as other miners, are present. Clay minerals are usually absent, except very close to the surface. Grades average 4.5% P2O5. Color varies considerably but is usually yellowish to dark brown. The resource is estimated to total 1.1 billion mt of rock.
Detailed studies have been made for two mining areas, the Elenas and the Prados, using Longyear drilling equipment for sampling. The Elenas zone is the southern-most of the two and measures 6 km north-south by 3.5 km east-west. Its southern limit lies just north of the fishing port of Lopez Mateos. The Prados zone is contiguous with the northern limit of the Elenas zone and is 9 km north-south. The western limit of the ore is an estuary formed by the barrier island, Isla Magdalena.
Both ore zones have been explored on a grid of 500 m, using truck-mounted rigs drilling 4-in dia holes. About 5,000 samples were taken from these holes at 1.5-m vertical intervals. Maximum hole depth was 70 m and average hole depth was 30-m. Ore tests followed the practice established in the Florida phosphate industry.
Dual Dredge Mining
Analysis of mining systems for the Santo Domingo project included consideration of bucketwheel excavators, draglines, scrapers, shovels, and dredges. Selection of a dredge-based system hinged primarily on the fact that the other systems could not operate effectively below sea level. Low operating and maintenance costs prompted the final selection of floating hydraulic cutter suction dredges with 27-in diameter suction heads.
Two Ellicott dredges were used, each capable of pumping about 2,000 mt/hr of slurried solids to the floating primary beneficiation plant. Each dredge is connected to the plant by a flexible, 600-m, 24-in dia pipe. The dredge heads have a lower reach of 15 m. The two dredges combined to work a single mining front averaging 21 m wide. A single dredge operating at capacity is able to keep the plant at near capacity. In normal operation, the two dredges work together at about 70% of their rated capacity.
Suction mining was performed at a cutoff grade of 3% P2O5 producing an average ore grade of 4.56% P2O5in the Prados mining area and 4.29% in the Elenas area. No removal of overburden will be required; however, internal wastes in the ore will dilute the feed grade to the beneficiation plant to 4.05% P2O5. With an average CaO of 9.34%. The mining rate was planned at 16.5 million mt/yr, working three shifts per day, 330 days per year.
The mining face averaged 6 m above sea level and 12 m below sea level. Selective mining was not always possible, but areas of high CaO content were avoided when feasible. Sample drillings were made on a 100-m grid, with each hole representing about 225,000 mt of ore.
The dredges operated on an electric power supply of 4,160v, 60 hertz. A substation was located on a floating barge.
Concentrating Phosphate Sands
Because ores located above and below sea level have somewhat different characteristics, mainly CaO content, Rofomex developed a process that is compatible to both ores. The system allowed disposal of tails into the mined-out area at all times. As at San Juan de la Costa, seawater was used throughout the process, with only the product washed with fresh water to eliminate chlorine.
The main elements of the process included a primary flotation plant located on a barge behind the dredges and a secondary flotation plant, washing tanks, and filters located on shore. The ore was pumped from the barge to the shore plant through pipelines supported by pontoons.
Slurry from the dredges fed the barge-mounted primary plant at a rate of 2,200 mt/hr, with the discharge to the barge first fed across two single-deck screens to scalp plus 1/4-inch organic debris, such as shells, and oversized ore from the slurry. Screen undersize was cycloned to remove part of the minus 150-mesh solids and slimes and to concentrate the slurry. The fines flowed by gravity to a disposal area, and the underflow dropped to storage tanks, which provided 1/2 hr surge capacity.
From the surge tanks, pumps move the slurry to hydro-separators, where plus 28-mesh solids were removed and pumped to waste. The slurry was then advanced to a second bank of cyclones, which removed any remaining minus 150-mesh solids and densify the slurry to 65% solids. In preparation for flotation, the cyclone underflow was conditioned in several stages. Diesel oil was used at the emulsion.
The conditioned slurry was diluted to 32-35% solids with seawater and fed to four banks of 500-ft3 flotation cells. Tails were pumped to waste, and the primary concentrate flowed to deactivation tanks, where it was agitated with sulphuric acid to deactivate the flotation reagents. The concentrate is then pumped ashore at a rate of 360 mt/hr.
A transfer barge equipped with reagent tanks delivered primary flotation reagents and 95% sulphuric acid to the barge to sustain the process.
On shore, the concentrate slurry is washed again, dewatered in cyclones, and pumped to a hydraulic classifier for separation of minus 28/plus 48 mesh solids. The plus 48 mesh fraction is diluted and fed to vibrating tables to remove calcitic solids. The minus 48 mesh fraction is cycloned to produce another particle size split at 100 mesh. The minus 100 mesh fraction is then advanced through densifying cyclones to banks of Humphrey spirals, which remove ilmenite, zircon, and magnetite. These heavy minerals are pumped to a separate storage pond. The phosphate concentrates produced at the vibrating tables and the spirals combine with the plus 100/minus 48 mesh fraction to produce the feed for secondary flotation.
The pulp is conditioned with diamine, acetic acid, and 5% pine oil in seawater to prepare it for inverse secondary flotation, which produces a silica concentrate waste product and phosphate tails as product. The phosphate slurry is densified to 60% solids through cyclones, washed with fresh water, cycloned again, and filtered. The filters are also equipped with washing sprays.
A belt conveyor advances the filter cake to the concentrate storage facility. If necessary, a bulldozer will churn the stored material to produce additional drying.
A water treatment plant at the shore plant site will clean up brackish well water to produce fresh water for washing and to supply the boiler used to prepare diamine reagent. Fresh water specifications set a maximum chlorine content of 350 ppm.
The phosphate process in use at Santo Domingo produces several waste streams, all of which are deposited either into mined-out areas or behind dikes constructed of sand. At the shore, the natural terrain is left as a continuous berm to isolate waste areas from the waters by the bay. Waste waters are recycled. The waste streams include 300 mt/hr at the floating plant ahead of the flotation cells; primary flotation tails containing 1,340 mt/hr of slurry at 30% solids; silica waste concentrate from secondary flotation and wash water, containing a combined 3,000 mt/hr of slurry at 5% solids; and heavy minerals concentrated at the Humphrey spirals, totaling about 72/mt hr of slurry at 25% solids.
Concentrates produced at Santo Domingo are transported 100 km to shipping facilities at Punta Belcher on Magdalena Island via a tug-pulled barge. At Punta Belcher storage facilities were built for 100,000 mt of concentrate. The dock handles ships of up to 40,000 DWT, and the concentrate loading system has a capacity of 3,000 mt/hr.
Reprinted from Engineering & Mining Journal