Bauxite The extraction process, storage, loading and transformation of bauxite is developed in three basic areas: Mine, Homogenization Area (Pie de Cerro) and Storage and Shipping Area (El Jobal). Generally the infrastructure for the bauxite extraction and processing was previously designed for the production of six Million/Tons per year covering: 1) Mine, 2) Crushing Station, 3) Conveyor Belt (Supported by 2 cables) of 4.5 kilometers length with a capacity of 1.600 t/hr and with a descending trajectory of 650 meters high; 4) Railway of 52 Km . And with a capacity of 3.600 t/h and a terminal with a gabbards loader, 6) Gabbards Fleet for the transport through the Orinoco River.

MINE

The bauxite production process is initiated with the explotation by conventional methods of the open sky mines (stripping mine) after been removed and piled up the vegetal mantle for its subsequent afforestation.

The bauxite is directly extracted from the different mine blocks looking forward to obtain the mineral requested quality. The mine operations are planned and controlled through the MINTEC Program “Medsystem”.

Mine operation sequence: 1)Removal of the vegetal mantle (< 1 Meter ), 2) Ripping for breaking the hard laterite mantle, 3) Loading without blasting with hydraulic shovels; 4) Carrying with rocky truck with capacity up to 5-100 tons, 5) Material crushing – crushing station (Normal capacity 1.600 t/h.).

At the milling station the bauxite is transferred through a plate carrier up to the mill that reduces the material to a granulation lower to 100 m .m. for its transportation and better handling. Once the material has been rippingned, is transferred to the descendent conveyor belt which is supported by 2 steel cables (telpher or cable type technology) and it has a 4.2 Km . Length.

The main task to increase the production from 1.000.000 up to 2.000.000 t/year was to increase the productivity, efficiency and operating factor, as well as to increase the mineral processing capacity, using the Los Pijiguaos Bauxite at 100% this new capacity places Bauxilum as the third biggest worldwide plant.

The Alumina Plant applies The Bayer Process (low digestion process to low pressure and low temperature) in order to assure a good production and efficiency for the high grade Alumina extraction from the bauxite mineral. This process is divided in three high areas: Materials handling, red side and white side.

The Processed Bauxite at the Alumina Plant is 100% trihydrated Bauxite from Los Pijiguaos. Some plant areas were modified so the Los Pijiguaos bauxite with its specific properties (high content of sand and quartz could be used as plant stock raw material.

The plant arrangement includes two steps, in such a way that allows compensating plant suspensions for maintenance, repairing, cleaning, etc. This plant fixing is conceived to allow a later expansion.

 

MATERIALS HANDLING

The handling materials area is adjusted by the teams that allow the handling of bauxite and caustic soda and the exportation of the end product. The Alumina Plant has units for the pile up and bauxite recovering.

Now a days it holds an unit with buckets systems that joints. The piling up as the recovery, with a media capacity of 2400 t/h for the piling up and of 900 T/H for the recovery. This last material handling system ads enough transport capacity and storage on or around 1.500.000 t. to guarantee a continuous bauxite feeding from Los Pijiguaos. Besides It counts with two additional bauxite bins (a 200.000/t open Pile) and an alumina bin with capacity for 1.500.000 t.

 

RED SIDE

The red side allows the reduction of the mineral particles size, the extraction of the alumina inside the bauxite and the separation of the impurities inside the alumina.

At the red side, the process is made in two steps. This starts at the size reduction area, compound by 5 crushers and 5 bal grinders, the bauxite should be adjusted to an particle specific size with an adequate distribution for its later treatment (80% minor to 0,3 mm ) the predessilication area is constituted by 4 heating tanks ( 1.700 m 3 ea) in series and transference pumps to control the silica levels (Si 02) in the process liquor and in the alumina process.

 

The predessilication process consist in increasing the mud temperature or the bauxite pula up to 100 ° C, keeping it during 8 hours, at the same time that the material is stired. In order to extract the maximum alumina quantity from the bauxite, the mineral (bauxite suspension) and the caustic soda (pre-heated liquor) have to be mixed in an adequate proportion in the digesters which are well dimensionated to allow a higher stay in order to improve the descilication process the resulting suspension from the mud digestion is reduced to the atmospheric pressure through a series of expansion tanks for it is later pumping to the remove sand area.

At the remove sand area, the hydrocyclonics together with the set of three (3) classifiers in spiral are used for the bauxite digestion know as red mud must be separated of the alumina suspension before been recovered by precipitation. This can be obtained due to the settling into the thickeners and washers tanks (mud classification and washing) the polymerics are added in the mud suspensions in various points to increase the settling speed.

The mud filtration is now requested. The over flow from the thickeners tanks is filtrated in an eight filters batch battery looking forward to eliminate the red mud particles that could stay in the sodium aluminate solution.

The crystal hydrates settled at the bottom of the primary classifiers are sent to the product filtration area, where the hydrate is washed and separated from the drained caustic liquor through and empty filtration in horizontal filters. The filtrated hydrate has to reach a low content of free humidity, in order to minimize the required heat for the calcinators termic dried. With the trihydrate washing it is wished to reduce the caustic soda contented at the hydrate in order to obtain a higher reduction of the loss if said substance and to avoid the contamination of the final product with caustic soda. The filtered hydrate is unloaded through an endless feed screw to the feeding hopper of the calcinators ventury dryers. The hydrate is calcinated with the purpose of removing the humidity and the chemically mixed water.

This is done in a flowing cough calcinatory (wo by each stage) at a maximum temperature of 1100 ° C the water is removed by heating interchange of the cyclones between the hydrate and the exhausted gases. Later the material gets into a flowinged couch. Finally the calcinated alumina is cooled in cyclones with heat interchange in countercurrent with the combustion air. A flowinged couch cooler provides the final cooling. To separate the dragged down solids in the cyclones with wasted gases and to increase the efficiency, an electronic precipitators were installed. The Alumina production cycle is a closed circuit regarding the caustic liquor which is handled up to different concentration levels an instantly evaporation plant is settled for restauring. The original caustic concentration and to reduce the specific vapor consumption.

 

RED MUD

The red mud is the Alumina's production sub-product and contains those bauxite components that are dissolved in digestion. This sub-product is contaminated with Alumina-Sodium formed during the desilication and the calcium and sodium aluminate components from the mother liquor dragging. The red mud is diluted in water and pumped to the lagoons which dikes are specially prepared and are continuously inspected. The remaining water in the lagoons is recollected and returned to plant to be use for the mud flowing and to facilitate it is delivery through the pipes as well as to the mud washing. The sand originated from the unsanding process is deposited in an equal way.

 

CENTRAL CONTROL ROOM

The electronic and instrumentation is used at the large of the process. Distributed control systems are used as primary control with a computers system as a support for its supervision graphic screens and programming controllers with interlocks are used for the motors control the whole operation supervised from a Central Area and three periphery control rooms vapor station, product filtering and materials handling.

 

ENLARGING PLAN

The CVG BAUXILUM, Matanzas , enlarging project increased in 1992 it is settled capacity from 1.000.000 to 2.000.000 metric tons. It was due to its eminently good processes and new unit construction.

This project was developed I two big areas. The first one was oriented to increase of the productivity, efficiency and the plant operative factor, upon applying the elimination of congesting points (PEPCO), this program was issued to modify minor congestion points, this increasing the operative factor. These improvements allowed to assure a 1.300.000 tons per year production. As well as an oxalate control plant was settled to avoid impurities and another one for predesilication.

The second one allowed to reach a 2.000.000 tons per year in order to reach this task the alternative of the increase of the plant productivity was taken, which was and already proved technology. Owing to this production had to be acquainted using the Los Pijiguaos Bauxite, many important modifications had to be done oriented to an end of supporting the original plant one hundred per cent operative. Among these modifications the following can be considered: •  Extension and up dating of the bauxite storage systems. Due to the variations of the Orinoco River water levels it was unfailing to guarantee the bauxite supply at the dry station, thus it originated the need of stocking 1.800.000 tons of this material in order to cover this period, the gabbards to carry the have specific and appropriated dimensions to rise the proficiency of the Orinoco River and its pier.

•  The additional extension of conveyor belts. This extension was an indispensable requirement to drive the bauxite from the pier up to the new storage points, allowing the pier to keep its loading and unloading capacity of caustic soda and alumina.

Among the more significant modifications in the process the following can be enumerated:

OPERATING AREAS

PREDESILICATION

Consist of 4, 1.700 cub/mt heating tanks oriented to control de silix (SI02) levels in the process liquor and the alumina. The process stands for increasing the bauxite pulp temperature form 650 m 3 /h to100 ° C supporting it during 8 hours, meantime the material is stirred.

CRUSHING AND MILLING

This function looks forward to reduce the bauxite mineral to an appropriated particles size for the alumina extraction.

DESAND

It removes the indissoluble residues of sizes between 0.1 and 0.5 mm . Which are produced in the alumina dissolution stage in the caustic liquor.

MUD SEPARATION AND WASHING

This area has as function the separation of the higher part of the indissoluble residues commonly named red mud, which are a product from the alumina dissolution in the caustic liquor and the recovering of the higher caustic soda quantity associated to this residues using for it a washing operation with stop water.

CARBONATES CAUSTIFICATION

It has control over the liquor process levels of contamination through the sodium carbonate (Na 2 CO 3 ). Capacity: 600 cub/mt. Per hour for the 4 tons conversion from sodium carbonate to calcium carbonate (CaCo 3 ) per hour, which is eliminated from the process.

SLAKED LIME

It has the function of to slaking the caustic lime and to produce a calcium hidroxyde slurry which is usaged in the mud separation and washing, in the carbonates causticity and the security filtration.

SECURITY FILTRATION

It removes the red mud sketches in the mother liquor saturated in alumina.

EXPANSION COOLING

It operates the temperature reduction from the required mother liquor for the alumina precipitation process.

PRECIPITATION

It this area the alumina is dissolved in the mother liquor and in super saturated stage is inducted to crystallize as aluminium trihidroxide over a seed of the same compound.

HYDRATE CLASSIFICATION

Classification by size of aluminium trihydoxide particles, know as hydrate, product use a for calcinating fine seed and gross seed.

PRODUCT FILTRATION AND CALCINATION

In this area the aluminium trihydoxide (AL 2 0 3 .3H 2 0) is converted in metallurgic grade alumina (AL 2 0 3 ), with maxim soluble sodium reduction associated to the hydrate.

FINE SEED FILTRATION

There is a filtration and washing with hot water of the fine seed to be recycled in the precipitation area, in order to eliminate the sodium oxalate and other impurities precipitated in it self thus to guarantee the hydrate granulometry control.

GROSS SEED FILTRATION

There is also a gross seed filtration oriented to reduce the exhausted liquor recycling, with few capacities for the hydrate precipitation.

ENCLOSURES

Flujograma Proceso Producción de Alumina