Design of asphalt-concrete plants
An asphalt-concrete plant (ACP) is a complex of process, power and auxiliary equipment designed for preparation of asphalt-concrete mixes (hot, cold, casting types). In addition, ACP is used to produce crushed stone coated with bitumen (coated chippings) and to reprocess old asphalt concrete.
In terms of process equipment operating principle ACP and installations are divided into cyclical and continuous. Cyclical ACP use discontinuous operation plants and batchers for dosing mixture components. Batching, mixing and finished mixture feeding operations are combined at continuous operation plants.
In terms of capacity asphalt mixing plants of ACP are divided into the following types: low — up to 40 t/h; medium — 50 to 100 t/h; high — 150 to 350 t/h and super high capacity — 400 t/h and more.
In terms of process equipment layout ACP and installations are divided into tower and parterre types. The most common plants are those with tower position of units.
In terms of inventory degree the plants are divided into three types: stationary, prefabricated and mobile.
An important problem in ACP design is selecting a location for it on the road being built. Solution of this problem influences not only the mixture cost, but also the plant operation success and road construction timeframe.
The plant location depends on demand in the mixture, construction time, relief of the terrain, locations of materials supply bases and sources for mixture preparation, railway stations, availability of power and water supply, production or residential facilities, etc.
A decisive factor for ACP deployment is taking into consideration of process peculiarities of hot-mixed asphalt-concrete mixes – cooling en route and respectively time of delivery from the plant to the construction project.
Location of ACP and its capacity shall ensure the lowest cost of asphalt-concrete mix at the location where it is placed, continuity of its flow and scheduled construction speed.
Preparation of asphalt-concrete mixtures consists of the following process stages: drying and heating of rock materials; their sizing, batching of mineral and binding materials, mixing the mix components and feeding the finished mix.
The hot-mixed asphalt-concrete mix process chart:
1 — feeder bin; 2 — feeders; 3 — belt conveyor; 4 — inclined conveyor; 5 — drying drum; 6 — de-dusting unit (cyclones); 7 — bitumen batcher; 8 — elevator; 9 — screen; 10 — mineral material bins; 11 — mineral dust bin; 12— mineral material weight batcher; 13 — mixer; 14 — skip; 15 — skip track; 16 — mineral dust storage bin; 17—dump truck
To improve quality, some surfactants and activators are added into asphalt-concrete mixes. Cation-active and anion-active agents are used as surfactants. Cation-active agents include salts of high primary, secondary and tertiary aliphatic amines, tetrasubstituted ammonium bases; anion-active agents include high carbolic acids, salts (soaps) if heavy and alkaline-earth metals of high carbolic acids.
Using surfactants allows decreasing mineral material and mix heating temperature, increasing degree of coating of mineral particles surface with bitumen, reducing mixing time, improving placeability and compactibility of mixes. At ACP surfactants are added to bitumen or into the mixer directly during asphalt-concrete mix mixing. The maximum effect is achieved when a surfactant is introduced into a bitumen melting unit.
Properties and qualities of surfactants introduced into asphalt-concrete mix shall comply with GOST requirements and other respective specifications for surfactants and activators.
At ACP asphalt-concrete mix is prepared in continuous and cyclic operation units with gravity or forced mixing of materials. If it is possible to select, cyclic operation units are used on a first-priority basis, then continuous operation units with forced mixing and, as the lowest-priority choice, continuous asphalt mixing units with gravity mixing.
When asphalt-concrete mix is prepared in a discontinuous operation mixer, it is easier to regulate mixing time and obtain the required quality mix.
Drying and heating of rock materials shall ensure not only specified temperature, but also moisture removal. When surfactants are used for hot-mixed mixes, the permissible moisture of mineral materials is no more than 1%; cold-mixed – no more than 3%.
Also the heat calculation of the drum includes determination of hot gas temperature over the drum length, hourly fuel consumption, heat efficiency, volume of flue gases and determination of the main drum specifications. Thermal balance of drying drum takes into consideration useful heat input in three drum zones, losses of heat by furnace, drum walls, with flue gases due to incomplete fuel combustion and other undeterminable losses. Table 5.1 contains specifications of drying chambers.
Automation of drying unit is monitoring and maintenance of the set material temperature at the inlet, and monitoring presence of flame in the nozzle. Thermocouples installed in discharging (scree) trays of the drying drum or non-contact thermistors heat indicators are used as temperature sensors. Temperature is regulated using an electronic device which, in case of any material temperature deviations from the set value acts on magnetic reversing-type starter of the auxiliary motor controlling delivery of fuel into the nozzle.
The automatic nozzle flame monitoring system is based on the use of photoelectric sensors. If flame is out, the photocell signal is amplified and transmitted to the auxiliary motor acting on the cock delivering fuel to the nozzle. An electric gas igniter triggered by an electric discharger is used in up-to-date asphalt mixing units for remote lighting-up of furnaces.
Continuous and discontinuous operation bitumen melting equipment is used for heating and dewatering of bitumen. Dewatering occurs in a thin layer in continuous operation units with gas or electric heating. Discontinuous operation units consist of several bitumen melting pots. Viscous bitumen is prepared using a two-stage cycle: in one type of pots bitumen is heated to 110–120 °С and if necessary, water is evaporated, then it is pumped to other feeding pots and heated to operating temperature.
If it is necessary to introduce a surfactant or liquefier, bitumen is prepared using a three-stage cycle: after heating and evaporation of water bitumen is pumped into empty pots where surfactants or liquefiers are added to it, and then it is pumped into feeding pots and heated to operating temperature.
Components of asphalt-concrete mix are batched based on weight; an exception is only permissible for bitumen and surfactant additives which are batched based on volume. Feeder units are used for preliminary batching of mineral materials (upstream of drying drum). Accuracy of preliminary material batching is ± 5%. When using very moist materials an adjustment for their moisture shall be made.
A representative of the plant laboratory checks accuracy of batching of the asphalt-concrete mix components and operability of batchers. Department control agencies check operation and batchers each 3 months.
Special attention is paid on modes of asphalt-concrete mixing in cyclical and continuous operation mixers.
The mode of mixing mineral materials with bitumen is the main stage of the mix preparation process. The thoroughly mixed mix is characterised by uniform distribution of all its components and complete coating of the particles with bitumen. Mixing duration depends on the mixer type and type of the mix to be prepared. It varies from 20 to 180 C.
The Samara Strommashina plant offers for sale asphalt-concrete plants, both new and used ones. Besides, it upgrades the existing asphalt plants. An upgraded ACP is the best price/quality ratio. In terms of its technical state it is as good as a new plant, while it is twice as cheap. The Strommashina has its own extensive manufacturing capabilities and high-skilled engineering department; it allows solving any technological problems of the customer.