Patent GB-1400721-A (GB)
Title: Method of and apparatus for production of coated roadstone
IPC: E01C19/10
Filed: 1971-04-15
Abstract: 1400721 Mixing apparatus REDLAND ROADSTONE Ltd and UNDERGROUND MINING MACHINERY Ltd 17 July 1972 [15 April 1971] 9515/71 Heading B1C [Also in Division F4] A process for the production of coated roadstone comprises charging a cold, moist aggregate and a bituminous binder into a mixer, "cold" mixing the binder and aggregate in the mixer to form a mixture of solid particles of binder and the aggregate, and continuing the mixing while a current of hot gas e.g. air is passed through the mixer so that the moisture evaporates from the aggregate and the binder liquifies and coats the aggregate. The moisture content of the aggregate may be adjusted prior to the "cold" mix to be between 0.02 and 15% (preferably 1-4%) by weight. The binder may be added to the aggregate while the latter is in a hopper before charging into the mixer, or on a conveyor belt leading to the hopper, or the binder is charged directly into the mixer. The binder may be added in granular form but is preferably injected in a molten condition and solidifies in contact with the cold, moist aggregate. Cold gas may be passed through the mixer during or after discharge of the coated product. Apparatus for producing batches of coated roadstone comprises a rotary drum mixer 3 pivotable to a charging position (Fig. 4) and a mixing position (Fig. 1), a weigh hopper 22 having a gate 122, an air heater 12, an extractor fan 16, a binder spray nozzle 23, a water spray nozzle 25, an oil spray nozzle 24, and valves 113-115 and 117-120 for controlling air flow in the apparatus. Different grades of aggregate are fed from respective hoppers via belt weigh feeders or volumetric feeders onto the conveyor belt 21 in predetermined proportions. The moisture content of the aggregate mixture is checked prior to the weigh hopper 22, and additional water, if required, is supplied via the nozzle 25. When the weigh hopper has the required weight of aggregate the gate 122 is opened and the aggregate is fed into the mixer (Fig. 4) and at the same time binder B is injected via nozzle 23. The gate 122 is then closed and the mixer is pivoted by hydraulic ram 11 to its mixing position in which it is rotated about its axis in one sense by a hydraulic motor 9. During the "cold" mix air is drawn in through duct 18 and expelled via duct 19 by the extractor fan 16 and the valves are positioned as shown in Fig. 4. The heater flame 28 is brought to full flame condition and the valves are positioned to allow hot air to be drawn through the mixer for the "hot" mix. A stream of cold air with entrained oil from nozzle 24 may be drawn through the mixer (Fig. 7 not shown) prior to the mixer being pivoted to one of its discharge positions R 1 , R 2 . In the discharge position the mixer is rotated about its axis in the opposite sense to facilitate discharge into a waiting truck and the valves are again in the Fig. 4 positions. After discharge the positions of valves 119 and 120 are reversed to permit cold air to be passed through the mixer. If discharge is effected in position R 2 , the mixer is returned to the R 1  position for cooling. At the completion of cooling the mixer and valves are conditioned for a further cycle. The operations of the apparatus may be controlled manually or automatically in a timed sequence. In a modified arrangement of the above apparatus the contents of the weigh hopper 22 (Fig. 1A) when gate 222 is opened are fed via a vibratory feeder and chute 34 into a holding hopper 35, the outlet of which is controlled by the gate 122. The nozzles 23, 25 in this arrangement extend into the chute 34.