Abstract: The mixer has a chamber with a shaft supported for rotation along an axis along the length of the chamber. Two sets of outer ribbon members are coupled to the shaft such that the two set of outer ribbon members are located about 180 degrees apart and extend helically around the shaft. Two sets of inner ribbon members are coupled to the shaft such that the two sets of inner ribbon members are located about 180 degrees apart and extend helically around the shaft. A drive is coupled to the shaft for rotating the shaft and hence the ribbon members about the axis. The two sets of outer ribbon members are located radially outwardly of the two sets of inner ribbon members. Each set of outer ribbon members has at least two radially spaced apart outer ribbon members. Each set of inner ribbon members has at least two radially spaced apart inner ribbon members. One of the two sets of ribbon members has a left hand pitch and the other of the two sets of outer ribbon members has a right hand pitch.