we refer now to the drawings wherein like reference characters in the several views refer to similar parts . referring to fig1 designates generally the dispersing apparatus or mill embodying the invention herein . the dispersing apparatus 10 which is substantially like that disclosed in my u . s . pat . no . 3 , 653 , 600 , comprises a generally cylindrical vessel 12 , a rotatable agitator shaft 14 , a drive unit 16 for driving the shaft 14 , a plurality of impellers or agitator discs 18 mounted on the shaft 14 and a rotor separator 20 secured to the shaft 14 . the vessel 12 and drive unit 16 usually are mounted on a suitable supporting frame 22 . a 24 22 is associated with the mill to introduce premixed material into the vessel 12 to be processed . the vessel is partially filled with a dispersing media 25 such as metal shot , for example , but other types of dispersing media can also be used . when steel shot is used , the vessel preferably is filled only about half way . in the illustrated embodiment the vessel 12 is closed at both its upper and lower ends . an outlet 26 is disposed at the upper end of the vessel . an inlet 27 is disposed at the lower end of the vessel through which product to be processed is pumped from pump 24 via inlet pipe 28 and conduit 29 , the conduit 29 being connected to the inlet pipe by way of a connecting nipple 30 or the like . the unattached end of the inlet pipe 28 has a normally closed shut - off valve 32 which is opened when the mixing vessel is to be drained . a valve is disposed between the inlet 27 and inlet pipe 28 . one embodiment of such a valve is valve 34 which comprises a valve body 36 and a coil compression spring 38 which is screwed into a threaded portion of the upper part of an elongated bore 40 which is formed in and extends through the valve body . a solid plate 42 is secured to the upper end of spring 38 to close the open end . the coils of the compression spring 38 are normally tightly fitted against one another as seen in fig2 so that material may not pass therethrough in either direction in the absence of some force which is sufficient to pull them apart as seen in fig3 . the inlet pipe 28 is appropriately connected to an inlet opening 43 which communicates with bore 40 . the lower end of the valve body 36 may be formed with a counterbore 44 formed concentrically with bore 40 . the counterbore 44 receives a flange 46 which may be held in place by a retainer ring 48 . a seal such as an o - ring seal 50 may be disposed in the upper surface of the flange 46 for sealing contact with the face 52 of the valve body . the flange 46 may be formed with a central threaded bore 54 for threadedly receiving therethrough an elongated pin or stem 56 which extends through bore 40 . the stem 56 is long enough so that the upper end thereof may be moved into engagement with the underside of plate 42 . rotation of the stem 56 by means of associated lever 57 to move it upwardly against plate 42 will stretch spring 38 opening the coils thereof . an annular flange 58 may be secured to the lower end of the stem 56 by suitable means such as welding . the flange 58 is designed for slidable engagement in the bore 40 . the flange 58 may have a seal member such as an o - ring seal 59 disposed on its outer periphery to thereby provide a seal with the bore 40 . a seal member such as an o - ring seal 60 may be associated with the upper surface 62 of the valve body for sealing engagement with the base 64 of the mixing vessel 12 . the base 64 may be formed with a central bore 66 through which the coil spring 38 extends and a counterbore 68 receiving the flange 70 of the valve body . the valve 34 preferably is positioned at the center of the base 64 so that the coil spring valve 38 is positioned directly under the end of the shaft 14 . it will be apparent that when product is pumped into the mixing vessel under pressure by way of conduits 28 and 29 and bore 40 from the pump 24 , the coils of the coil spring valve 38 will open in response to the pressure of the product on the plate 42 attached to the end of the spring 38 . when the pressure is reduced below a certain amount , the spring check valve will close preventing product from backing into the conduits 28 and 29 . after a product run , if it becomes desirable to remove the remaining product from the mixing vessel , the valve 34 may be used as a dump valve . to do this , the stem 56 is advanced against the underside of the plate 42 by rotating it in the threaded connection at 54 , thereby pulling apart the coils of the compression spring 38 . this permits the product to drain through the spring 38 , the bore 40 and the conduit 28 through valve 32 to drain outlet 72 . an easily releasable clamping means such , for example , as a bar clamp 74 may be associated with the base to clamp the combination check and dump valve in place in the base of the mixing vessel . this permits easy removal of the valve . a second embodiment 134 of a combination check and dump valve is shown in fig4 and 5 . it operates in substantially the same manner as the valve 34 shown in fig2 and 3 . the primary difference is that in the valve of fig4 and 5 the stem for opening the coil spring valve for draining is advanced against the plate 42 by means of a cam arrangement associated with the lower end of the valve body . the valve 134 comprises a valve body 136 having a bore 140 into the upper end of which the coil spring valve 38 is threadedly secured . the lower end of the valve body 136 is formed with a smaller bore 142 coaxial with bore 140 . the lower end of the valve body may be formed with a substantially cylindrical boss 144 which has formed therein a spiral or helical camming slot 146 which may extend approximately half way around the boss . the slot 146 is adapted to accommodate a small pin or lever 148 which extends through the slot 146 into the lower end of stem 156 . the bore 142 has slidably disposed therein the enlarged end portion 157 of stem 156 . an o - ring seal 160 is positioned around the end portion 157 to provide a seal between the valve body and the stem . a radially extending opening 150 in the lower end portion 157 of the stem 156 is adapted to receive the pin 148 . rotation of the stem 156 and simultaneous advancement or retraction thereof is accomplished by moving the pin 148 circumferentially in the spirally formed camming slot 146 . the upper end of the valve body generally is provided with a seal arrangement for cooperation with the base of the mixing vessel . the upper end of the valve body may be constructed with a boss 160 which extends into a bore 162 of the base and around which an o - ring seal 164 is disposed in an undercut 166 . this arrangement could also be used on the embodiment shown in fig2 and 3 and the upper flange arrangement 70 could be used on the valve of fig4 and 5 . the coil spring valve is particularly desirable in this environment both as an inlet check valve and as a drain valve . since it is essential both during the movement of material into the vessel and draining of material ( at some other time ) that no clogging of the inlet take place , the coils of the compression spring valve open just enough to permit product to pass between the coils but not so much as to allow the dispersing media to lodge between the coils . the opening between the coils , of course , is controllable . it will be quite apparent that the embodiments of a combination check and dump valve disclosed herein are simple to manufacture , and are inexpensive . the valves are easily installed and removed from their assembly with the mixing vessel . the parts of the valves are easily assembled and taken apart . while certain preferred embodiments of the invention have been disclosed , it will be appreciated that these are shown by way of example only , and the invention is not to be limited thereto as other variations will be apparent to those skilled in the art and the invention is to be given its fullest possible interpretation within the terms of the following claims .