A side pocket mandrel designed primarily for controlling the flow of production fluid into a tubing string includes a sliding sleeve valve in the side wall receptacle for alternatively closing and opening the entry port into the receptacle. The sleeve valve is latched in its port-closing position, but is designed to enable removal from the receptacle bore for servicing; and a pulling tool is provided for removing the sleeve valve from the receptacle when desired. The sleeve is shifted to its port-opening position when a standing flow control valve is placed in the receptacle; and the standing valve has a large valve chamber and side entry and exit ports for the production fluid to provide for maximum flow. The standing valve provides a check valve into the producing zone, the valve closure member being a modified ball consisting of a cylindrical body with spherical ends and coacting with a spherical valve seat. During flow through the valve, the closure member is moved completely out of the flow path for maximum flow.

Elemental sulfur in substantially pure form is recovered from an underground sulfur formation which has been previously been mined by the Frasch process by injection of hot water into the underground formation to melt at least a portion of the sulfur, removing the resulting mixture of sulfur and hot water to the surface, separating the mixture of sulfur and hot water, recovering the sulfur as molten sulfur, and recycling the water to the underground formation. In alternatives, the water may be treated with additives to prevent corrosion, and scale and to reduce hardness and enhance separation of sulfur and water. The process is practiced using a conventional Frasch sulfur well in combination with a bleed water well and/or injection well. Also provided is a method and apparatus for separating molten sulfur from a mixture of molten sulfur and water.